Natural selection

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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.

Contents

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

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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