Natural selection

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 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.

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 and especially misuse 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 will 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 is an example of what is sometimes called an 'arms race', in which natural selection continues to select bacteria that are less susceptible to antibiotics, while medical researchers continue to develop new antibiotics that can kill them. A similar situation occurs with pesticide resistance in plants and insects.

Background and context

Until the early 19th 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 19th century, radical 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 (see Lamarckism). 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 (see survival of the fittest). He realised that this simple and apparently inevitable 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.

For Darwin, '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, and Darwin realised that: "I am convinced that [it] has been the main, but not exclusive means of modification." Now, scientists use 'natural selection' mainly to describe the mechanism. In this sense, natural selection includes any selection by a natural agent, including sexual selection and kin selection. Sometimes, sexual selection is distinguished from natural selection, but a more useful distinction is between sexual selection and ecological selection.

Definitions of natural selection

Scientists use several, slightly different definitions of natural selection; the main differences are whether the effect of selection is an integral part of the definition. Mechanistic definitions are limited to the selection process; whether or not there is an effect on the gene-frequency is not part of the definition: natural selection acts on the phenotype, but has an effect on subsequent generations only if the phenotypic variation has a genetic basis. Inclusive definitions define natural selection as the process and its result, and this requires that the phenotypic variation has a genetic basis. Thus, with an inclusive definition, the same mechanism acting on the same trait in two different species can be called natural selection in one, but not in another species when the trait in the second species lacks heritable genetic variation. This article uses the mechanistic definition of natural selection to describe and explore how it operates.

Mechanism

Some traits are determined by just a single gene, but most are affected by many different genes. Variation in most of these genes has only a small effect on the phenotypic value of a trait, and the study of the genetics of these quantitative traits is called quantitative genetics.

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. Often, natural selection acts on specific traits of an individual, and the terms phenotype and genotype are sometimes used narrowly to indicate these specific traits.

The 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, 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 phenotyic trait, and any aspect of the environment, including mates and conspecifics, can result in a selective pressure. However, this does not imply that natural selection is always directional and result in adaptive evolution; natural selection often results in the maintenance of the situation through purifying selection. The 'unit of selection' is not limited to the level of individuals, but includes other levels within the hierarchy of biological organisation, such as genes, cells and relatives. There is still debate, however, about whether natural selection acts at the level of groups or species, (i.e. selection for adaptations that benefit the group or species, rather than the individual). Selection at a different level than the individual, for example the gene, can result in an increase in fitness for that gene, while at the same time reducing the fitness of the individuals carrying that gene (see 'Intragenomic conflict' for more details). Overall, the combined effect of all selection pressures at various levels determines the overall fitness of an individual, and hence the outcome of natural selection.

File:Selection forces.gif

Figure 2: The life cycle of a sexually reproducing organism. Various components of natural selection are indicated for each life stage.

Natural selection occurs at every life stage of an individual (see Figure 2), and selection at any of these stages can affect the likelihood that an individual will survive and reproduce. After an individual is born, it has to survive until adulthood before it can reproduce, and selection of those that reach this stage is called viability selection. Adults of many species must compete with each other for mates (sexual selection), and success in this competition determines who will parent the next generation. When species reproduce more than once, a longer survival in the reproductive phase increases the number of offspring (survival selection). The fecundity of both females (e.g. how many eggs a female bird can produce) and males (e.g. giant sperm in certain species of Drosophila) can be limited (fecundity selection). The viability of produced gametes can differ, while intragenomic conflict (meiotic drive) between the haploid gametes can result in gametic or genic selection. Finally, the union of certain combinations of eggs and sperm might be more compatible that others (compatibility selection).

"Ecological selection" and "sexual selection"

It is also useful to make a mechanistic distinction between ecological selection and sexual selection. Ecological selection covers any mechanism of selection as a result of the environment (including relatives (e.g. kin selection) and conspecifics (e.g. competition, infanticide)), while sexual selection refers specifically to competition between conspecifics 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, or 'intersexual selection', typically involves 'female choice', as it is usually the females who are most choosy, but in some sex-role reversed species it is the males that choose. Some features that are confined to one sex only of a particular species can be explained by selection exercised by the other sex in the choice of a mate, e.g. 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. More generally, intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.

Directionality of selection

Selection can be divided into three classes, on the basis of how it drives an allele (a specific version of a gene) to fixation.

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.

Far more common is purifying or stabilizing selection, which lowers the frequency of alleles with a deleterious phenotype (that is, a lower fitness) until they are fixed out of the population entirely. Purifying selection results in functional genetic features (e.g. protein-coding sequences or regulatory sequences) being conserved over time due to selective pressure against deleterious variants.

Finally, a number of forms of balancing selection exist, which do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in diploid species (with two pair of chromosomes) when individuals with a combination of two different alleles at a single position at the chomosome (heterozygote) have a higher fitness than individuals that have two of the same alleles (homozygote). This is called heterozygote advantage or overdominance. Maintenance of allelic variation can also occur through disruptive or diversifying selection, which 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. Finally, it can occur through frequency-dependent selection, where the fitness of an individual phenotype depends upon the distribution of other phenotypes in the population (see also Game theory).

Levels

Notes: Add: stage of development that selection works at — zygote on different levels or objects of natural selection — gene - organisms - species

Selection and genetic variation

A portion of all genetic variation (sequence polymorphism) is functionally neutral; i.e., it produces no phenotypic effect or significant differences in fitness. Previously, this was though to encompass most of the genetic variation in non-coding DNA, but recent studies have shown that large parts of those sequences are highly conserved and under strong purifying selection. When genetic variation is functionally neutral, selection does not directly affect the frequency of such variation. Nevertheless, because of genetic linkage (the fact that two alleles are more likely to segragate together the closer together they are on a chromosome), selection can have a strong influence on patterns of variation in the genome.

Generally speaking, sites that are themselves under selection should be expected to have a lower density of genetic variation than neutrally-evolving sites (for purifying selection - other forms of selection are exceedingly rare in actual populations, or proceed so quickly towards fixation that they have no appreciable effect on the density of variation). However, in some instances such sites may instead show higher-than-average density if they are experiencing mutation-selection balance; if selection has favored the retention of highly-mutable sequences that would be quickly destroyed by mutations were they evolving neutrally, then such sites, being more prone to mutation, would have a higher than expected level of variation. However, purifying selection will weed out such variants before they reach an appreciable frequency; thus, balance (Note - this is not the same as balancing selection).

However, selection also acts on linked, neutral variation, and one of the few ways to ascertain ongoing selection is via examination of patterns of neutral variation in a genomic locus.

Selective sweeps occur when an allele is suddenly pushed upwards in frequency as a result of positive selection. As its frequency increases, linked alleles (those that are proximal to it on the chromosome) will also increase in frequency, whether they are neutral or even slightly deleterious. This phenomenon is called genetic hitchhiking. A strong selective sweep results in a region of the genome where the positively selected allele and its linked variants (which together form a haplotype) are essentially the only ones that exist in the population; the total genetic variation in the locus is thus greatly reduced.

A useful tool of analysis is to measure linkage disequilibrium (LD), i.e., whether a given haplotype is overrepresented in the population. If the region is neutrally evolving, recombination will tend to break up haplotypes, so that no one haplotype will predominate in the population. Strong LD may be a signal of a selective sweep and can be used to identify sites recently under selection.

Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection (perhaps as a result of mutation-selection balance), linked variation will tend to be weeded out along with it. Background selection, however, acts as a result of new mutations, which can occur randomly in any haplotype. It therefore produces no linkage disequilibrium, though it reduces the amount of variation in the region.

Some forms of balancing selection may also result in an elevation of neutral variation in the surrounding genomic region.

'Selection for' versus 'selection of'

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 . This distinction is important, because an individual is more than the trait it is selected for. For example, sometimes two or more traits are genetically linked through mechanisms such as pleiotropy (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 has also 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.

Evolution by means of natural selection

Main articles: Evolution and Darwinism

A prerequisite for natural selection to result in adaptive 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 noncoding DNA or resulted in a synonymous substitution. 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 usually rapidly eliminated by natural selection

By the definition of fitness, 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 they 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. For example, mutations in some HOX genes in humans result in an increase in the number of fingers or toes or a cervical rib. 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. In these circumstances, in the absence of natural selection to preserve the trait, the trait will become more variable and will deteriorate over time. 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.

Notes: 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.

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?

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.

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."

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.

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.

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. 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.” 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. 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. 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. 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. 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.

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.

Speciation

Speciation requires selective mating, which result in a reduced gene flow. Selective mating can be the result of, for example, a change in the physical environment (physical isolation by an extrinsic barrier), or by sexual selection resulting in assortative mating. Over time, these subgroups might diverge radically to become different species, either because of differences in selection pressures on the different subgroups, or because different mutations arise spontaneously in the different populations, or because of 'founder effects' - some potentially beneficial alleles may, by chance, be present in only one or other of two subgroups when they first become separated. When the genetic changes result in increasing incompatibility between the genotypes of the two subgroups, gene flow between the groups will be reduced even more, and will stop altogether as soon as the mutations become fixed in the respective subgroups. As few as two mutations can result in speciation: if each mutation has a neutral or positive effect on fitness when they occur separately, but a negative effect when they occur together, then fixation of these genes in the respective subgroups will lead to two reproductively isolated populations. According to the 'biological species concept', these will be two different species.

History of the principle

Main articles: History of evolutionary thought, Inception and Development of Darwin's theory.

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

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 but powerful: 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.

Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding (artificial selection); in his early manuscripts he referred to a 'Nature' which would do the selection. 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. Not wanting to be 'scooped', 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, along with his evidence and detailed discussion. This became a topic of great dispute; evolutionary theories became the primary way of talking about speciation, but natural selection did not predominate as the mechanism by which it happened. What made natural selection controversial was doubt about whether it was powerful enough to result in speciation, and that it was 'unguided' rather than 'progressive', something that even Darwin's supporters balked at.

Darwin's ideas were inspired by the observations that he had made on the H.M.S. Beagle, and by the economic theories of Thomas Malthus, who noted that population (if unchecked) increases 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. However, 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 6th 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. Wells presented his hypothesis to explain the origin of human races in person at the Royal Society, and Matthew published his as an appendix to his book on arboriculture. 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. 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 articles: Modern evolutionary synthesis

Only after the integration of a 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), William Hamilton (who conceived of kin selection), Ernst Mayr (who recognised the key importance of reproductive isolation for speciation) and many others formed the modern evolutionary synthesis. This propelled 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 19th-century thought. Perhaps the most radical claim of the 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" evolved from the simplest forms of life by a few simple principles. This claim inspired some of Darwin's most ardent supporters—and provoked the most profound opposition. The radicalism of natural selection, according to Stephen Jay Gould , 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 civilisation 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 . (see article on Sociocultural evolution).

In 1922, Alfred Lotka proposed that natural selection might be understood as a physical principle which can be energetically quantified. Through the work of Howard T. Odum this became known as the maximum power principle whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation.

Natural selection need not apply only to biological organisms. In computer-based systems (e.g., artificial life), simulating natural selection can be very effective in 'adapting' entities to their environments. By combining this with simulated reproduction and random variation it is possible for instance to 'evolve' problem-solving abilities of computer-based systems. However, whether such systems show that evolution by means of natural selection per se can generate complexity is contested. The mathematician and science fiction writer Rudy Rucker explored the use of natural selection to create artificial intelligence in his best-known work, The Ware Tetralogy, and in his novel The Hacker and the Ants.

The role of natural selection in speciation and adaptation is well established, but whether natural selection played an important role in the origin of life on Earth remains a topic for speculation.

Trivia

• In a letter to Charles Lyell in September 1860, Darwin regrets the use of the term 'natural selection', preferring the term 'Natural Preservation'.

Further reading

• Darwin C (1859) On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life John Murray, London; modern reprint Charles Darwin, Julian Huxley (2003). The Origin of Species. Signet Classics. ISBN 0-45-152906-5. </
• Dawkins, R. 1976. The Selfish Gene. (1976; second edition 1989) ISBN 0192860925
• Maynard Smith J (1993) The Theory of Evolution Cambridge University Press
• Williams GC (1992) Natural Selection: Domains, Levels and Challenges Oxford University Press.

• ^  Darwin, Charles (1859). The Origin of Species (First Edition).
• ^  Wolfram, Stephen (2002). A New Kind of Science, p. 1001. Wolfram Media, Inc. ISBN 1579550088

• Endler, John A (1986). "Natural Selection in the Wild". Princeton University Press.
• Maynard Smith, John (1993). "The Theory of Evolution. Cambridge University Press.
• Williams, George C (1992). Natural Selection: Domains, Levels and Challenges. Oxford University Press

External links

• Introduction to evolutionary biology (has a section on natural selection in context of evolution)
• Evolution by Natural Selection - An introduction to the logic of the theory of natural selection
• Darwin's Precursors and Influences. Part 4 — Natural selection; by John Wilkins

Basic topics in evolutionary biology	(edit)
Processes of evolution: evidence - macroevolution - microevolution - speciation
Mechanisms: natural selection - genetic drift - gene flow - mutation - phenotypic plasticity
Modes: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics

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