Difference between revisions of "Natural selection" - New World Encyclopedia

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Note: This is only a very rough draft, with notes that may be useful in developing the article. Please do not edit this article until the actual article is complete — i.e., when this notice is removed. You may add comments on what you would like to see included. [[User:Rick Swarts|Rick Swarts]] 00:05, 28 Sep 2005 (UTC)
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'''Natural selection''' is the process by which biological [[organism]]s with favorable traits survive and [[reproduction|reproduce]] more successfully than organisms that do not possess such traits, and, conversely, organisms with deleterious traits survive and reproduce less successfully than organisms lacking such deleterious traits. This selection process is in response to forces in the natural world, as opposed to [[artificial selection]], whereby selection is made by a human being, such as a farmer selecting his breeding stock or variety of [[plant]]. Traditionally, natural selection has been applied to biological individuals; however, the process has also been applied to levels both below the individual (the gene) and above the individual ([[species]], higher taxa) (Dawkins 1988; Gould 2002).
  
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In Darwin's comprehensive theory of evolution, there can actually be elucidated at least five major, largely independent theories. The two basic theories, and the ones which I will treat here, are: (1) the theory of evolution by common descent, and (2) the theory of modification through natural selection. The first is a  kinematic theory which deals with non-causal relations between things — it deals with the pattern of evolution.  The latter is a dynamic theory which deals with mechanisms and causal relationships B it deals with the  process. Other theories offered by Darwin deal with (3) evolution as such (the fact of evolution), (4) the gradualness of evolution, and (5) populational speciation.
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Natural selection is a cornerstone of modern [[evolution|evolutionary]] theory. The term was introduced by [[Charles Darwin]] in his 1859 book ''The Origin of Species.'' The [[Evolution#Theory of natural selection|theory of evolution by natural selection]], as developed by Darwin, holds that natural selection results in favorable, heritable traits becoming more common in subsequent populations and, over time, is the creative force even in [[macroevolution|macroevolutionary]] changes, such as the development of new species, higher taxa, and major new designs.
  
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The "theory of descent with modification" essentially postulates that all organisms have descended from common ancestors by a continuous process of branching.  In other words, all life evolved from one kind of organism or from a few simple kinds, and each species arose in a single geographic location, from another species that preceded it in time.  Evolutionists have marshaled substantial evidence for the theory of descent with modification.  That is, the "pattern of evolution" is well documented by the fossil record, the distribution patterns of existing species, methods of dating fossils, and comparison of homologous structures. Interestingly, all of the classical arguments for evolution are fundamentally arguments for imperfections that reflect history.  They fit the pattern of observing that the leg of Reptile B is not the best for walking, because it evolved from Fish A. In other words, why would a rat run, a bat fly, a porpoise swim and a man type all with the same structures utilizing the same bones unless inherited from a common ancestor? 
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The existence of the mechanism of natural selection is nearly universally recognized, as is its ability to impact gene frequencies in populations ([[microevolution]]) and remove unfit [[phenotype]]s. However, the ability of natural selection to be the main creative force of changes on the macroevolutionary level, such as the development of higher taxa and major new designs, remains controversial.
  
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==Overview==
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Evidence is so overwhelming for the theory of descent with modification that only religious fundamentalists have attempted to challenge this theory. Among these are the Ascientific creationists.@  Scientific creationists@ are a specific group of creationists who maintain that modern organisms did not descend from common ancestors, and that their only historical connectedness is in the mind of God. Instead, scientific creationists promulgate the view that living organisms are immutable, and were all created by God in a short time period, on a earth whose age is generally measured in 1000s of years. The substantial fossil record is dismissed in various ways, including as a trick of God and as an artifact from the Great Flood (with some organisms sinking faster than others and thus on a lower fossil plane). Although some individual presentations by scientific creationists are quite sophisticated, the overall theory of scientific creationism runs counter to an enormous body of evidence and thus is strongly criticized by most of the scientific community.
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Evolutionist Ernst Mayr (2001) defines natural selection as "the process by which in every generation individuals of lower fitness are removed from the population."
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Natural selection generally is defined independently of whether or not there is actually an effect on the [[gene]]-frequency of a population. That is, it is limited to the selection process itself, whereby individuals in a population experience differential survival and reproduction based on a particular [[phenotype|phenotypic]] variation(s). If the phenotypic variation has a genetic basis, then natural selection will impact the gene-frequency of the population. If the variation does not have a genetic basis, then natural selection will not impact the gene-frequency. More inclusive definitions define natural selection as the process ''and'' a resulting change in the gene-frequency of the population. In these cases, only if there is a change in the gene-frequency can the mechanism be called natural selection.
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The ''theory of evolution by natural selection'' encompasses both minor changes in gene frequency in populations, brought about by the creative force of natural selection, and major evolutionary changes brought about through natural selection, such as the origin of new designs. For Darwin, the term ''natural selection'' was synonymous with ''[[Evolution|evolution by natural selection]].'' Other mechanisms of evolution, such as evolution by genetic drift, were not explicitly formulated at that time, but Darwin realized there may be other mechanisms at work: "I am convinced that it [natural selection] has been the main, but not exclusive means of modification" (Darwin 1859). Now, scientists use ''natural selection'' mainly to describe the ''mechanism,'' not the theory of evolution by natural selection. In this sense, natural selection includes ''any'' selection by a natural agent, including sexual selection (discussed below).
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Selection targets specific traits of an individual, and if such a trait has a heritable component, the frequency of that trait will increase in the next generation. So ''selection for'' a specific trait results in ''selection of'' certain individuals (Sober 1984). This distinction is important, because an individual is more than the trait ''selected for.'' For example, sometimes two or more traits are genetically linked through mechanisms such as pleiotropy (a single [[gene]] that affects multiple traits) and linkage disequilibrium (non-random association of two genes). Sometimes, selection of a trait relates to a specific function of that trait, while that trait also has other functions that are not affected by natural selection. In either case, direct ''selection for'' specific traits or functions results in indirect ''selection of'' other traits or functions.
  
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The second theory of Darwin, the "theory of modification through natural selection," is one explanation offered for how evolution might have occurred, i.e, the "process" by which evolution took place and arrived at the pattern. This theory of natural selection was the most revolutionary and controversial concept advanced by Darwin.  While the theory of descent with modification was accepted soon after its introduction, the theory of natural selection took until the mid-1900s to be accepted by the scientific community. By providing a purely non-teleogical, materialistic explanation for all phenomenon of living nature, it was said it "dethroned God." 
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'''Directionality of selection.''' Selection can be divided into diverse classes, on the basis of how it drives an [[allele]] (a specific version of a gene) to fixation or toward removal of the allele from the population. ''Positive'' or ''directional selection'' occurs when a certain allele confers a higher fitness than others, resulting in that allele increasing in frequency until it is fixed and the entire population expresses the more fit phenotype. Essentially, it causes the population distribution, with its range of phenotypes, to move in one direction, such as selecting for birds with larger beaks. Far more common is ''purifying'' or ''stabilizing selection,'' which lowers the frequency of alleles with deleterious phenotypes (that is, a lower fitness) until they are fixed out of the population entirely. It may remove phenotypes on both extremes of the population distribution, such as selecting for a certain birth size for newborns versus very large or very small offspring. ''Disruptive or diversifying selection'' favors [[genotype]]s that depart from the average in either direction (that is, the opposite of overdominance), and can result in a bimodal distribution of trait values, such as birds with small beaks and with large beaks, but not intermediate size beaks. This type of selection maintains variation in the population. ''Balancing selection'' refers to those selections that maintain an allele at intermediate frequencies in a population, rather than fixing the allele in the population. For example, this can occur in diploid species (with two pairs of [[chromosome]]s) when individuals with a combination of two different alleles at a single position on the chromosome (heterozygote) have a higher fitness than individuals that have two of the same alleles (homozygote). This is called heterozygote advantage or overdominance. For example, there is documented evidence of a heterozygote advantage of sickle cell anemia in humans in central African countries because of resistance conferred against [[malaria]].  
  
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According to this theory, natural selection is the directing or creative force of evolution. Natural selection is considered far more than just a minor force for weeding out unfit organisms. Even Paley and other natural theologians accepted natural selection, albeit as a devise for removing unfit organisms, rather than as a directive force for creating new species and new designs. Natural selection had three radical components-- (a) purposelessness (no higher purpose, just the struggle of individuals to survive and reproduce); (b) philosophical materialism (matter is seen as the ground of all existence with spirit and mind being produced by or a function of the material brain); and (c) the view that evolution is not progressive from lower to higher, but just an adaptation to local environments; it could form a man with his superior brain or a parasite, but no one could say which is higher or lower.
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'''Ecological selection and sexual selection.''' It is useful to make a distinction between ''ecological selection'' and ''sexual selection.'' Ecological selection covers any mechanism of selection as a result of the [[natural environment|environment]], such as temperature, predation, humidity, competition, and so forth. Sexual selection refers specifically to competition between organisms for mates. Sexual selection includes mechanisms such as ''mate choice'' and ''male-male competition,'' although the two forms can act in combination in some species, when females choose the winners of the male-male competition. Mate choice typically involves "female choice," but in some [[species]] it is the males that choose. Some features that are confined to one sex only of a particular species often are explained by selection exercised by the other sex in the choice of a mate, such as the extravagant plumage of some male [[bird]]s. Aggression between members of the same sex (intrasexual selection) is typically referred to as "male-male competition," and is sometimes associated with very distinctive features, such as the antlers of [[deer|stags]], which are used in combat with other stags. Sometimes, sexual selection is distinguished from natural selection, but it may also be considered one category of natural selection with ecological selection being another category.
  
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Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the systematic color change in the peppered moth, Biston  betularia which was observed over a 50-year period in England, or through artificial selection, whereby various breeds of animals and varieties of plants have been produced which are different in some respect from their ancestors.  The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level.  That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years.  If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors.  One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.
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==Mechanism==
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Natural selection acts on the outward form of an individual, the [[phenotype]]. The phenotype is the overall result of an individual's genetic make-up ([[genotype]]), the [[ecosystem|environment]], and the interactions between [[gene]]s and between genes and the environment.  
  
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Some of the confusion in the dialogue between evolutionists and creationists is what is being referred to by the term “evolution” or “theory of evolution.” For evolutionists, a working definition of the term "evolution" is generally descent with modification or a change of gene frequencies in populations.
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A key element in understanding natural selection is the concept of '''fitness.''' Natural selection acts on individuals, but its ''average'' effect on all individuals with a particular genotype is the fitness of that genotype. Fitness is measured as the proportion of progeny that survives, multiplied by the average fecundity (potential reproductive capacity), and it is equivalent to the ''reproductive success'' of a genotype. A fitness value of greater than one indicates that the frequency of that genotype in the population increases, while a value of less than one indicates that it decreases.
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Since there is considerable experimental and observational evidence of populations systematically changing over time, evolutionists speak of "the fact of evolution."  There is evidence on the microevolutionary level (change in gene frequencies within populations), in terms of artificial selection. On a macroevolutionary level (large-scale events such as speciation and origin of new designs), various evidences such as fossil records, biogeography, and studies of homologies have strongly supported the view that all organisms have descended from common ancestors. In fact, renowned evolutionist Mayr contends that “the facts of biogeography posed some of the most insoluble dilemmas for the creationists and were eventually used by Darwin as his most convincing evidence in favor of evolution.”
 
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Darwin helped to establish the "fact of evolution." In 1859, most scientists and laymen believed that the world was constant.  The massive evidence that Darwin presented was so convincing that within a few years every biologist became an evolutionist, believing that the world was the product of a continuing process of change. For most biologists today, evolution is no longer a theory but simply a fact. They may disagree with the mechanisms, but that evolution takes place — that there is a systematic change in populations — is unquestioned.
 
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The statement that “evolution is a fact,” draws the ire of scientific creationists, of course. However, scientific creationists represent only a small body of those individuals that do believe in a creation by a supreme being. Nonetheless, other religious adherents likewise often speak of opposition to evolution, despite having a belief system that allows descent with modification and change in gene frequencies in populations. There are a couple of ready explanations for this.
 
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For one, there is the case of terminological confusion. When some individuals and religious adherents use the term “evolution,” they are not referring to simply a systematic change in populations over time — which is a highly established fact — but are instead treating the word “evolution” as synonymous with the specific Darwinian theory of evolution by natural selection — a theory with which even some eminent evolutionists find troublesome as the sole explanation for observed changes.  Thus, religious adherents may reject “evolution” since they see the concept of randomness in natural selection as counter to their belief that a Supreme Being directs changes.
 
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Furthermore, popular writings often tend to create an artificial dichotomy : either belief in a Creator is correct or evolution is correct — an “either-or dichotomy” which tends to foster an erroneous view of the relationship between evolution and religion. By such means, evolution and religion (specifically creation by a God) are presented as if mutually exclusive alternatives. Thus, many religious adherents reject evolution out of hand, not wishing to reject God.
 
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Textbook authors have often confused the dialogue on evolution by treating the term as if it signified one unified whole — not only the fact of evolution having occurred, but also the specific Darwinian and neo-Darwinian theories regarding natural selection, gradualism, speciation, and so forth. Certain textbook authors, in particular, have exacerbated this terminological confusion by lumping “evidences of evolution” into a section placed immediately after a comprehensive presentation on Darwin's overall theory — thereby creating the misleading impression that the evidences are supporting all components of Darwin's theory, including natural selection. In reality, the confirming information is invariably limited to the phenomenon of evolution having occurred (descent from a common ancestor or change of gene frequencies in populations), or perhaps including evidence of natural selection within populations.  
 
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The issue has been further complicated by the fact that textbooks have persisted in presenting some proofs for evolution which are false or misleading, as pointed out by Jonathan Wells in his book Icons of Evolution. These widely-known but misleading teachings include the famous Miller –Urey experiment in which sparks are sent through a mixture of gases and yield the building blocks of amino acids, and the drawings by Ernst Haeckel of the early embryonic stages of such vertebrates as fish, chick, rabbits and humans, whereby it is exhibited that the earliest stages in all of these vertebrates are virtually identical. Wells reports that scientists have known for years both that the Miller-Urey experiments did not really approximate conditions of the early earth and that Haeckel had faked his drawings, since in reality the vertebrate embryos never look as similar as he made them look. These errors are well-known, yet textbook authors persist in using these examples. Another interesting case is the classic example of natural selection as seen in the case of the peppered moth (Biston belularia) in England, known as a case of industrial melanism, whereby a shift toward darker melanic forms is seen and attributed to an heightened predation by birds of the light-colored moths, because the lighter forms could  more easily be seen on the tree trunks which have been increasingly darkened from pollution. In these cases, individuals have known that peppered moths do not normally alight on tree trunks, and there are even inverse correlations with pollution in many situations. Textbook photos are generally staged by gluing or pinning the moths to tree trunks. Some authors have responded that they knew the peppered moth case had problems, but they were good examples because they were easily grasped by the students. Use of such flawed cases has the unfortunate consequence of causing distrust of science by the students.
 
  
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Natural selection can act on any phenotypic trait, and any aspect of the environment, including mates and competitors, can result in a selective pressure. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection is considered to often result in the maintenance of the situation.
  
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'''Natural selection''' is a process by which [[biology|biological]] [[population]]s are altered over time, as a result of the propagation of [[Heritability|heritable traits]] that affect the capacity of individual [[organism]]s to survive and [[reproduction|reproduce]]. It is one of several mechanisms that give rise to the [[evolution]] of biological [[species]] (other mechanisms include [[genetic drift]] and [[gene flow]].) However, natural selection has a special significance because it is believed to be the one responsible for organisms being [[Adaptation (biology)|adapted]] to their environment. The theory of natural selection was proposed by [[Charles Darwin]] and [[Alfred Russel Wallace]] in [[1858]], though vaguer and more obscure formulations had been arrived at by earlier workers.
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===Levels of selection===
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Natural selection is often discussed in terms of a struggle among individual organisms for reproductive success. However, other objects of natural selection have been suggested on levels both below and above the individual.
  
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Natural selection can be subdivided into two types: (i) [[ecological selection]], which arises from the portion of an organism's environment not related to direct sexual competition (such as the availability of [[food]], the presence of [[predator]]s, and so forth); and (ii) [[sexual selection]], which arises from the competition for mates between individuals of the same sex. The reason for this division is that the effects of sexual selection can produce results that seem counterintuitive from the point of view of ecological selection alone (a famous example being the tails of [[peacock]]s, which, though cumbersome, serve an important purpose in [[courtship]] displays.)
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Some have proposed the [[gene]] as the principal object of selection. Dawkins (1976) argued that "the fundamental unit of selection, and therefore of self-interest, is not the [[species]], nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity…. Selection occurs at only one lowest level—the gene." Gene selection theory, or the selfish gene theory, holds that natural selection acts through differential survival of genes, increasing the frequency of those alleles whose phenotypic effects successfully promote and allow for replication. While a number of evolutionists support this view, Mayr (2001), for one, considers gene selection as invalid, both because a gene is only one part of the [[genotype]] and natural selection acts on the [[phenotype]], and because it fails to recognize that genes do not act independently of other genes. Likewise, [[Stephen Jay Gould|Gould]] (2002) insists that only individuals can reproduce or die, and hence genes could not be the unit of selection.  
  
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Natural selection is distinguished from [[artificial selection]], which is the alteration of [[Domestication|domesticated]] species resulting from [[human]] intervention as opposed the "natural environment". However, the mechanisms of natural and artificial selection are essentially identical, and in fact the observed effects of artificial selection were used by Darwin to illustrate how natural selection works.
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Some, such as Gould (2002), recognize other hierarchical levels of selection, including groups of individuals, species, and higher taxa. Species selection also has been tied to the theory of [[punctuated equilibrium]], developed by Gould and Eldredge. Such levels of selection remain controversial. Many evolutionists recognize "kin selection," that being selection for traits that favor the survival and reproduction of close relatives who share similar genotypes (Mayr 2001).
  
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The modern theory of natural selection is formulated in terms of [[genetic]] differences between individuals, resulting in differences in the frequency of [[allele]]s in a population over successive generations. The [[genetic variation]] on which natural selection acts are now understood to arise from random [[mutation]]s.
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== An example: antibiotic resistance==
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[[Image:Antibiotic_resistance.gif|thumb|size=180px|right| '''Figure 1:''' Schematic representation of how antibiotic resistance is enhanced by natural selection. The top section represents a population of [[bacteria]] before exposure to an antibiotic. The middle section shows the population directly after exposure, the phase in which selection took place. The last section shows the distribution of resistance in a new generation of bacteria. The legend indicates the resistance levels of individuals.]]
  
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==Overview==
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A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. [[Antibiotic]]s have been used to fight [[bacteria]]l diseases since the discovery of [[penicillin]] in 1928 by Alexander Fleming. However, the widespread use of antibiotics has led to increased microbial resistance against antibiotics, to the point that the methicillin-resistant ''Staphylococcus aureus'' (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.
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Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of [[mutation]]s. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them a little less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. This selective elimination of "maladapted" individuals from a population is natural selection in action.
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These surviving bacteria will then reproduce again, producing the next generation. Due to the elimination of the maladapted individuals in the past generation, this population contains more bacteria that have some resistance against the antibiotic. At the same time, new mutations occur, contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, very few have any effect at all, and usually any effect is deleterious. However, populations of bacteria are enormous, and so a few individuals may have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic. Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge.
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Recently, several new strains of MRSA have emerged that are resistant to vancomycin and teicoplanin. This exemplifies a situation where medical researchers continue to develop new antibiotics that can kill the bacteria, and this leads to resistance to the new antibiotics. A similar situation occurs with pesticide resistance in [[plant]]s and [[insect]]s.
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==Evolution by means of natural selection==
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''See also:'' [[Evolution]] and [[Darwinism]]
  
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The basic concept of natural selection is that conditions (or "nature") determine (or "select") how well particular traits of organisms can serve the survival and reproduction of the organism; organisms lacking these traits might die before reproducing, or be less prolific.  As long as environmental conditions remain the same, or similar enough that these traits continue to be adaptive, such traits will become more common within populations. Loss of the species' [[ecological niche]] or crowding-out due to [[population growth]] can change drastically the adaptive traits required to survive—in such conditions, or in any circumstance where survival is determined by ecology more than by the secondary sexual characteristics, an [[ecological selection]] is taking place (this term is used solely to differentiate processes irrelevant to mating, and is of modern usage, having grown up with the field of [[ecology]] itself).
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The [[Evolution#Theory of natural selection|theory of modification through natural selection]], or the theory of natural selection, postulates a process by which the mechanism of natural selection can lead to biological [[evolution]]. This theory is used to explain both evolution ''at or below the level of species'' ([[microevolution]]), such as changes in gene frequencies in populations and [[speciation]] phenomena, as well as major genetic changes ''above the species level'' ([[macroevolution]]), such as the development of novel traits (wings, feathers, jaws, etc.) and higher taxa (phyla, orders, etc.)
  
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Darwin's scientific theory of the [[evolution]] of [[species]] through natural selection starts from the premise that an organism's traits vary in a [[nondeterminism|nondeterministic]] way from parent to offspring, a process Darwin called "individuation".  This theory does not make any specific claims as to how this process works, although more recent scientific
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In the theory of natural selection, a prerequisite for natural selection to result in evolution, novel traits, and speciation is the presence of heritable genetic variation. Genetic variation is the result of [[mutation]]s, [[genetic recombination|recombination]]s, and alterations in the karyotype (the number, shape, size, and internal arrangement of the [[chromosome]]s). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are very rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in non-coding [[DNA]]. However, recent research suggests that many mutations in non-coding DNA do have slight deleterious effects. Overall, of those mutations that do affect the fitness of the individual, most are slightly deleterious, some reduce the fitness dramatically, and some increase the fitness.  
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discoveries in [[genetics]] explain several mechanisms that occur in the process of reproduction: in the case of both asexual and sexual reproduction, random mutation (including [[DNA]] [[transcription]] errors); in the case of sexual reproduction (which mixes the DNA of two parents into an offspring), [[gene flow]] and [[genetic drift]] are also important mechanisms. Competition (typically among males to impregnate females) for mates produces [[sexual selection]]—a process which Darwin considered secondary to ecological in most species.
 
  
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Natural selection does not distinguish between [[ecological selection]] and [[sexual selection]], as it is concerned with traits, for example, dexterity of movement, on which both may operate simultaneously. If a particular variation makes the offspring which manifest it better suited to survival or to successful reproduction, that offspring and its descendants will be more likely to survive than those offspring without the variation.  The original traits, as well as any maladaptive variations, will disappear as the offspring who carry them are replaced by their more successful relatives.
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[[Image:Pavo cristatus albino001xx.jpg|right|thumb|280px|The exuberant tail of the [[peacock]] is thought to be the result of sexual selection by females. This peacock is an albino—it carries a mutation that makes it unable to produce [[melanin]]. Selection against albinos in nature is intense because they are easily spotted by predators or are unsuccessful in competition for mates, and so these mutations are considered to be rapidly eliminated by natural selection.]]
  
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Therefore, certain traits are preserved due to the selective advantage they provide to their holders, allowing the individual to leave more offspring than individuals without the trait(s). Eventually, through many iterations of this process, organisms will develop more and more complex adaptive traits.
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Individuals with greater fitness are more likely to contribute offspring to the next generation, while individuals with lesser fitness are more likely to die early or fail to reproduce. As a result, genotypes with greater fitness become more abundant in the next generation, while genotypes with a lesser fitness become rarer.  
  
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==Mechanisms of natural selection==
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If the selection forces remain the same for many generations, beneficial genotypes become more and more abundant, until they dominate the population, while genotypes with a lesser fitness disappear. In every generation, new mutations and recombinations arise spontaneously, producing a new spectrum of phenotypes. Therefore, each new generation will be enriched by the increasing abundance of alleles that contribute to those traits that were favored by selection, enhancing these traits over successive generations.
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[[Image:Polydactyly 01 Lhand AP.jpg|left|thumb|150px|X-ray of the left hand of a ten year old boy with polydactyly.]]
  
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In Chapter 4 of ''The Origin of Species'', Darwin wrote:
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Some mutations occur in so-called regulatory genes. Changes in these can have large effects on the phenotype of the individual because they regulate the function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable zygotes. Mutations in some HOX genes in [[human]]s result in polydactyly, an increase in the number of fingers or toes (Zakany et al. 1997) or a cervical rib (Galis 1999). According to the theory of natural selection, when such mutations result in a higher fitness, natural selection will favor these phenotypes and the novel trait will spread in the population.
  
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:''It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.''
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Established traits are not immutable: an established trait may lose its fitness if environmental conditions change. The power of natural selection will also inevitably depend upon prevailing environmental factors; in general, the number of offspring is (far) greater than the number of individuals that can survive to the next generation, and there will be intense selection of the best-adapted individuals for the next generation.  
  
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What makes one trait more likely to succeed is highly dependent on environmental factors, including the species' predators, food sources, [[abiotic stress]], physical environment, and so on. When members of a species become separated, such as geographically, they face different environments and tend to develop in different directions. After a long period of time, their traits will have developed along different paths to such an extent that they can no longer interbreed, at which point they are considered separate species. This is why a species will sometimes separate into multiple species, rather than simply being replaced by a newer form of the species (from this fact Darwin suggested that all species today have evolved from a common ancestor).
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The theory of natural selection is one of two major theories presented by [[Charles Darwin|Darwin]], the other being the [[Evolution#Theory of descent with modification|theory of descent with modification]]. The theory of descent with modification deals with the pattern of evolution, while the theory of natural selection deals with the cause of evolutionary change. In other words, the theory of natural selection is an explanation offered for how evolution might have occurred, i.e, the "process" by which evolution took place and arrived at the pattern. It was the most revolutionary and controversial concept advanced by Darwin.  
  
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Additionally, some scientists have theorized that an adaptation which serves to make the organism more adaptable in the future will also tend to supplant its competitors even though it provides no specific advantage in the near term. Descendants of that organism will be more varied and therefore more resistant to extinction due to environmental catastrophes and [[extinction event|extinction events]]. This has been proposed as one reason for the rise of [[mammalia|mammals]]. While this form of selection is possible, it is more likely to play an important role in cases where selection for adaptation is continuous.  For example, the [[Red Queen]] hypothesis suggests that [[sex]] might have evolved to help organisms adapt to deal with parasites.
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According to this theory, natural selection is the directing or creative force of evolution. That is, it is more than just a force for weeding out unfit organisms—a concept accepted by natural theologians, who accepted it as a force for removing unfit organisms, but not for directing major evolutionary change.  
  
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Natural selection can be expressed as the following general law (taken from the conclusion of ''[[The Origin of Species]]''):
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Evidence for the theory of modification by natural selection is seen on the [[microevolution|microevolutionary]] level, such as the development of bacterial resistance. However, the view that natural selection is the primary causal agent in macroevolutionary change remains controversial. There are evolutionists, such as Gould (2002), who question whether one can extrapolate from microevolutionary change to macroevolutionary change (see [[macroevolution]]).
  
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#IF there are organisms that reproduce, and
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===History of the principle===
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#IF offspring inherit traits from their progenitor(s), and
 
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#IF there is variability of traits, and
 
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#IF the environment cannot support all members of a growing population,
 
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#THEN those members of the population with less-adaptive traits (determined by the environment) will die out, and
 
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#THEN those members with more-adaptive traits (determined by the environment) will thrive
 
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The result is the evolution of [[species]].
 
  
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This is a continuing process—it accounts for how species change, and can account for both the extinction of one species and the creation of a new one. The formulation does not rule out selection occurring at all biological levels (e.g., gene, organism, group), and the particular process of introducing new traits does not matter.
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[[Image:Charles Darwin aged 51.jpg|right|thumb|250px|The modern theory of natural selection derives from the work of [[Charles Darwin]] in the nineteenth century.]]
  
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Darwin did not maintain that natural selection was the only mechanism of evolution, however, as he pronounced in the introduction to ''The Origin of Species'': "I am convinced that [it] has been the most important, but not the exclusive means of modification."
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Until the early nineteenth century, the established view was that differences between individuals of a species were uninteresting departures from their [[Plato|Platonic]] ideal (or typus) of created kinds. However, growing awareness of the [[fossil]] record led to the recognition that [[species]] that lived in the distant past were often very different from those that exist today. Naturalists of the time tried to reconcile this with the emerging ideas of uniformitarianism in [[geology]]—the notion that simple, weak forces, acting continuously over very long periods of time, could have radical consequences, shaping the landscape as we know it today. Most importantly perhaps, these notions led to the awareness of the immensity of geological time, which makes it possible for slight causes to produce dramatic consequences. This opened the door to the notion that species might have arisen by descent with modification from ancestor species.  
  
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== History of the principle ==
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In the early years of the nineteenth century, evolutionists such as [[Jean-Baptiste Lamarck]] had proposed that characteristics (adaptations) acquired by individuals might be inherited by their progeny, causing, in enough time, transmutation of species (Lamarck 1809). By contrast, Darwin postulated that adaptation is a passive process in which the selective culling by nature of maladapted individuals results in an increase of the fittest individuals. He postulated that this simple process might be powerful enough to explain the evolution of the astounding ways in which organisms are adapted to their environments and the origins of the millions of species that exist.
  
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Charles Darwin's discovery of the principle of natural selection, as his explanation for the origin of species, occurred in about 1838.  Over the next twenty years, he shared it with only a very small number of acquaintances, while he amassed evidence in its favor. He first outlined his theory in two unpublished manuscripts, written in 1842 and 1844.  In 1858, Alfred Russel Wallace independently discovered the principle, and wrote a letter to Darwin, explaining his hypothesis.  This prompted a reading, at the [[Linnean Society]], of tracts from both men describing the principle that year.  Darwin published his detailed theory the following year, in ''[[The Origin of Species]]''.
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Evolutionary change can also happen without any selection, as a result of [[genetic drift]] or gene flow. However, ''adaptive'' change needs more than this, because it is very unlikely that favorable characteristics will consistently become more common in successive generations simply as a result of random fluctuations in occurrence. Favorable characteristics that can be attributed to [[gene]]s that become more common through evolution by natural selection are called ''adaptations.''  
  
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Unbeknownst to both Darwin and Wallace, the principle of natural selection had been previously hypothesized by others.  [[Pierre Louis Moreau de Maupertuis]] in 1745, [[Erasmus Darwin]] in 1794–1796, [[William Charles Wells]] in [[1813]], and [[Patrick Matthew]] in [[1831]] were amongst the first to grasp the idea. Maupertuis' discovery is in dispute, but has enough substantial evidence in its favor to warrant mention.  Erasmus Darwin was a contemporary and colleague of Wells—not to mention the grandfather of Charles Darwin—and he expressed much of his theory of evolution in poetic verse. His formal exposition of the hypothesis lacks a structured formulation, but has enough merit to be considered a possibility. Wells' hypothesis, applied solely to explain the origin of human races, had been presented in person at the [[Royal Society]].  Matthew's hypothesis had appeared in an appendix to his book on [[arboriculture]]. [[Richard Owen]] also claimed precedence over Darwin.  [[Edward Blyth]] had also proposed natural selection, as a mechanism of keeping species constant.
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Between 1842 and 1844, [[Charles Darwin]] outlined his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved." The concept was simple: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are inherited, then differential reproductive success will lead to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different might eventually become different species. Given enough time, novel new designs can originate.
  
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==Scope and role of natural selection==
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In the next twenty years, he shared these theories with just a few friends, while gathering evidence and trying to address all possible objections. In 1858, [[Alfred Russel Wallace]], a young naturalist, independently conceived the principle and described it in a letter to Darwin. Darwin contacted scientific friends to find an honorable way to handle this potentially embarrassing situation, and two short papers by the two were read at the Linnean Society announcing co-discovery of the principle. The following year, Darwin published ''The Origin of Species,'' outlining his theory in detail. It was quite controversial, both out of concern to whether it was powerful enough to result in speciation, and that it was "unguided" rather than "progressive."
  
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Natural selection need not apply solely to biological organisms; in theory, it applies to all systems in which entities reproduce in a way that includes both inheritance and variation. Thus, a form of natural selection can occur in the nonbiological realm.  Computer-based systems (e.g., [[artificial life]]) have shown that natural selection can be highly effective in adapting entities to their environments; whether such systems have demonstrated that natural selection ''per se'' can generate complexity is contested.{{ref|wolfram_complexity}}
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Darwin's ideas were inspired by the observations that he had made on the HMS ''Beagle,'' and by the economic theories of [[Thomas Malthus]], who noted that populations (if unchecked) increase exponentially whereas the food supply grows only arithmetically. Thus, inevitable limitations of resources would have demographic implications, leading to a struggle for existence, in which only the "fittest" would survive.  
  
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==Impact of the idea==
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Similar ideas go back to ancient times. The Ionian physician [[Empedocles]] said that many races "must have been unable to beget and continue their kind. For in the case of every species that exists, either craft or courage or speed has from the beginning of its existence protected and preserved it." Several eighteenth-century thinkers wrote about similar theories, including Pierre Louis Moreau de Maupertuis in 1745 and Darwin's grandfather Erasmus Darwin in 1794–1796. In the sixth edition of ''The Origin of Species,'' Darwin acknowledged that others—notably William Charles Wells in 1813, and Patrick Matthew in 1831—had proposed similar theories, but had not presented them fully or in notable scientific publications. Edward Blyth had also proposed a method of natural selection as a mechanism of keeping species constant. However, these precursors had little influence on evolutionary thought.
  
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Perhaps the most radical claim of Darwin's theory of evolution through natural selection is that "elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner" have evolved out of the simplest forms of life and according to a few simple principles.  It is this fundamental claim that has inspired some of Darwin's most ardent supporters—and that has provoked the most profound opposition.
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Within a decade of ''The Origin of Species,'' most educated people had begun to accept that evolution had occurred in some form or another. However, of the many ideas of evolution that emerged, only August Weismann's saw natural selection as the main evolutionary force (see [[modern evolutionary synthesis]]). Even [[Thomas Henry Huxley|T. H. Huxley]] believed that there was more "purpose" in evolution than natural selection afforded, and neo-[[Jean-Baptiste Lamarck|Lamarckism]] was also popular. After reading Darwin, [[Herbert Spencer]] introduced the term "survival of the fittest"; this became popular, and Wallace marked up an entire edition of ''The Origin of Species,'' replacing every instance of "natural selection" with Spencer's phrase. Although the phrase is still often used by non-biologists, modern biologists avoid it because it is tautological unless "fittest" is not read to mean "functionally superior."
  
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In addition, many theories of [[Artificial selection]] have been proposed to suggest that economic or social fitness factors assessed by other humans or their built environments are somehow biological or inevitable—[[Social Darwinism]].  Others held that there was an [[evolution of societies]] analogous to that of species. Many theories of [[eugenics]] were created in an attempt to address these issues.  Darwin's ideas, along with those of [[Adam Smith]] and [[Karl Marx]], are considered by most historians to have had a profound influence on [[19th-century]] thought.
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====The modern evolutionary synthesis====
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{{main article|Modern evolutionary synthesis}}
  
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==References==
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Only after the integration of Darwin's theory of evolution with a complex statistical appreciation of [[Gregor Mendel|Mendel]]'s "re-discovered" laws of inheritance did natural selection become generally accepted by scientists. The work of [[Ronald Fisher]] (who first attempted to explain natural selection in terms of the underlying genetic processes), [[J.B.S. Haldane]] (who introduced the concept of the "cost" of natural selection), [[Sewall Wright]] (one of the founders of population genetics), [[Theodosius Dobzhansky]] (who established the idea that mutation, by creating genetic diversity, supplied the raw material for natural selection), [[Ernst Mayr]] (who stressed the key importance of reproductive isolation for speciation), and many others formed the [[modern evolutionary synthesis]]. This propelled the theory of natural selection to the forefront of evolutionary theories, where it remains today.
  
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* {{note|darwin_origin}} Darwin, Charles (1859). ''The Origin of Species'' (First Edition).
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===Impact of the idea===
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* {{note|wolfram_complexity}} Wolfram, Stephen (2002). ''[[A New Kind of Science]]'', [http://www.wolframscience.com/nksonline/page-1001 p. 1001]. Wolfram Media, Inc. ISBN 1579550088
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Darwin's ideas, along with those of [[Adam Smith]] and [[Karl Marx]], had a profound influence on nineteenth-century thought. The radicalism of natural selection, according to [[Stephen Jay Gould]] (1997) lay in its power to "dethrone some of the deepest and most traditional comforts of Western thought." In particular, it challenged beliefs in nature's benevolence, order, and good design: the belief that humans occupy a summit of power and excellence; belief in an omnipotent, benevolent creator; and belief that nature has any meaningful direction, or that humans fit into any sensible pattern.
  
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== Further reading ==
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The social implications of the theory of evolution by natural selection also became the source of continuing controversy. [[Friedrich Engels|Engels]] in 1872 wrote that "Darwin did not know what a bitter satire he wrote on mankind when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of the animal kingdom." That natural selection had apparently led to "advancement" in intelligence and civilization also became used as a justification for [[colonialism]] and policies of [[eugenics]]—see [[social Darwinism]]. [[Konrad Lorenz]] won the Nobel Prize in 1973 for his analysis of animal behavior in terms of the role of natural selection (particularly group selection). However, in Germany in 1940, in writings that he subsequently disowned, he used the theory as a justification for policies of the [[Nazi]] state. He wrote "…selection for toughness, heroism, and social utility…must be accomplished by some human institution, if mankind, in default of selective factors, is not to be ruined by domestication-induced degeneracy. The racial idea as the basis of our state has already accomplished much in this respect."
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* Endler, John A (1986). "Natural Selection in the Wild". Princeton University Press.
 
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* Maynard Smith, John (1993). "The Theory of Evolution. Cambridge University Press.
 
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* Williams, George C (1992). Natural Selection: Domains, Levels and Challenges. Oxford University Press
 
  
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== External links ==
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Others have developed ideas that human societies and culture evolve by mechanisms that are analogous to those that apply to evolution of species (Wilson 2002).
  
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* [http://www.talkorigins.org/faqs/faq-intro-to-biology.html Introduction to evolutionary biology] (has a section on natural selection in context of evolution)
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While natural selection is widely accepted as a force in nature, and the theory of modification by natural selection has been demonstrated on the [[microevolution|microevolutionary]] level, the theory remains controversial as an explanation for [[macroevolution|macroevolutonary]] change.
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* [http://www.chains-of-reason.org/chains/evolution-by-natural-selection/introduction.htm Evolution by Natural Selection] - An introduction to the logic of the theory of natural selection
 
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* [http://www.talkorigins.org/faqs/precursors/precursnatsel.html#r2 Darwin's Precursors and Influences. Part 4 — Natural selection]; by John Wilkins
 
  
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==See also==
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==References==
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* [[adaptation]]
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* Darwin, C. 1859. ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.'' London: John Murray.
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* [[artificial selection]]
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**Modern reprint: Darwin, C. 2003. ''The Origin of Species.'' Signet Classics. ISBN 0-45-152906-5
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* [[directional selection]]
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* Dawkins, R. 1976 [1989]. ''The Selfish Gene,'' 2nd reprint. Oxford: Oxford University Press. ISBN 0192860925
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* [[disruptive selection]]
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* Dawkins, R. 1982. ''The Extended Phenotype.'' Oxford: Oxford University Press.
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* [[ecological selection]]
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* Dobzhansky, T. 1937. ''Genetics and the Origin of Species.'' New York: Columbia University Press (2nd ed., 1941; 3rd ed., 1951).
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* [[evolution]]
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* Eisenberg, L. 2005. Which image for Lorenz? ''Am J Psychiatry'' 162:1760 [http://ajp.psychiatryonline.org/cgi/content/full/162/9/1760].
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* [[fitness (biology)|Fitness]]
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* Endler, J. A. 1986. ''Natural Selection in the Wild.'' Princeton, NJ: Princeton University Press.
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* [[genetic drift]]
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* Engels, F. 1873. ''Dialectics of Nature.'' Moscow: Progress, 1964.
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* [[negative selection]]
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* Fisher, R. A. 1930. ''The Genetical Theory of Natural Selection.'' Oxford: Clarendon Press.
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* [[selection]]
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* Futuyma, D. J. 2005. ''Evolution.'' Sunderland, MA: Sinauer Associates. ISBN 0878931872.
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* [[sexual selection]]
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* Galis, F. 1999. Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. ''J Exp Zool'' 285:19–26.
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* [[stabilizing selection]]
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* Gould, S. J. 1997. Darwinian fundamentalism. ''New York Review of Books'' 44(10): June 12, 1997. http://www.nybooks.com/articles/1151 (accessed May 25, 2006).
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* [[survival of the fittest]]
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* Gould, S. J. 2002. ''The Structure of Evolutionary Thought.'' Cambridge, MA: Belknap Press of Harvard University Press.
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* Haldane, J. B. S. 1932. ''The Causes of Evolution.'' London: Longmans, Green & Co.
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* Haldane, J. B. S. 1957. The cost of natural selection. ''J Genet'' 55:511–524.
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* Lande, R., and S. J. Arnold. 1983. The measurement of selection on correlated characters. ''Evolution'' 37:1210–1226.
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* Maynard Smith, J. 1993. ''The Theory of Evolution.'' Cambridge University Press.
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* Mayr, E. 1942. ''Systematics and the Origin of Species.'' New York: Columbia University Press.
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* Mayr, E. 1982. ''The Growth of Biological Thought: Diversity, Evolution, and Inheritance.'' Cambridge, MA: Belknap Press of Harvard University Press.
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*Mayr, E. 2002. ''What Evolution Is.'' New York: Basic Books.
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* Schito, G. C. 2006. The importance of the development of antibiotic resistance in ''Staphylococcus aureus.'' ''Clin Microbiol Infect'' 12:3–8.
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* Sober, E. 1984. ''The Nature of Selection: Evolutionary Theory in Philosophical Focus.'' University of Chicago Press. ISBN 0226767485.
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* Williams, G. C. 1992. ''Natural Selection: Domains, Levels and Challenges.'' Oxford University Press.
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* Wilson, D. S. 2002. ''Darwin's Cathedral: Evolution, Religion, and the Nature of Society.'' University of Chicago Press. ISBN 0226901343
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* Wolfram, S. 2002. ''A New Kind of Science.'' Wolfram Media [http://www.wolframscience.com/nksonline/page-1001]. ISBN 1579550088
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* Wright, S. 1932. The roles of mutation, inbreeding, crossbreeding and selection in evolution. ''Proc 6th Int Cong Genet'' 1:356–366.
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* Zakany J., C. Fromental-Ramain, X. Warot, and D. Duboule. 1997. Regulation of number and size of digits by posterior Hox genes: a dose-dependent mechanism with potential evolutionary implications. ''Proc Natl Acad Sci USA'' 94:13695–13700.
  
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{{credit2|Natural_selection|54759635|Gene-centered_view_of_evolution|54978529}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
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[[Category:Evolution]]

Latest revision as of 15:29, 3 July 2010


Natural selection is the process by which biological organisms with favorable traits survive and reproduce more successfully than organisms that do not possess such traits, and, conversely, organisms with deleterious traits survive and reproduce less successfully than organisms lacking such deleterious traits. This selection process is in response to forces in the natural world, as opposed to artificial selection, whereby selection is made by a human being, such as a farmer selecting his breeding stock or variety of plant. Traditionally, natural selection has been applied to biological individuals; however, the process has also been applied to levels both below the individual (the gene) and above the individual (species, higher taxa) (Dawkins 1988; Gould 2002).

Natural selection is a cornerstone of modern evolutionary theory. The term was introduced by Charles Darwin in his 1859 book The Origin of Species. The theory of evolution by natural selection, as developed by Darwin, holds that natural selection results in favorable, heritable traits becoming more common in subsequent populations and, over time, is the creative force even in macroevolutionary changes, such as the development of new species, higher taxa, and major new designs.

The existence of the mechanism of natural selection is nearly universally recognized, as is its ability to impact gene frequencies in populations (microevolution) and remove unfit phenotypes. However, the ability of natural selection to be the main creative force of changes on the macroevolutionary level, such as the development of higher taxa and major new designs, remains controversial.

Overview

Evolutionist Ernst Mayr (2001) defines natural selection as "the process by which in every generation individuals of lower fitness are removed from the population."

Natural selection generally is defined independently of whether or not there is actually an effect on the gene-frequency of a population. That is, it is limited to the selection process itself, whereby individuals in a population experience differential survival and reproduction based on a particular phenotypic variation(s). If the phenotypic variation has a genetic basis, then natural selection will impact the gene-frequency of the population. If the variation does not have a genetic basis, then natural selection will not impact the gene-frequency. More inclusive definitions define natural selection as the process and a resulting change in the gene-frequency of the population. In these cases, only if there is a change in the gene-frequency can the mechanism be called natural selection.

The theory of evolution by natural selection encompasses both minor changes in gene frequency in populations, brought about by the creative force of natural selection, and major evolutionary changes brought about through natural selection, such as the origin of new designs. For Darwin, the term natural selection was synonymous with evolution by natural selection. Other mechanisms of evolution, such as evolution by genetic drift, were not explicitly formulated at that time, but Darwin realized there may be other mechanisms at work: "I am convinced that it [natural selection] has been the main, but not exclusive means of modification" (Darwin 1859). Now, scientists use natural selection mainly to describe the mechanism, not the theory of evolution by natural selection. In this sense, natural selection includes any selection by a natural agent, including sexual selection (discussed below).

Selection targets specific traits of an individual, and if such a trait has a heritable component, the frequency of that trait will increase in the next generation. So selection for a specific trait results in selection of certain individuals (Sober 1984). This distinction is important, because an individual is more than the trait selected for. For example, sometimes two or more traits are genetically linked through mechanisms such as pleiotropy (a single gene that affects multiple traits) and linkage disequilibrium (non-random association of two genes). Sometimes, selection of a trait relates to a specific function of that trait, while that trait also has other functions that are not affected by natural selection. In either case, direct selection for specific traits or functions results in indirect selection of other traits or functions.

Directionality of selection. Selection can be divided into diverse classes, on the basis of how it drives an allele (a specific version of a gene) to fixation or toward removal of the allele from the population. Positive or directional selection occurs when a certain allele confers a higher fitness than others, resulting in that allele increasing in frequency until it is fixed and the entire population expresses the more fit phenotype. Essentially, it causes the population distribution, with its range of phenotypes, to move in one direction, such as selecting for birds with larger beaks. Far more common is purifying or stabilizing selection, which lowers the frequency of alleles with deleterious phenotypes (that is, a lower fitness) until they are fixed out of the population entirely. It may remove phenotypes on both extremes of the population distribution, such as selecting for a certain birth size for newborns versus very large or very small offspring. Disruptive or diversifying selection favors genotypes that depart from the average in either direction (that is, the opposite of overdominance), and can result in a bimodal distribution of trait values, such as birds with small beaks and with large beaks, but not intermediate size beaks. This type of selection maintains variation in the population. Balancing selection refers to those selections that maintain an allele at intermediate frequencies in a population, rather than fixing the allele in the population. For example, this can occur in diploid species (with two pairs of chromosomes) when individuals with a combination of two different alleles at a single position on the chromosome (heterozygote) have a higher fitness than individuals that have two of the same alleles (homozygote). This is called heterozygote advantage or overdominance. For example, there is documented evidence of a heterozygote advantage of sickle cell anemia in humans in central African countries because of resistance conferred against malaria.

Ecological selection and sexual selection. It is useful to make a distinction between ecological selection and sexual selection. Ecological selection covers any mechanism of selection as a result of the environment, such as temperature, predation, humidity, competition, and so forth. Sexual selection refers specifically to competition between organisms for mates. Sexual selection includes mechanisms such as mate choice and male-male competition, although the two forms can act in combination in some species, when females choose the winners of the male-male competition. Mate choice typically involves "female choice," but in some species it is the males that choose. Some features that are confined to one sex only of a particular species often are explained by selection exercised by the other sex in the choice of a mate, such as the extravagant plumage of some male birds. Aggression between members of the same sex (intrasexual selection) is typically referred to as "male-male competition," and is sometimes associated with very distinctive features, such as the antlers of stags, which are used in combat with other stags. Sometimes, sexual selection is distinguished from natural selection, but it may also be considered one category of natural selection with ecological selection being another category.

Mechanism

Natural selection acts on the outward form of an individual, the phenotype. The phenotype is the overall result of an individual's genetic make-up (genotype), the environment, and the interactions between genes and between genes and the environment.

A key element in understanding natural selection is the concept of fitness. Natural selection acts on individuals, but its average effect on all individuals with a particular genotype is the fitness of that genotype. Fitness is measured as the proportion of progeny that survives, multiplied by the average fecundity (potential reproductive capacity), and it is equivalent to the reproductive success of a genotype. A fitness value of greater than one indicates that the frequency of that genotype in the population increases, while a value of less than one indicates that it decreases.

Natural selection can act on any phenotypic trait, and any aspect of the environment, including mates and competitors, can result in a selective pressure. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection is considered to often result in the maintenance of the situation.

Levels of selection

Natural selection is often discussed in terms of a struggle among individual organisms for reproductive success. However, other objects of natural selection have been suggested on levels both below and above the individual.

Some have proposed the gene as the principal object of selection. Dawkins (1976) argued that "the fundamental unit of selection, and therefore of self-interest, is not the species, nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity…. Selection occurs at only one lowest level—the gene." Gene selection theory, or the selfish gene theory, holds that natural selection acts through differential survival of genes, increasing the frequency of those alleles whose phenotypic effects successfully promote and allow for replication. While a number of evolutionists support this view, Mayr (2001), for one, considers gene selection as invalid, both because a gene is only one part of the genotype and natural selection acts on the phenotype, and because it fails to recognize that genes do not act independently of other genes. Likewise, Gould (2002) insists that only individuals can reproduce or die, and hence genes could not be the unit of selection.

Some, such as Gould (2002), recognize other hierarchical levels of selection, including groups of individuals, species, and higher taxa. Species selection also has been tied to the theory of punctuated equilibrium, developed by Gould and Eldredge. Such levels of selection remain controversial. Many evolutionists recognize "kin selection," that being selection for traits that favor the survival and reproduction of close relatives who share similar genotypes (Mayr 2001).

An example: antibiotic resistance

Figure 1: Schematic representation of how antibiotic resistance is enhanced by natural selection. The top section represents a population of bacteria before exposure to an antibiotic. The middle section shows the population directly after exposure, the phase in which selection took place. The last section shows the distribution of resistance in a new generation of bacteria. The legend indicates the resistance levels of individuals.

A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. Antibiotics have been used to fight bacterial diseases since the discovery of penicillin in 1928 by Alexander Fleming. However, the widespread use of antibiotics has led to increased microbial resistance against antibiotics, to the point that the methicillin-resistant Staphylococcus aureus (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.

Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them a little less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. This selective elimination of "maladapted" individuals from a population is natural selection in action.

These surviving bacteria will then reproduce again, producing the next generation. Due to the elimination of the maladapted individuals in the past generation, this population contains more bacteria that have some resistance against the antibiotic. At the same time, new mutations occur, contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, very few have any effect at all, and usually any effect is deleterious. However, populations of bacteria are enormous, and so a few individuals may have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic. Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge.

Recently, several new strains of MRSA have emerged that are resistant to vancomycin and teicoplanin. This exemplifies a situation where medical researchers continue to develop new antibiotics that can kill the bacteria, and this leads to resistance to the new antibiotics. A similar situation occurs with pesticide resistance in plants and insects.

Evolution by means of natural selection

See also: Evolution and Darwinism

The theory of modification through natural selection, or the theory of natural selection, postulates a process by which the mechanism of natural selection can lead to biological evolution. This theory is used to explain both evolution at or below the level of species (microevolution), such as changes in gene frequencies in populations and speciation phenomena, as well as major genetic changes above the species level (macroevolution), such as the development of novel traits (wings, feathers, jaws, etc.) and higher taxa (phyla, orders, etc.)

In the theory of natural selection, a prerequisite for natural selection to result in evolution, novel traits, and speciation is the presence of heritable genetic variation. Genetic variation is the result of mutations, recombinations, and alterations in the karyotype (the number, shape, size, and internal arrangement of the chromosomes). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are very rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in non-coding DNA. However, recent research suggests that many mutations in non-coding DNA do have slight deleterious effects. Overall, of those mutations that do affect the fitness of the individual, most are slightly deleterious, some reduce the fitness dramatically, and some increase the fitness.

The exuberant tail of the peacock is thought to be the result of sexual selection by females. This peacock is an albino—it carries a mutation that makes it unable to produce melanin. Selection against albinos in nature is intense because they are easily spotted by predators or are unsuccessful in competition for mates, and so these mutations are considered to be rapidly eliminated by natural selection.

Individuals with greater fitness are more likely to contribute offspring to the next generation, while individuals with lesser fitness are more likely to die early or fail to reproduce. As a result, genotypes with greater fitness become more abundant in the next generation, while genotypes with a lesser fitness become rarer.

If the selection forces remain the same for many generations, beneficial genotypes become more and more abundant, until they dominate the population, while genotypes with a lesser fitness disappear. In every generation, new mutations and recombinations arise spontaneously, producing a new spectrum of phenotypes. Therefore, each new generation will be enriched by the increasing abundance of alleles that contribute to those traits that were favored by selection, enhancing these traits over successive generations.

X-ray of the left hand of a ten year old boy with polydactyly.

Some mutations occur in so-called regulatory genes. Changes in these can have large effects on the phenotype of the individual because they regulate the function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable zygotes. Mutations in some HOX genes in humans result in polydactyly, an increase in the number of fingers or toes (Zakany et al. 1997) or a cervical rib (Galis 1999). According to the theory of natural selection, when such mutations result in a higher fitness, natural selection will favor these phenotypes and the novel trait will spread in the population.

Established traits are not immutable: an established trait may lose its fitness if environmental conditions change. The power of natural selection will also inevitably depend upon prevailing environmental factors; in general, the number of offspring is (far) greater than the number of individuals that can survive to the next generation, and there will be intense selection of the best-adapted individuals for the next generation.

The theory of natural selection is one of two major theories presented by Darwin, the other being the theory of descent with modification. The theory of descent with modification deals with the pattern of evolution, while the theory of natural selection deals with the cause of evolutionary change. In other words, the theory of natural selection is an explanation offered for how evolution might have occurred, i.e, the "process" by which evolution took place and arrived at the pattern. It was the most revolutionary and controversial concept advanced by Darwin.

According to this theory, natural selection is the directing or creative force of evolution. That is, it is more than just a force for weeding out unfit organisms—a concept accepted by natural theologians, who accepted it as a force for removing unfit organisms, but not for directing major evolutionary change.

Evidence for the theory of modification by natural selection is seen on the microevolutionary level, such as the development of bacterial resistance. However, the view that natural selection is the primary causal agent in macroevolutionary change remains controversial. There are evolutionists, such as Gould (2002), who question whether one can extrapolate from microevolutionary change to macroevolutionary change (see macroevolution).

History of the principle

The modern theory of natural selection derives from the work of Charles Darwin in the nineteenth century.

Until the early nineteenth century, the established view was that differences between individuals of a species were uninteresting departures from their Platonic ideal (or typus) of created kinds. However, growing awareness of the fossil record led to the recognition that species that lived in the distant past were often very different from those that exist today. Naturalists of the time tried to reconcile this with the emerging ideas of uniformitarianism in geology—the notion that simple, weak forces, acting continuously over very long periods of time, could have radical consequences, shaping the landscape as we know it today. Most importantly perhaps, these notions led to the awareness of the immensity of geological time, which makes it possible for slight causes to produce dramatic consequences. This opened the door to the notion that species might have arisen by descent with modification from ancestor species.

In the early years of the nineteenth century, evolutionists such as Jean-Baptiste Lamarck had proposed that characteristics (adaptations) acquired by individuals might be inherited by their progeny, causing, in enough time, transmutation of species (Lamarck 1809). By contrast, Darwin postulated that adaptation is a passive process in which the selective culling by nature of maladapted individuals results in an increase of the fittest individuals. He postulated that this simple process might be powerful enough to explain the evolution of the astounding ways in which organisms are adapted to their environments and the origins of the millions of species that exist.

Evolutionary change can also happen without any selection, as a result of genetic drift or gene flow. However, adaptive change needs more than this, because it is very unlikely that favorable characteristics will consistently become more common in successive generations simply as a result of random fluctuations in occurrence. Favorable characteristics that can be attributed to genes that become more common through evolution by natural selection are called adaptations.

Between 1842 and 1844, Charles Darwin outlined his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved." The concept was simple: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are inherited, then differential reproductive success will lead to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different might eventually become different species. Given enough time, novel new designs can originate.

In the next twenty years, he shared these theories with just a few friends, while gathering evidence and trying to address all possible objections. In 1858, Alfred Russel Wallace, a young naturalist, independently conceived the principle and described it in a letter to Darwin. Darwin contacted scientific friends to find an honorable way to handle this potentially embarrassing situation, and two short papers by the two were read at the Linnean Society announcing co-discovery of the principle. The following year, Darwin published The Origin of Species, outlining his theory in detail. It was quite controversial, both out of concern to whether it was powerful enough to result in speciation, and that it was "unguided" rather than "progressive."

Darwin's ideas were inspired by the observations that he had made on the HMS Beagle, and by the economic theories of Thomas Malthus, who noted that populations (if unchecked) increase exponentially whereas the food supply grows only arithmetically. Thus, inevitable limitations of resources would have demographic implications, leading to a struggle for existence, in which only the "fittest" would survive.

Similar ideas go back to ancient times. The Ionian physician Empedocles said that many races "must have been unable to beget and continue their kind. For in the case of every species that exists, either craft or courage or speed has from the beginning of its existence protected and preserved it." Several eighteenth-century thinkers wrote about similar theories, including Pierre Louis Moreau de Maupertuis in 1745 and Darwin's grandfather Erasmus Darwin in 1794–1796. In the sixth edition of The Origin of Species, Darwin acknowledged that others—notably William Charles Wells in 1813, and Patrick Matthew in 1831—had proposed similar theories, but had not presented them fully or in notable scientific publications. Edward Blyth had also proposed a method of natural selection as a mechanism of keeping species constant. However, these precursors had little influence on evolutionary thought.

Within a decade of The Origin of Species, most educated people had begun to accept that evolution had occurred in some form or another. However, of the many ideas of evolution that emerged, only August Weismann's saw natural selection as the main evolutionary force (see modern evolutionary synthesis). Even T. H. Huxley believed that there was more "purpose" in evolution than natural selection afforded, and neo-Lamarckism was also popular. After reading Darwin, Herbert Spencer introduced the term "survival of the fittest"; this became popular, and Wallace marked up an entire edition of The Origin of Species, replacing every instance of "natural selection" with Spencer's phrase. Although the phrase is still often used by non-biologists, modern biologists avoid it because it is tautological unless "fittest" is not read to mean "functionally superior."

The modern evolutionary synthesis

Main article: Modern evolutionary synthesis

Only after the integration of Darwin's theory of evolution with a complex statistical appreciation of Mendel's "re-discovered" laws of inheritance did natural selection become generally accepted by scientists. The work of Ronald Fisher (who first attempted to explain natural selection in terms of the underlying genetic processes), J.B.S. Haldane (who introduced the concept of the "cost" of natural selection), Sewall Wright (one of the founders of population genetics), Theodosius Dobzhansky (who established the idea that mutation, by creating genetic diversity, supplied the raw material for natural selection), Ernst Mayr (who stressed the key importance of reproductive isolation for speciation), and many others formed the modern evolutionary synthesis. This propelled the theory of natural selection to the forefront of evolutionary theories, where it remains today.

Impact of the idea

Darwin's ideas, along with those of Adam Smith and Karl Marx, had a profound influence on nineteenth-century thought. The radicalism of natural selection, according to Stephen Jay Gould (1997) lay in its power to "dethrone some of the deepest and most traditional comforts of Western thought." In particular, it challenged beliefs in nature's benevolence, order, and good design: the belief that humans occupy a summit of power and excellence; belief in an omnipotent, benevolent creator; and belief that nature has any meaningful direction, or that humans fit into any sensible pattern.

The social implications of the theory of evolution by natural selection also became the source of continuing controversy. Engels in 1872 wrote that "Darwin did not know what a bitter satire he wrote on mankind when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of the animal kingdom." That natural selection had apparently led to "advancement" in intelligence and civilization also became used as a justification for colonialism and policies of eugenics—see social Darwinism. Konrad Lorenz won the Nobel Prize in 1973 for his analysis of animal behavior in terms of the role of natural selection (particularly group selection). However, in Germany in 1940, in writings that he subsequently disowned, he used the theory as a justification for policies of the Nazi state. He wrote "…selection for toughness, heroism, and social utility…must be accomplished by some human institution, if mankind, in default of selective factors, is not to be ruined by domestication-induced degeneracy. The racial idea as the basis of our state has already accomplished much in this respect."

Others have developed ideas that human societies and culture evolve by mechanisms that are analogous to those that apply to evolution of species (Wilson 2002).

While natural selection is widely accepted as a force in nature, and the theory of modification by natural selection has been demonstrated on the microevolutionary level, the theory remains controversial as an explanation for macroevolutonary change.

References
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  • Darwin, C. 1859. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray.
    • Modern reprint: Darwin, C. 2003. The Origin of Species. Signet Classics. ISBN 0-45-152906-5
  • Dawkins, R. 1976 [1989]. The Selfish Gene, 2nd reprint. Oxford: Oxford University Press. ISBN 0192860925
  • Dawkins, R. 1982. The Extended Phenotype. Oxford: Oxford University Press.
  • Dobzhansky, T. 1937. Genetics and the Origin of Species. New York: Columbia University Press (2nd ed., 1941; 3rd ed., 1951).
  • Eisenberg, L. 2005. Which image for Lorenz? Am J Psychiatry 162:1760 [1].
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  • Engels, F. 1873. Dialectics of Nature. Moscow: Progress, 1964.
  • Fisher, R. A. 1930. The Genetical Theory of Natural Selection. Oxford: Clarendon Press.
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  • Gould, S. J. 1997. Darwinian fundamentalism. New York Review of Books 44(10): June 12, 1997. http://www.nybooks.com/articles/1151 (accessed May 25, 2006).
  • Gould, S. J. 2002. The Structure of Evolutionary Thought. Cambridge, MA: Belknap Press of Harvard University Press.
  • Haldane, J. B. S. 1932. The Causes of Evolution. London: Longmans, Green & Co.
  • Haldane, J. B. S. 1957. The cost of natural selection. J Genet 55:511–524.
  • Lande, R., and S. J. Arnold. 1983. The measurement of selection on correlated characters. Evolution 37:1210–1226.
  • Maynard Smith, J. 1993. The Theory of Evolution. Cambridge University Press.
  • Mayr, E. 1942. Systematics and the Origin of Species. New York: Columbia University Press.
  • Mayr, E. 1982. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge, MA: Belknap Press of Harvard University Press.
  • Mayr, E. 2002. What Evolution Is. New York: Basic Books.
  • Schito, G. C. 2006. The importance of the development of antibiotic resistance in Staphylococcus aureus. Clin Microbiol Infect 12:3–8.
  • Sober, E. 1984. The Nature of Selection: Evolutionary Theory in Philosophical Focus. University of Chicago Press. ISBN 0226767485.
  • Williams, G. C. 1992. Natural Selection: Domains, Levels and Challenges. Oxford University Press.
  • Wilson, D. S. 2002. Darwin's Cathedral: Evolution, Religion, and the Nature of Society. University of Chicago Press. ISBN 0226901343
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  • Wright, S. 1932. The roles of mutation, inbreeding, crossbreeding and selection in evolution. Proc 6th Int Cong Genet 1:356–366.
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