Natural selection is the differential survival and reproduction of individuals due to differences in the relative fitness endowed on them by their own particular complement ofobservable characteristics. It is a key law or mechanism ofevolution which changes theheritable traits characteristic of apopulation orspecies over generations.Charles Darwin popularised the term "natural selection", contrasting it withartificial selection, which is intentional, whereas natural selection is not.
For Darwin natural selection was a law or principle which resulted from three different kinds of process:inheritance, including thetransmission of heritable material from parent to offspring and itsdevelopment (ontogeny) in the offspring;variation, which partly resulted from an organism's own agency (seephenotype;Baldwin effect); and thestruggle for existence, which included both competition between organisms and cooperation or 'mutual aid' (particularly in 'social' plants and social animals).[1][2]
Variation of traits, bothgenotypic and phenotypic, exists within all populations oforganisms. However, some traits are more likely to facilitatesurvival andreproductive success. Thus, these traits are more likely to be passed on to the next generation. These traits can also become morecommon within a population if the environment that favours these traits remains fixed. If new traits become more favoured due to changes in a specificniche,microevolution occurs. If new traits become more favoured due to changes in the broader environment,macroevolution occurs. Sometimes,new species can arise especially if these new traits are radically different from the traits possessed by their predecessors.
The likelihood of these traits being 'selected' and passed down are determined by many factors. Some are likely to be passed down because theyadapt well to their environments. Others are passed down because these traits are actively preferred by mating partners, which is known assexual selection. Female bodies also prefer traits that confer the lowest cost to their reproductive health, which is known asfecundity selection.
Aristotle considered whether different forms could have appeared, only the useful ones surviving.
Several philosophers of theclassical era, includingEmpedocles[3] and his intellectual successor, theRoman poetLucretius,[4] expressed the idea that nature produces a huge variety of creatures, randomly, and that only those creatures that manage to provide for themselves and reproduce successfully persist. Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised byAristotle in Book II ofPhysics.[5] He posited naturalteleology in its place, and believed that form was achieved for a purpose, citing the regularity of heredity in species as proof.[6][7] Nevertheless, he acceptedin his biology that new types of animals,monstrosities (τερας), can occur in very rare instances (Generation of Animals, Book IV).[8] As quoted in Darwin's 1872 edition ofThe Origin of Species, Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only the useful forms survived:
So what hinders the different parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity, and whatsoever things were not thus constituted, perished, and still perish.
But Aristotle rejected this possibility in the next paragraph, making clear that he is talking about thedevelopment of animals as embryos with the phrase "either invariably or normally come about", not the origin of species:
... Yet it is impossible that this should be the true view. For teeth and all other natural things either invariably or normally come about in a given way; but of not one of the results of chance or spontaneity is this true. We do not ascribe to chance or mere coincidence the frequency of rain in winter, but frequent rain in summer we do; nor heat in the dog-days, but only if we have it in winter. If then, it is agreed that things are either the result of coincidence or for an end, and these cannot be the result of coincidence or spontaneity, it follows that they must be for an end; and that such things are all due to nature even the champions of the theory which is before us would agree. Therefore action for an end is present in things which come to be and are by nature.
Thestruggle for existence was later described by theIslamic writerAl-Jahiz in the 9th century, particularly in the context of top-down population regulation, but not in reference to individual variation or natural selection.[11][12]
At the turn of the 16th centuryLeonardo da Vinci collected a set of fossils of ammonites as well as other biological material. He extensively reasoned in his writings that the shapes of animals are not given once and forever by the "upper power" but instead are generated in different forms naturally and then selected for reproduction by their compatibility with the environment.[13]
Until the early 19th century, theprevailing view inWestern societies was that differences between individuals of a species were uninteresting departures from theirPlatonic ideals (ortypus) ofcreated kinds. However, the theory ofuniformitarianism in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the Earth's landscape. The success of this theory raised awareness of the vast scale ofgeological time and made plausible the idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species.[15]
The early 19th-century zoologistJean-Baptiste Lamarck suggested theinheritance of acquired characteristics as a mechanism for evolutionary change; adaptive traits acquired by an organism during its lifetime could be inherited by that organism's progeny, eventually causingtransmutation of species.[16] This theory,Lamarckism, was an influence on the Soviet biologistTrofim Lysenko's ill-fated antagonism to mainstream genetic theory as late as the mid-20th century.[17]
Between 1835 and 1837, the zoologistEdward Blyth worked on the area of variation, artificial selection, and how a similar process occurs in nature. Darwin acknowledged Blyth's ideas in the first chapter on variation ofOn the Origin of Species.[18]
Modern biology began in the nineteenth century withCharles Darwin's work onevolution by natural selection.
In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation foradaptation and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".[19] 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 and that variation isheritable, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to the evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.[20][21]
Darwin's ideas were inspired by the observations that he had made on thesecond voyage of HMSBeagle (1831–1836), and by the work of a political economist,Thomas Robert Malthus, who, inAn Essay on the Principle of Population (1798), noted that population (if unchecked)increases exponentially, whereas the food supply grows onlyarithmetically; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence".[22] When Darwin read Malthus in 1838 he was already primed by his work as anaturalist to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species."[23] Darwin wrote:
If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.
Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called "artificial selection"; in his early manuscripts he referred to a "Nature" which would do the selection. At the time, mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, but, even in 1859, Darwin clearly stated that selection was only part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification".[28] The final edition ofThe Origin of Species documented several other contributors to evolutionary modification:sexual selection; the inherited effects of the use and disuse of parts (seeBaldwin effect); "the direct action of external conditions" (a process which has been revived in some 21st century evolutionary biologies);[29] and "variations which seem to us in our ignorance to arise spontaneously" (seemutation).[30] In a letter to Charles Lyell in September 1860, Darwin regretted the use of the term "Natural Selection", preferring the term "Natural Preservation".[31]
For Darwin and his contemporaries, evolution was in essence synonymous with evolution by natural selection. After the publication ofOn the Origin of Species,[32] educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a law or mechanism, partly because it was perceived to be too weak to explain the range of observed characteristics of living organisms, and partly because even supporters of evolution balked at its "unguided" and non-progressive nature,[33] a response that has been characterised as the single most significant impediment to the idea's acceptance.[34] However, some thinkers enthusiastically embraced natural selection; after reading Darwin,Herbert Spencer introduced the phrasesurvival of the fittest, which became a popular summary of the theory.[35][36] The fifth edition ofOn the Origin of Species published in 1869 included Spencer's phrase as an alternative to natural selection, with credit given: "But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient."[37] Although the phrase is still often used by non-biologists, modern biologists avoid it because it istautological if "fittest" is read to mean "functionally superior" and is applied to individuals rather than considered as an averaged quantity over populations.[38]
Natural selection relies crucially on the idea of heredity, but developed before the basic concepts ofgenetics were invented. Although theMoravian monkGregor Mendel, the father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900.[39] With the early 20th-century integration of evolution withMendel's laws of inheritance, the so-calledmodern synthesis, scientists generally came to accept natural selection.[40][41] The synthesis grew from advances in different fields. Ronald Fisher developed the required mathematical language and wroteThe Genetical Theory of Natural Selection (1930).[42]J. B. S. Haldane introduced the concept of the "cost" of natural selection.[43][44]Sewall Wright elucidated the nature of selection and adaptation.[45]In his bookGenetics and the Origin of Species (1937),Theodosius Dobzhansky established the idea that mutation,once seen as a rival to selection, actually supplied the raw material for natural selection by creating genetic diversity.[46][47]
Darwin's argument inOn the Origin of Species portrayed natural selection as a law which resulted from other processes:inheritance (including both thetransmission anddevelopment of heritable material); what we now call'phenotypic' variation; and the metaphoricalstruggle for existence among living organisms. The 20th century's dominanttheories of evolutionary biology treated natural selection differently, as if it were itself acausal mechanism, the agency of which was attributed either to the machinations of selfish genes or to 'the environment'. Which meant that livingorganisms themselves dropped out of scientists' theoretical picture. Under the pressure of evidence, 21st centuryevolutionary biology has seen growing criticism of the 20th century'sgene-centred view of evolution. In consequence we now have an array ofextended evolutionary syntheses which have returned the agency of living organisms to the heart of the theory of natural selection.[55][56]
Terminology
The termnatural selection is most often defined as the differential survival and reproduction of differentphenotypic variations, where these are supported by heritable traits. It is sometimes helpful to distinguish between the processes or mechanisms which result in selection and selection's effects. Traits that endow greater reproductive success on an organism are said to beselected for, while those that reduce success areselected against.[57]
Mechanism
Heritable variation, differential reproduction
During theIndustrial Revolution, pollution killed manylichens, leaving tree trunks dark. Adark (melanic) morph of thepeppered moth largely replaced the formerly usual light morph (both shown here). Since the moths are subject topredation by birds hunting by sight, the colour change offers bettercamouflage against the changed background, suggesting natural selection at work.
Natural orphenotypic variation occurs among the individuals of any population of organisms. Some variations may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring. If the variations that give these individuals a reproductive advantage are also supported byheritable traits which are passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of flying squirrels,[58] fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. In this way thenatural environment of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described.[59] This gives the appearance of purpose, but in natural selection there is no intentional choice.[a] Artificial selection ispurposive where natural selection is not, thoughbiologists often use teleological language to describe it.[60]
Thepeppered moth exists in both light and dark colours in Great Britain, but during theIndustrial Revolution, many of the trees on which the moths rested became blackened bysoot, giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrialManchester were dark. The balance was reversed by the effect of theClean Air Act 1956, and the dark moths became rare again, demonstrating the influence of natural selection onpeppered moth evolution.[61] A recent study, using image analysis and avian vision models, shows that pale individuals more closely match lichen backgrounds than dark morphs and for the first time quantifies thecamouflage of moths topredation risk.[62] Modern genetic studies show that the switch from light to dark coloration is due to a transposable element insertion into the first intron of the gene cortex.[63]
An example of natural selection in the wild involving a much larger number of genes is given by ash trees in Britain, under selection by an invasive fungus causingash dieback.[64] This fungus has killed large numbers of ash trees in Europe,[65] and damaged many others, though some trees remain healthy.[66] The genetic basis of health under ash dieback pressure has been shown to be quantitative and highly polygenic.[67] Using genomic prediction models trained on planted trials,[67] geneticists have shown that natural selection is acting on a woodland in Surrey England, causing the new generation of ash trees to be, on average, more genetically resistant to ash dieback than their parents generation.[68] This is due to selection for beneficial gene combinations from among the variation present in the parents.[68]
The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "survival of the fittest", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particulargenotype corresponds to the average effect on all individuals with that genotype.[69]A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". "Survival of the fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection".[70] Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness was described by Kleinman.[71] An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment.[72] In this experiment, "improvement in fitness" depends on the number of replications of the particular variant for a new variant to appear that is capable of growing in the next higher drug concentration region. Fixation or substitution is not required for this "improvement in fitness". On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting.Richard Lenski's classicE. coli long-term evolution experiment is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest").[73] The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness is slowed by the competition. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring.[74]
In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. This may be because both rely on alimited supply of a resource such as food, water, orterritory.[75] Competition may bewithin orbetween species, and may be direct or indirect.[76] Species less suited to compete shouldin theory either adapt or die out, since competition plays a powerful role in natural selection, but according to the "room to roam" theory it may be less important than expansion among largerclades.[76][77]
wherer is thegrowth rate of the population (N), andK is thecarrying capacity of its local environmental setting. Typically,r-selected species exploit emptyniches, and produce many offspring, each with a relatively lowprobability of surviving to adulthood. In contrast,K-selected species are strong competitors in crowded niches, andinvest more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood.[80]
Foreshadowing a central theme in 21st centuryevolutionary biology, Darwin argued that natural selection operated differently in social than in non-social species. The members of social species aided their conspecifics to survive, either passively (as insocial plants) or both passively and actively, as insocial animals. Darwin called plants like grasses and thistles social, because, in a "somewhat strained sense", they help each other by increasing their mutual chances of cross-fertilization (and hence vigour), and by reducing the depredations of their "devourers" (e.g. birds eating their seeds). This meant that often, if social plants "did not live in numbers, they could not live at all."[81]
When it came to animals, Darwin said a truly social animal sought society beyond its own family. Unlike marmosets and tamarins, gorillas, lions, and tigers were not social in Darwin's sense, because, while they "no doubt" felt sympathy for the suffering of their young, they did not sympathize with "any other animal" beyond their own family.[82][83]
In addition to the passive kinds ofmutual aid[84] that advantaged social plants, social animals could gain additional benefits through efficiencies due to divisions of labour like those found insocial insects. Beyond this, some social species of bird and mammalactively signaled danger to other members of their community, some even posting sentinels to warn the group of approaching enemies. Thus rabbits stamp their hind-feet, and female seals act as look-outs. Social creatures may also actively groom each other, removing parasites, or licking each other’s wounds. Animals like wolves, killer whales, and pelicans hunt in concert, sometimes with a combined strategy. Social animals mutually defend each other too, and thereby show their "heroism."[85] All these advantages mean that, in social animals, unlike non-social species,natural selection "will adapt the structure of each individual for the benefit of the whole community; if the community profits by the selected change."[86] InThe Descent of Man, Darwin attributes the evolution of all the most human of human characteristics—rationality, intellect, language, conscience, moral qualities, and culture—to the fact that our pre-human ancestors were group-living social animalspar excellence.
1:directional selection: a single extremephenotype favoured. 2,stabilizing selection: intermediate favoured over extremes. 3: disruptive selection: extremes favoured over intermediate. X-axis:phenotypic trait Y-axis: number of organisms Group A: original population Group B: after selection
Natural selection can act on any heritablephenotypic trait,[88] and selective pressure can be altered by any aspect of the environment, including sexual selection andcompetition orcooperation with members of the same or other species.[89][90] However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants.[59]
Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for.
By effect on a trait
Selection has different effects on phenotypic traits.Stabilizing selection acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous.Directional selection favours extreme values of a trait. The uncommondisruptive selection also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative andunivariate then both higher and lower trait levels are favoured. Disruptive selection can be a precursor tospeciation.[59]
By effect on genetic diversity
Alternatively, selection can be divided according to its effect ongenetic diversity.Purifying or negative selection acts to remove genetic variation from the population (and is opposed byde novo mutation, which introduces new variation.[91][92] In contrast,balancing selection acts to maintain genetic variation in a population, even in the absence ofde novo mutation, by negativefrequency-dependent selection. One mechanism for this isheterozygote advantage, where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the humanABO blood group locus has been explained in this way.[93]
Different types of selection act at eachlife cycle stage of a sexually reproducing organism.[94]
By life cycle stage
Another option is to classify selection by thelife cycle stage at which it acts. Some biologists recognise just two types:viability (or survival) selection, which acts to increase an organism's probability of survival, and fecundity (or fertility or reproductive) selection, which acts to increase the rate of reproduction, given survival. Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting ongamete survival, and compatibility selection, acting onzygote formation.[94]
By unit of selection
Selection can also be classified by the level orunit of selection. Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals.Gene selection acts directly at the level of the gene. Inkin selection andintragenomic conflict, gene-level selection provides a more apt explanation of the underlying process.Group selection, if it occurs, acts on groups of organisms, on the assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature.[95]
Finally, selection can be classified according to theresource being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability.Ecological selection is natural selection via any means other than sexual selection, such as kin selection, competition, andinfanticide. Following Darwin, natural selection is sometimes defined as ecological selection,[98] in which case sexual selection is considered a separate mechanism.[99]
Sexual selection as first articulated by Darwin (using the example of thepeacock's tail)[96] refers specifically to competition for mates,[100] which can beintrasexual, between individuals of the same sex, that is male–male competition, orintersexual, where one genderchooses mates, most often with males displaying and females choosing.[101] However, in some species, mate choice is primarily by males, as in some fishes of the familySyngnathidae.[102][103]
Phenotypic traits can bedisplayed in one sex and desired in the other sex, causing apositive feedback loop called aFisherian runaway, for example, the extravagant plumage of some male birds such as the peacock.[97] An alternate theory proposed by the sameRonald Fisher in 1930 is thesexy son hypothesis, that mothers want promiscuous sons to give them large numbers of grandchildren and so choose promiscuous fathers for their children. Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers ofstags, which are used in combat with other stags. More generally, intrasexual selection is often associated withsexual dimorphism, including differences in body size between males and females of a species.[101]
Arms races
Selection in action:resistance to antibiotics grows through the survival of individuals less affected by the antibiotic. Their offspring inherit the resistance.
Natural selection is seen in action in the development ofantibiotic resistance inmicroorganisms. Since the discovery ofpenicillin in 1928,antibiotics have been used to fight bacterial diseases. The widespread misuse of antibiotics has selected for microbial resistance to antibiotics in clinical use, to the point that themethicillin-resistantStaphylococcus aureus (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.[104] Response strategies typically include the use of different, stronger antibiotics; however, newstrains of MRSA have recently emerged that are resistant even to these drugs.[105] This is anevolutionary arms race, in which bacteria develop strains less susceptible to antibiotics, while medical researchers attempt to develop new antibiotics that can kill them. A similar situation occurs withpesticide resistance in plants and insects. Arms races are not necessarily induced by man; a well-documented example involves the spread of a gene in the butterflyHypolimnas bolina suppressing male-killing activity byWolbachia bacteria parasites on the island ofSamoa, where the spread of the gene is known to have occurred over a period of just five years.[106][107]
Withoutphenotypic variation, there would be no evolution by natural selection. A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that affects phenotypic fitness differences. Genetic variation is the result of mutations,genetic recombinations and alterations in thekaryotype (the number, shape, size and internal arrangement of thechromosomes). Any of these changes might have an effect that is highly advantageous or highly disadvantageous forphenotypic variations, but large effects on phenotypes are rare.
In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred innoncoding DNA or resulted in asynonymous substitution. However, many mutations innon-coding DNA have deleterious effects.[108][109] Although both mutation rates and average fitness effects of mutations are dependent on the organism, a majority of mutations in humans are slightly deleterious.[110]
Some mutations occur in"toolkit" or regulatory genes. Changes in these often 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 embryos. Some nonlethal regulatory mutations occur inHOX genes in humans, which can result in acervical rib[111] orpolydactyly, an increase in the number of fingers or toes.[112] When such mutations result in a higher fitness, natural selection favours these phenotypes and the novel trait spreads in the population.Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. In the absence of natural selection to preserve such a trait, it becomes more variable and deteriorate over time, possibly resulting in avestigial manifestation of the trait, also calledevolutionary baggage. In many circumstances, the apparently vestigial structure may retain a limited functionality, or may be co-opted for other advantageous traits in a phenomenon known aspreadaptation. A famous example of a vestigial structure, the eye of theblind mole-rat, is believed to retain function inphotoperiod perception.[113]
Speciation requires a degree ofreproductive isolation—that is, a reduction in gene flow. However, it is intrinsic to the concept of aspecies thathybrids are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. The problem does not occur inallopatric speciation with geographically separated populations, which can diverge with different sets of mutations.E. B. Poulton realized in 1903 that reproductive isolation could evolve through divergence, if each lineage acquired a different, incompatible allele of the same gene. Selection against the heterozygote would then directly create reproductive isolation, leading to theBateson–Dobzhansky–Muller model, further elaborated byH. Allen Orr[114] andSergey Gavrilets.[115] Withreinforcement, however, natural selection can favour an increase in pre-zygotic isolation, influencing the process of speciation directly.[116]
Natural selection results from the ways an organism'sphenotypes, or observable characteristics, bear on its capacity to reproduce. Phenotypes areplastic which means they are less directly determined by a given organism's genetic make-up (genotype) than by the way that particular organism develops and behaves in the theatre of agency which constitutes itshabitat or environment. When different organisms in a population possess different versions of a gene affecting a certain phenotypic trait, each of these versions is known as anallele. (An example is theABO blood typeantigens in humans, where three alleles govern the phenotype.[117]) It is these genetic variations which affect fitness-relevant differences in phenotypic traits and so underpin the evolution of new adaptations and, ultimately, new species.
Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can support a continuum of possible phenotypic values.[118]
When some component of a phenotypic trait is heritable, selection alters the frequencies of the different alleles, or variants of the gene that affect the variants of the observed trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies:directional,stabilizing, anddisruptive selection.[119] Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population. This process can continue until the allele isfixed and the entire population shares the fitter phenotype.[120] Far more common is stabilizing selection, which lowers the frequency of alleles that have a deleterious effect on the phenotype—that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Stabilizing selectionconserves functional genetic features, such asprotein-coding genes orregulatory sequences, over time by selective pressure against deleterious variants.[121] Disruptive (or diversifying) selection is selection favouring extreme trait values over intermediate trait values. Disruptive selection may causesympatric speciation throughniche partitioning.
Some forms ofbalancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur indiploid species (with pairs of chromosomes) whenheterozygous individuals (with just one copy of the allele) have a higher fitness than homozygous individuals (with two copies). This is called heterozygote advantage or over-dominance, of which the best-known example is the resistance to malaria in humans heterozygous forsickle-cell anaemia. Maintenance of allelic variation can also occur throughdisruptive or diversifying selection, which favours genotypes that depart from the average in either direction (that is, the opposite of over-dominance), and can result in abimodal distribution of trait values. Finally, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles ofgame theory have been applied to understand the fitness distributions in these situations, particularly in the study of kin selection and the evolution ofreciprocal altruism.[50][122]
A portion of all genetic variation is functionally neutral, producing no phenotypic effect or significant difference in fitness.Motoo Kimura'sneutral theory of molecular evolution bygenetic drift proposes that this variation accounts for a large fraction of observed genetic diversity.[123] Neutral events can radically reduce genetic variation throughpopulation bottlenecks,[124] which among other things can cause thefounder effect in initially small new populations.[125] When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites is higher than at sites where variation does influence fitness.[119] However, after a period with no new mutations, the genetic variation at these sites is eliminated due to genetic drift. Natural selection reduces genetic variation by eliminating maladapted individuals, and consequently the mutations that caused the maladaptation. At the same time, new mutations occur, resulting in amutation–selection balance. The exact outcome of the two processes depends both on the rate at which new mutations occur and on the strength of the natural selection, which is a function of how unfavourable the mutation proves to be.[126]
Genetic linkage occurs when theloci of two alleles are close on a chromosome. During the formation of gametes, recombination reshuffles the alleles. The chance that such a reshuffle occurs between two alleles is inversely related to the distance between them.Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, closely linked alleles can also become more common by "genetic hitchhiking", whether they are neutral or even slightly deleterious. A strong selective sweep results in a region of the genome where the positively selectedhaplotype (the allele and its neighbours) are in essence the only ones that exist in the population. Selective sweeps can be detected by measuringlinkage disequilibrium, or whether a given haplotype is overrepresented in the population. Since a selective sweep also results in selection of neighbouring alleles, the presence of a block of strong linkage disequilibrium might indicate a 'recent' selective sweep near the centre of the block.[127]
Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection, linked variation tends to be weeded out along with it, producing a region in the genome of low overall variability. Because background selection is a result of deleterious new mutations, which can occur randomly in any haplotype, it does not produce clear blocks of linkage disequilibrium, although with low recombination it can still lead to slightly negative linkage disequilibrium overall.[128]
Darwin's ideas, along with those ofAdam Smith andKarl Marx, had a profound influence on 19th century thought, including his radical claim 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.[129] This inspired some of Darwin's most ardent supporters—and provoked the strongest opposition. Natural selection had the power, according toStephen Jay Gould, to "dethrone some of the deepest and most traditional comforts of Western thought", such as the belief that humans have a special place in the world.[130]
How life originated from inorganic matter remains an unresolved problem in biology. One prominent hypothesis is that life first appearedin the form of short self-replicating RNA polymers.[133] On this view, life may have come into existence whenRNA chains first experienced the basic conditions, as conceived by Charles Darwin, for natural selection to operate. These conditions are: heritability,variation of type, and competition for limited resources. The fitness of an earlyRNA replicator would likely have been a function of adaptive capacities that were intrinsic (i.e., determined by thenucleotide sequence) and the availability of resources.[134][135] The three primary adaptive capacities could logically have been: (1) the capacity to replicate with moderate fidelity (giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) the capacity to acquire and process resources.[134][135] These capacities would have been determined initially by the folded configurations (including those configurations withribozyme activity) of the RNA replicators that, in turn, would have been encoded in their individual nucleotide sequences.[136]
Cell and molecular biology
In 1881, the embryologistWilhelm Roux publishedDer Kampf der Theile im Organismus (The Struggle of Parts in the Organism) in which he suggested that the development of an organism results from a Darwinian competition between the parts of the embryo, occurring at all levels, from molecules to organs.[137] In recent years, a modern version of this theory has been proposed byJean-Jacques Kupiec. According to this cellular Darwinism,random variation at the molecular level generates diversity in cell types whereas cell interactions impose a characteristic order on the developing embryo.[138]
The social implications of the theory of evolution by natural selection also became the source of continuing controversy.Friedrich Engels, a Germanpolitical philosopher and co-originator of the ideology ofcommunism, wrote in 1872 that "Darwin did not know what a bitter satire he wrote on mankind, and especially on his countrymen, when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of theanimal kingdom."[139] Herbert Spencer and the eugenics advocateFrancis Galton's interpretation of natural selection as necessarily progressive, leading to supposed advances in intelligence and civilisation, became a justification forcolonialism,eugenics, andsocial Darwinism. For example, in 1940,Konrad Lorenz, in writings that he subsequently disowned, used the theory as a justification for policies of theNazi 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."[140] Others have developed ideas that human societies and cultureevolve by mechanisms analogous to those that apply to evolution of species.[141]
More recently, work among anthropologists and psychologists has led to the development ofsociobiology and later of evolutionary psychology, a field that attempts to explain features ofhuman psychology in terms of adaptation to the ancestral environment. The most prominent example of evolutionary psychology, notably advanced in the early work ofNoam Chomsky and later bySteven Pinker, is the hypothesis that the human brain has adapted toacquire thegrammatical rules ofnatural language.[142] Other aspects of human behaviour and social structures, from specific cultural norms such asincest avoidance to broader patterns such asgender roles, have been hypothesised to have similar origins as adaptations to the early environment in which modern humans evolved. By analogy to the action of natural selection on genes, the concept ofmemes—"units of cultural transmission," or culture's equivalents of genes undergoing selection and recombination—has arisen, first described in this form byRichard Dawkins in 1976[143] and subsequently expanded upon by philosophers such asDaniel Dennett as explanations for complex cultural activities, including humanconsciousness.[144]
Information and systems theory
In 1922,Alfred J. Lotka proposed that natural selection might be understood as a physical principle that could be described in terms of the use of energy by a system,[145][146] a concept later developed byHoward T. Odum as themaximum power principle inthermodynamics, whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation.[147]
The principles of natural selection have inspired a variety of computational techniques, such as "soft"artificial life, that simulate selective processes and can be highly efficient in 'adapting' entities to an environment defined by a specifiedfitness function.[148] For example, a class of heuristicoptimisation algorithms known asgenetic algorithms, pioneered byJohn Henry Holland in the 1970s and expanded upon byDavid E. Goldberg,[149] identify optimal solutions by simulated reproduction and mutation of a population of solutions defined by an initialprobability distribution.[150] Such algorithms are particularly useful when applied to problems whoseenergy landscape is very rough or has many local minima.[151]
Darwinian evolution by natural selection is pervasive in literature, whether taken optimistically in terms of how humanity may evolve towards perfection, or pessimistically in terms of the dire consequences of the interaction of human nature and the struggle for survival. Among major responses isSamuel Butler's 1872 pessimisticErewhon ("nowhere", written mostly backwards). In 1893H. G. Wells imagined "The Man of the Year Million", transformed by natural selection into a being with a huge head and eyes, and shrunken body.[152]
Notes
^In natural selection, the purposive agency of living organisms may often lead to new and adaptive phenotypic variations which are supported by heritable traits, as in theBaldwin effect. Likewise, insexual selection, a female animal making a choice of mate may be argued to be intending to get the best mate. But, in neither case is there a suggestion that the organism has any intention to improve the bloodline in the manner of an animal breeder.
^Scott, Gilbert.; Epel, David (2015).Ecological Developmental Biology: The Environmental Regulation of Development, Health, and Evolution – Second Edition. Sinauer Ass.ISBN1605353442.
^Stucke, Maurice E. (Summer 2008)."Better Competition Advocacy".St. John's Law Review.82 (3). Jamaica, NY:951–1036. Archived fromthe original on 30 April 2011. Retrieved3 November 2008.This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection, or the preservation of favoured races in the struggle for life.'—Herbert Spencer,Principles of Biology (1864), vol. 1, pp. 444–445
^Carroll, Sean B.; Grenier, Jennifer K.; Weatherbee, Scott D. (2005).From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design – Second Edition. Blackwell Publishing. pp. 66–67.ISBN978-1-4051-1950-4.
^Carroll, Sean B.; Grenier, Jennifer K.; Weatherbee, Scott D. (2005).From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design – Second Edition. Blackwell Publishing. p. 13.ISBN978-1-4051-1950-4.
^Walsh, Dennis (2015).Organisms, Agency, and Evolution. Cambridge University Press.ISBN1107122104.
^Stauffer, R.C. (1975),Charles Darwin's Natural Selection: being the second part of his big species book written from 1856 to 1858, Cambridge University Press, p. 203,ISBN0521348072
^Gavrilets, S. (2004),Fitness Landscapes and the Origin of Species, Princeton University Press,ISBN978-0-691-11983-0
^Schuler, Hannes; Hood, Glen R.; Egan, Scott P.; Feder, Jeffrey L. (2016). "Modes and Mechanisms of Speciation".Reviews in Cell Biology and Molecular Medicine.2 (3):60–93.
^McKusick, Victor A.; Gross, Matthew B. (18 November 2014)."ABO Glycosyltransferase; ABO".Online Mendelian Inheritance in Man. National Library of Medicine. Retrieved7 November 2016.
^abBernstein, Harris; Byerly, Henry C.; Hopf, Frederick A.; et al. (June 1983). "The Darwinian Dynamic".The Quarterly Review of Biology.58 (2):185–207.doi:10.1086/413216.JSTOR2828805.S2CID83956410.
Haldane, J.B.S. (1954)."The Measurement of Natural Selection". In Montalenti, Giuseppe; Chiarugi, A. (eds.).Atti del IX Congresso Internazionale di Genetica, Bellagio (Como) 24–31 agosto 1953 [Proceedings of the 9th International Congress of Genetics]. Caryologia. Vol. 6 (1953/54) Suppl. Florence, Italy:University of Florence. pp. 480–487.OCLC9069245.
Mayr, Ernst (2006) [Originally published 1972; Chicago, IL: Aldine Publishing Co.]. "Sexual Selection and Natural Selection". In Campbell, Bernard G. (ed.).Sexual Selection and the Descent of Man: The Darwinian Pivot. New Brunswick, NJ:AldineTransaction.ISBN978-0-202-30845-6.LCCN2005046652.OCLC62857839.
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