Natural selection 21147 225642156 2008-07-14T18:18:05Z RGTraynor 270405 Replacing per MOS, Replaced: July of → July using [[Project:AutoWikiBrowser|AWB]] {{otheruses}} {{evolution3}} '''Natural selection''' is the process by which favorable [[heritable]] [[trait (biology)|traits]] become more common in successive [[generation]]s of a [[population]] of [[Reproduction|reproducing]] [[organism]]s, and unfavorable heritable traits become less common, due to differential reproduction of [[genotype]]s. Natural selection acts on the [[phenotype]], or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and [[biological reproduction|reproduce]] than those with less favorable phenotypes. The phenotype's [[Genetics|genetic]] basis, [[genotype]] associated with the favorable phenotype, will increase in [[gene frequency|frequency]] over the following generations. Over time, this process may result in [[adaptation]]s that specialize organisms for particular [[ecological niche]]s and may eventually result in the [[speciation|emergence of new species]]. In other words, natural selection is the mechanism by which evolution may take place in a population of a specific organism. Natural selection is one of the cornerstones of modern [[biology]]. The term was introduced by [[Charles Darwin]] in his groundbreaking 1859 book ''[[The Origin of Species]]''<ref name=origin> Darwin C (1859) ''[[The Origin of Species|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 {{cite book|author = Charles Darwin, Julian Huxley|year = 2003|title = The Origin of Species| publisher = Signet Classics|id = ISBN 0-451-52906-5}} Published online at [http://darwin-online.org.uk/ The complete work of Charles Darwin online]: [http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=2 On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life]. </ref> in which natural selection was described by analogy to [[artificial selection]], a process by which animals with traits considered desirable by human breeders are systematically favored for reproduction. The concept of natural selection was originally developed in the absence of a valid theory of [[inheritance]]; at the time of Darwin's writing, nothing was known of modern [[genetics]]. Although [[Gregor Mendel]], the father of modern genetics, was a contemporary of Darwin's, his work would lie in obscurity until the early 20th century. The union of traditional [[Darwinism|Darwinian evolution]] with subsequent discoveries in [[classical genetics|classical]] and [[molecular genetics]] is termed the ''[[modern evolutionary synthesis]]''. Although other mechanisms of molecular evolution, such as the [[neutral theory of molecular evolution|neutral theory]] advanced by [[Motoo Kimura]], have been identified as important causes of [[genetic diversity]], natural selection remains the single primary explanation for adaptive evolution. ==General principles== [[Image:Darwin's finches.jpeg|thumb|220px|[[Charles Darwin|Darwin's]] illustrations of [[beak]] variation in the [[Darwin's finches|finches]] of the [[Galápagos Islands]], which hold 13 closely related [[species]] that differ most markedly in the shape of their beaks. The beak of each species is suited to its preferred food, suggesting that beak shapes evolved by natural selection. ''See also [[character displacement]], [[adaptive radiation]], [[divergent evolution]].'']] :''See also: [[Genotype-phenotype distinction]].'' Natural selection acts on an organism's [[phenotype]], or physical characteristics. Phenotype is determined by an organism's genetic make-up ([[genotype]]) and the [[ecosystem|environment]] in which the organism lives. Often, natural selection acts on specific [[trait (biology)|traits]] of an individual, and the terms phenotype and genotype are used narrowly to indicate these specific traits. When different organisms in a population possess different versions of a [[gene]] for a certain trait, each of these versions is known as an [[allele]]. It is this genetic variation that underlies phenotypic traits. A typical example is that certain combinations of genes for [[eye color]] in humans which, for instance, give rise to the phenotype of blue eyes. (On the other hand, when all the organisms in a population share the same allele for a particular trait, and this state is stable over time, the allele is said to be ''[[fixation (population genetics)|fixed]]'' in that population.) 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 produce a continuum of possible phenotypic values.<ref>Falconer DS & Mackay TFC (1996) ''Introduction to Quantitative Genetics'' Addison Wesley Longman, Harlow, Essex, UK ISBN 0-582-24302-5</ref> ===Nomenclature and usage=== The term "natural selection" has slightly different definitions in different contexts. In simple terms, "natural selection" is most often defined to operate on ''heritable'' traits, but can sometimes refer to the differential reproductive success of phenotypes regardless of whether those phenotypes are heritable. Natural selection is "blind" in the sense that individuals' level of reproductive success is a function of the phenotype and not of whether or to what extent that phenotype is heritable. Following Darwin's primary usage<ref name="origin" /> the term is often used to refer to both the ''consequence'' of blind selection and to its mechanisms.<ref name=fisher>Fisher RA (1930) ''[[The Genetical Theory of Natural Selection]]'' Clarendon Press, Oxford</ref><ref name="nomenclature1">Works employing or describing this usage:<br>{{cite book| author=Endler JA | year=1986| title=Natural Selection in the Wild| publisher=[[Princeton University Press]]| location=[[Princeton, New Jersey|Princeton]], [[New Jersey]]| id=ISBN 0-691-00057-3}}<br>{{cite book| author=Williams GC| year=1966| title=[[Adaptation and Natural Selection]]| publisher=Oxford University Press}}</ref> It is sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; when this distinction is important, scientists define "natural selection" specifically as "those mechanisms that contribute to the selection of individuals that reproduce," without regard to whether the basis of the selection is heritable. This is sometimes referred to as 'phenotypic natural selection.'<ref name="nomenclature2">Works employing or describing this usage:<br>Lande R & Arnold SJ (1983) The measurement of selection on correlated characters. [[Evolution (journal)|''Evolution'']] 37:1210-26<br>[[Douglas J. Futuyma|Futuyma DJ]] (2005) ''Evolution''. Sinauer Associates, Inc., [[Sunderland, Massachusetts|Sunderland]], [[Massachusetts]]. ISBN 0-87893-187-2<br>Haldane, J.B.S. 1953. The measurement of natural selection. Proceedings of the 9th International Congress of Genetics. 1: 480-487</ref> Traits that cause greater reproductive success of an organism are said to be selected ''for'' whereas those that reduce success are selected ''against''. Selection ''for'' a trait may also result in the selection ''of'' other correlated traits that do not themselves directly influence fitness. This may occur as a result of [[pleiotropy]] or [[gene linkage]].<ref>[[Elliott Sober|Sober E]] (1984; 1993) ''The Nature of Selection: Evolutionary Theory in Philosophical Focus'' [[University of Chicago Press]] ISBN 0-226-76748-5</ref> ===Fitness=== {{main|Fitness (biology)}} The concept of ''[[Fitness (biology)|fitness]]'' is central to natural selection. However, as with Natural selection above, there is serious divergence of opinion over the precise meaning of the term, and [[Richard Dawkins]] manages in his later books to avoid it entirely. (He devotes a chapter of his The Extended Phenotype to discussing the various senses in which the term is used.) Although fitness is sometimes colloquially understood as a quality that promotes survival of a particular individual - as illustrated in the well-known phrase ''[[survival of the fittest]]'' - modern evolutionary theory defines fitness in terms of individual reproduction. The basis of this approach is: if an organism lives half as long as others of its species, but has twice as many offspring surviving to productive adulthood, its genes will become more common in the adult population of the next generation. This is known as ''differential reproduction''. Though natural selection acts on individuals, its ''average'' effect on all individuals with a particular genotype corresponds to the fitness of that genotype. Very low-fitness genotypes cause their bearers to have few or no offspring on average; examples include many human [[genetic disorder]]s like [[cystic fibrosis]]. Conditions like [[sickle-cell anemia]] may have low fitness in the general human population, but because it confers immunity from malaria, it has high fitness value in populations which have high malaria infection rates. Broadly speaking, an organism's fitness is a function of its alleles' fitnesses. Since fitness is an averaged quantity, however, it is possible a favorable mutation may arise in an individual that does not survive to adulthood for unrelated reasons. ===Types of selection=== Natural selection can act on any phenotypic trait, and selective pressure can be produced by any aspect of the environment, including [[Sexual selection|mates]] and [[Competition#Competition in biology and ecology|conspecifics]], or members of the same species. 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. The ''[[unit of selection]]'' can be the individual or it can be another level within the hierarchy of biological organisation, such as genes, cells, and [[kin selection|kin groups]]. There is still debate about whether natural selection acts at the level of [[group selection|groups or species]] to produce adaptations that benefit a larger, non-kin group. Selection at a different level such as 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, in a process called [[intragenomic conflict]]. 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. [[Image:Life cycle of a sexually reproducing organism.svg|thumb|280px|right|The life cycle of a sexually reproducing organism. Various components of natural selection are indicated for each life stage.<ref>Modified from Christiansen FB (1984) The definition and measurement of fitness. In: ''[[Evolutionary ecology]]'' (ed. Shorrocks B) pp65-79. [[Blackwell Publishing|Blackwell Scientific]], [[Oxford]] by adding survival selection in the reproductive phase</ref>]] Natural selection occurs at every life stage of an individual. An individual organism must survive until adulthood before it can reproduce, and selection of those that reach this stage is called ''viability selection''. In many species, adults must compete with each other for mates via ''[[sexual selection]]'', and success in this competition determines who will parent the next generation. When individuals can reproduce more than once, a longer survival in the reproductive phase increases the number of offspring, called ''survival selection''. The [[fecundity]] of both females and males (for example, giant [[spermatozoon|sperm]] in certain species of ''[[Drosophila]]''<ref>Pitnick S & Markow TA (1994) Large-male advantage associated with the costs of sperm production in ''Drosophila hydei'', a species with giant sperm. [[PNAS|''Proc Natl Acad Sci USA'']] 91:9277-81; Pitnick S (1996) Investment in testes and the cost of making long sperm in ''Drosophila''. [[American Naturalist|''Am Nat'']] 148:57-80</ref>) can be limited via ''fecundity selection''. The viability of produced [[gamete]]s can differ, while [[intragenomic conflict]]s such as meiotic drive between the [[haploid]] gametes can result in ''gametic'' or ''genic selection''. Finally, the union of some combinations of eggs and sperm might be more compatible than others; this is termed ''compatibility selection''. ===Sexual selection=== {{main|Sexual selection}} It is also useful to make a mechanistic distinction between [[ecological selection]] and the narrower term [[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 (biology)|competition]], [[infanticide]]), while ''sexual selection'' refers specifically to competition between conspecifics for mates.<ref>{{cite book| author=Andersson, M| year=1995| title=Sexual Selection| publisher=Princeton University Press| location=Princeton, New Jersey| id=ISBN 0-691-00057-3}}</ref> Sexual selection can be ''intrasexual'', as in cases of competition among individuals of the same sex in a population, or ''intersexual'', as in cases where one sex controls reproductive access by choosing among a population of available mates. Most commonly, intrasexual selection involves male-male competition and intersexual selection involves female choice of suitable males, due to the generally greater investment of resources for a female than a male in a single offspring organism. However, some species exhibit sex-role reversed behavior in which it is males that are most selective in mate choice; the best-known examples of this pattern occur in some [[fish]]es of the family ''[[Syngnathidae]]'', though likely examples have also been found in [[amphibian]] and [[bird]] species.<ref name="Eens">Eens M, Pinxten R. (2000). Sex-role reversal in vertebrates: behavioural and endocrinological accounts. ''Behav Processes'' 51(1-3):135-147. PMID 11074317 </ref> 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, for example, the extravagant plumage of some male birds. Similarly, aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of [[stag]]s, 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.<ref name="Barlow">Barlow GW. (2005). How Do We Decide that a Species is Sex-Role Reversed? ''[[The Quarterly Review of Biology]]'' 80(1):28–35. PMID 15884733 </ref> == An example: antibiotic resistance== [[Image:Antibiotic resistance.svg|thumb|size=180px|right|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 [[microorganism]]s. Since the discovery of [[penicillin]] in 1928 by [[Alexander Fleming]], [[antibiotic]]s have been used to fight [[bacteria]]l diseases. Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of [[mutation]]s. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them slightly 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''. 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, and advantageous mutations are even rarer. However, populations of bacteria are large enough that 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. The widespread use and misuse of antibiotics has resulted in increased microbial resistance to antibiotics in clinical use, to the point that the [[Methicillin-resistant Staphylococcus aureus|methicillin-resistant ''Staphylococcus aureus'']] (MRSA) has been described as a '[[Antibiotic resistance|superbug]]' because of the threat it poses to health and its relative invulnerability to existing drugs.<ref>{{cite web|url=http://www.inboxrobot.com/news/MRSASuperbug|title=MRSA Superbug News|accessdate=2006-05-06}}</ref> Response strategies typically include the use of different, stronger antibiotics; however, new [[strain (biology)|strains]] of MRSA have recently emerged that are resistant even to these drugs.<ref name=Schito_2006>{{cite journal | author=Schito GC | title=The importance of the development of antibiotic resistance in ''Staphylococcus aureus'' | journal=Clin Microbiol Infect | year=2006|pages=3–8|volume=12 Suppl 1|pmid=16445718 | doi=10.1111/j.1469-0691.2006.01343.x }} [http://www.blackwell-synergy.com/doi/abs/10.1111/j.1469-0691.2006.01343.x]</ref> This is an example of what is known as an [[evolutionary arms race]], in which bacteria continue to develop strains 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. Arms races are not necessarily induced by man; a well-documented example involves the elaboration of the [[RNA interference]] pathway in plants as means of [[innate immunity]] against [[virus]]es.<ref name="Lucy">Lucy A, Guo H, Li W, Ding S (2000). "Suppression of post-transcriptional gene silencing by a plant viral protein localized in the nucleus". ''[[EMBO Journal|EMBO J]]'' '''19''' (7): 1672–80. PMID 10747034. </ref> ==Genetical theory of natural selection== Natural selection by itself is a simple concept, in which fitness differences between phenotypes play a crucial role. It is the union of natural selection as a mechanism with genetic material as a substrate that offers most of the theory's explanatory power ===Directionality of selection===<!-- This section is linked from [[Race and intelligence]] --> When some component of a trait is heritable, selection will alter the frequencies of the different [[allele]]s, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies.<ref name="Rice">Rice SH. (2004). Evolutionary Theory: Mathematical and Conceptual Foundations. Sinauer Associates: Sunderland, Massachusetts, USA. ISBN 0-87893-702-1 See esp. ch. 5 and 6 for a quantitative treatment.</ref> [[Directional selection]] occurs when a certain allele has a greater fitness than others, resulting in an increase in frequency of that allele. This process can continue until the allele is [[fixation (population genetics)|fixed]] and the entire population shares the fitter phenotype. It is directional selection that is illustrated in the antibiotic resistance example [[#An example: antibiotic resistance|above]]. Far more common is [[stabilizing selection]] (also known as ''purifying 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. Purifying selection results in functional genetic features, such as [[protein biosynthesis|protein-coding genes]] or [[regulatory sequence]]s, being [[conservation (genetics)|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 (that is, those that have two pairs of [[chromosome]]s) when [[heterozygote]] individuals, who have different alleles on each chromosome at a single [[Locus (genetics)|genetic locus]], have a higher fitness than [[homozygote]] individuals that have two of the same alleles. This is called [[heterozygote advantage]] or overdominance, of which the best-known example is the [[malaria]]l resistance observed in heterozygous humans who carry only one copy of the gene for [[sickle cell anemia]]. Maintenance of allelic variation can also occur through [[disruptive selection|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, 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 of [[game theory]] have been applied to understand the fitness distributions in these situations, particularly in the study of [[kin selection]] and the evolution of [[reciprocal altruism]].<ref name="Hamilton">Hamilton WD. (1964). The genetical evolution of social behaviour I and II. ''Journal of Theoretical Biology'' 7: 1-16 and 17-52. PMID 5875341 PMID 5875340 </ref><ref name="Trivers">Trivers RL. (1971). The evolution of reciprocal altruism. ''Q Rev Biol'' 46: 35-57.</ref> ===Selection and genetic variation=== A portion of all [[genetic variation]] is functionally neutral in that it produces no phenotypic effect or significant difference in fitness; the hypothesis that this variation accounts for a large fraction of observed [[genetic diversity]] is known as the [[neutral theory of molecular evolution]] and was originated by [[Motoo Kimura]]. Neutral variation was once thought to encompass most of the genetic variation in [[non-coding DNA]], which was hypothesized to be composed of "[[junk DNA]]". However, more recently, the functional roles of non-coding DNA, such as the regulatory and developmental functions of [[RNA]] [[gene product]]s, has been studied in depth;<ref name="He">He L, Hannon GJ. (2004). MicroRNAs: small RNAs with a big role in gene regulation. ''Nat Rev Genet'' 5(7):522-31. PMID 15211354 </ref> large parts of non-protein-coding DNA sequences are highly conserved under strong purifying selection and thus do not vary much from individual to individual, indicating that mutations in these regions have deleterious consequences.<ref name=NCFitnessEffects>Kryukov GV, Schmidt S & Sunyaev S (2005) Small fitness effect of mutations in highly conserved non-coding regions. ''Human Molecular Genetics'' 14:2221-9</ref><ref name=NCFitnessEffects2>Bejerano G, Pheasant M, Makunin I, Stephen S, Kent WJ, Mattick JS & Haussler D (2004) Ultraconserved elements in the human genome. [[Science (journal)|''Science'']] 304:1321-5</ref> 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 will be higher than at sites where variation does influence fitness.<ref name="Rice" /> ====Mutation selection balance==== Natural selection results in the reduction of genetic variation through the elimination of maladapted individuals and consequently of the mutations that caused the maladaptation. At the same time, new mutations occur, resulting in a [[mutation-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 unfavorable the mutation proves to be. Consequently, changes in the mutation rate or the selection pressure will result in a different mutation-selection balance. ====Genetic linkage==== [[Genetic linkage]] occurs when the [[locus (genetics)|loci]] of two alleles are ''linked'', or in close proximity to each other on the [[chromosome]]. During the formation of [[gametes]], [[genetic recombination|recombination]] of the genetic material results in reshuffling of the alleles. However, the chance that such a reshuffle occurs between two alleles depends on the distance between those alleles; the closer the alleles are to each other, the less likely it is that such a reshuffle will occur. Consequently, when selection targets one allele, this automatically results in selection of the other allele as well; through this mechanism, selection can have a strong influence on patterns of variation in the genome. [[Selective sweep]]s occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, linked alleles can also become more common, whether they are neutral or even slightly deleterious. This is called ''[[genetic hitchhiking]]''. A strong selective sweep results in a region of the genome where the positively selected [[haplotype]] (the allele and its neighbours) are essentially the only ones that exist in the population. Whether a selective sweep has occurred or not can be investigated by measuring [[linkage disequilibrium]], or whether a given haplotype is overrepresented in the population. Normally, [[genetic recombination]] results in a reshuffling of the different alleles within a haplotype, and none of the haplotypes will dominate the population. However, during a selective sweep, selection for a specific allele will also result in selection of neighbouring alleles. Therefore, the presence of strong linkage disequilibrium might indicate that there has been a 'recent' selective sweep, and this 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, linked variation will tend 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 produces no linkage disequilibrium. ==Evolution by means of natural selection== {{main|Evolution|Darwinism}} A prerequisite for natural selection to result in [[adaptation|adaptive evolution]], novel traits and [[speciation]], is the presence of heritable [[genetic variation]] that results in fitness differences. Genetic variation is the result of [[mutation]]s, [[genetic recombination|recombination]]s and alterations in the [[karyotype]] (the number, shape, size and internal arrangement of the [[chromosome]]s). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are very rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in [[noncoding DNA]] or resulted in a [[synonymous substitution]]. However, recent research suggests that many mutations in non-coding DNA do have slight deleterious effects.<ref name=NCFitnessEffects/><ref name=NCFitnessEffects2/> Although both mutation rates and average fitness effects of mutations are dependent on the organism, estimates from data in [[human]]s have found that a majority of mutations are slightly deleterious.<ref name="Eyre-Walker">Eyre-Walker A, Woolfit M, Phelps T. (2006). The distribution of fitness effects of new deleterious amino acid mutations in humans. ''Genetics'' 173(2):891-900. PMID 16547091 </ref> [[Image:Pavo cristatus albino001xx.jpg|right|thumb|280px|The exuberant tail of the [[peacock]] is thought to be the result of sexual selection by females. This peacock is an [[albino]] - it carries a mutation that makes it unable to produce [[melanin]]. Selection against albinos in nature is intense because they are easily spotted by predators or are unsuccessful in competition for mates, and so these mutations are 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 fail to reproduce. As a result, alleles which on average result in greater fitness become more abundant in the next generation, while alleles which generally reduce fitness become rarer. If the selection forces remain the same for many generations, beneficial alleles become more and more abundant, until they dominate the population, while alleles 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. [[Image:Polydactyly 01 Lhand AP.jpg|left|thumb|150px|X-ray of the left hand of a ten year old boy with [[polydactyly]].]] Some mutations occur in so-called [[regulatory sequence|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 [[zygote]]s. Examples of nonlethal regulatory mutations occur in [[Homeobox|HOX genes]] in humans, which can result in a [[cervical rib]]<ref>Galis F (1999) Why do almost all mammals have seven cervical [[vertebrae]]? developmental constraints, Hox genes, and cancer. ''J Exp Zool'' 285:19-26</ref> or [[polydactyly]], an increase in the number of fingers or toes.<ref>Zakany J, FromentalRamain C, Warot X & Duboule D (1997) Regulation of number and size of digits by posterior Hox genes: a dose-dependent mechanism with potential evolutionary implications. [[PNAS|''Proc Natl Acad Sci USA'']] 94:13695-700</ref> 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; 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 will become more variable and deteriorate over time, possibly resulting in a [[vestigial structure|vestigial]] manifestation of the trait. 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 as [[preadaptation]]. A famous example of a vestigial structure, the [[eye]] of the [[blind mole rat]], is believed to retain function in [[photoperiod]] perception.<ref name="Sanyal">Sanyal S, Jansen HG, de Grip WJ, Nevo E, de Jong WW. (1990). The eye of the blind mole rat, Spalax ehrenbergi. Rudiment with hidden function? ''Invest Ophthalmol Vis Sci.'' 1990 31(7):1398-404. PMID 2142147 </ref> ===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 effect]]s - some potentially beneficial alleles may, by chance, be present in only one or other of two subgroups when they first become separated. A lesser-known mechanism of speciation occurs via [[Hybrid (biology)|hybrid]]ization, well-documented in [[plant]]s and occasionally observed in species-rich groups of animals such as [[cichlid]] fishes.<ref name="Salzburger">Salzburger W, Baric S, Sturmbauer C. (2002). Speciation via introgressive hybridization in East African cichlids? ''Mol Ecol'' 11(3): 619–625. PMID 11918795 </ref> Such mechanisms of rapid speciation can reflect a mechanism of evolutionary change known as [[punctuated equilibrium]], which suggests that evolutionary change and particularly speciation typically happens quickly after interrupting long periods of stasis. Genetic changes within groups result in increasing incompatibility between the genomes of the two subgroups, thus reducing gene flow between the groups. Gene flow will effectively cease when the distinctive mutations characterizing each subgroup become fixed. 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. ==Historical development== {{main|History of evolutionary thought|Inception of Darwin's theory|Development of Darwin's theory}} [[Image:Charles Darwin aged 51.jpg|right|thumb|250px|The modern theory of natural selection derives from the work of [[Charles Darwin]] in the nineteenth century.]] ===Pre-Darwinian theories=== Several ancient philosophers expressed the idea that [[Nature]] produces a huge variety of creatures, apparently randomly, and that only those creatures survive that manage to provide for themselves and reproduce successfully; well-known examples include [[Empedocles]]<ref>{{citation | last = Empedocles | authorlink = Empedocles | title = [http://history.hanover.edu/texts/presoc/emp.htm On Nature] | volume = Book II}}</ref> and his intellectual successor, [[Lucretius]],<ref>{{citation | author = Lucretius | authorlink = Lucretius | title = [http://classics.mit.edu/Carus/nature_things.5.v.html De rerum natura] | volume = Book V}}</ref> while related ideas were later refined by [[Aristotle]].<ref>{{citation | last = Aristotle | title = [http://classics.mit.edu/Aristotle/physics.2.ii.html Physics] | volume = Book II, Chapters 4 and 8}}</ref> The struggle for existence was later described by [[al-Jahiz]] in the 9th century.<ref>Conway Zirkle (1941). Natural Selection before the "Origin of Species", ''Proceedings of the American Philosophical Society'' '''84''' (1), p. 71-123.</ref><ref>Mehmet Bayrakdar (Third Quarter, 1983). "Al-Jahiz And the Rise of Biological Evolutionism", ''The Islamic Quarterly''. [[London]].</ref> Such classical arguments were reintroduced in the 18th century by [[Pierre Louis Maupertuis]]<ref>{{ cite journal | last = Maupertuis | first = Pierre Louis | authorlink = Pierre Louis Moreau de Maupertuis | date = 1748 | title = [[s:Derivation of the laws of motion and equilibrium from a metaphysical principle#I. Assessment of the Proofs of God.27s Existence that are Based on the Marvels of Nature|Derivation of the laws of motion and equilibrium from a metaphysical principle]] ([[s:fr:Les loix du mouvement et du repos déduites d'un principe metaphysique#I. Examen des preuves de l.27existence de Dieu.2C tir.C3.A9es des merveilles de la Nature|Original French text]]) | journal = Histoire de l'academie des sciences et belle lettres de Berlin | volume = 1746 | pages = 267–294}}</ref> and others, including [[Charles Darwin]]'s grandfather [[Erasmus Darwin]]. While these forerunners had an influence on [[Darwinism]], they later had little influence on the trajectory of evolutionary thought after Charles Darwin. [[History of evolutionary thought|Until the early 19th century]], the [[History of creationism#Renaissance to Darwin|prevailing view]] in Western societies was that differences between individuals of a species were uninteresting departures from their [[Platonic idealism|Platonic ideal (or typus)]] of [[created kind]]s. However, the theory of [[uniformitarianism (science)|uniformitarianism]] 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 of [[geological time]] and made plausible the idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species. Early 19th century [[evolutionism|evolutionists]] such as [[Jean Baptiste Lamarck]] suggested the [[inheritance 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 causing [[transmutation of species]].<ref>[[Jean-Baptiste Lamarck|Chevalier de Lamarck J-B, de Monet PA]] (1809) ''Philosophie Zoologique''</ref> This theory has come to be known as [[Lamarckism]] and was an influence on the anti-genetic ideas of the [[Stalinist]] [[Soviet Union|Soviet]] biologist [[Trofim Lysenko]].<ref name="Joravsky">Joravsky D. (1959). Soviet Marxism and Biology before Lysenko. ''Journal of the History of Ideas'' 20(1):85-104.</ref> ===Darwin's hypothesis=== 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".<ref>[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=76 Origin of Species, Chapter 3, page 61]</ref> The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce.<ref>[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=77 Origin of Species, Chapter 3, page 62]</ref> 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's ideas were inspired by the observations that he had made on [[the Voyage of the Beagle]], and by the work of two economists. The first was [[Thomas Malthus]], who in ''[[An Essay on the Principle of Population]]'', noted that population (if unchecked) increases [[exponential growth|exponentially]] whereas the food supply grows only [[linear function|arithmetically]]; thus inevitable limitations of resources would have demographic implications, leading to a "struggle for existence", in which only the ''fittest'' would survive. The second was [[Adam Smith]] who, in ''[[The Wealth of Nations]]'', identified a regulating mechanism in free markets, which he referred to as the "[[invisible hand]]", which suggests that prices self-adjust according to supplies and demand.<ref>[[David Orrell|Orrell, David]] (2007) ''Apollo's Arrow'' Toronto: HarperCollins Publishers Ltd. [http://www.apollosarrow.ca/]</ref> Thus for Darwin, the disaster that was supposed to occur according to Malthus was kept in check and constantly improved by competition (or law of selection). Once the theory had been formulated, Darwin was meticulous about gathering and refining evidence, sharing his ideas only with a few friends; he was inspired to publish after the naturalist [[Alfred Russel Wallace]] independently conceived of the principle and described it in an essay he sent to Darwin. An arrangement was made (without Wallace's knowledge) to present his essay and two short unpublished writings of Darwin's to the [[Linnean Society]] announcing co-discovery of the principle in July 1858;<ref>[[Alfred Russel Wallace|Wallace, Alfred Russel]] (1870) ''Contributions to the Theory of Natural Selection'' New York: Macmillan & Co. [http://www.hti.umich.edu/cgi/t/text/text-idx?c=moa&idno=AJP5195.0001.001&view=toc]</ref> Darwin published a more detailed account of his evidence and conclusions in ''[[The Origin of Species]]'' in 1859. In the 6th edition of ''The Origin of Species'' Darwin acknowledged that others &mdash; notably [[William Charles Wells]] in 1813, and [[Patrick Matthew]] in 1831 &mdash; had proposed similar theories, but had not presented them fully or in notable scientific publications. 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, other mechanisms of evolution such as [[genetic drift|evolution by genetic drift]] were not yet explicitly formulated, and Darwin realized that selection was likely only part of the story: "I am convinced that [it] has been the main, but not exclusive means of modification."<ref>[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=21 Origin of Species, Introduction, page 6]</ref> For Darwin and his contemporaries, ''natural selection'' was thus essentially synonymous with [[Evolution|evolution by natural selection]]. After the publication of ''The Origin of Species'', educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a 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,<ref name="Eisley">Eisley L. (1958). ''Darwin's Century: Evolution and the Men Who Discovered It.'' Doubleday & Co: New York, USA.</ref> a response that has been characterized as the single most significant impediment to the idea's acceptance.<ref name="Kuhn">Kuhn TS. [1962] (1996). ''The Structure of Scientific Revolution'' 3rd ed. University of Chicago Press: Chicago, Illinois, USA. ISBN 0-226-45808-3</ref> However, some thinkers enthusiastically embraced natural selection; after reading Darwin, [[Herbert Spencer]] introduced the term ''[[survival of the fittest]]'', which became a popular summary of the theory. Although the phrase is still often used by non-biologists, modern biologists avoid it because it is [[Tautology (rhetoric)|tautological]] if ''fittest'' is read to mean ''functionally superior'' and is applied to individuals rather than considered as an averaged quantity over populations.<ref name="Mills">Mills SK, Beatty JH. [1979] (1994). ''The Propensity Interpretation of Fitness''. Originally in ''Philosophy of Science'' (1979) 46: 263-286; republished in '' Conceptual Issues in Evolutionary Biology'' 2nd ed. Elliott Sober, ed. MIT Press: Cambridge, Massachusetts, USA. pp3-23. ISBN 0-262-69162-0.</ref> In a letter to [[Charles Lyell]] in September 1860, Darwin regrets the use of the term 'Natural Selection', preferring the term 'Natural Preservation'.<ref>{{cite web | url=http://www.darwinproject.ac.uk/darwinletters/calendar/entry-2931.html | title=Darwin Correspondence Online Database: Darwin, C. R. to Lyell, Charles, 28 September 1860 | accessdate=2006-05-10}}</ref> ===Modern evolutionary synthesis=== {{main|Modern evolutionary synthesis}} Only after the integration of a theory of evolution with a complex statistical appreciation of Austrian monk [[Gregor Mendel]]'s 're-discovered' laws of [[inheritance]] did natural selection become generally accepted by scientists. The work of [[Ronald Fisher]] (who developed the language of mathematics and [[The Genetical Theory of Natural Selection|natural selection in terms of the underlying genetic processes]]),<ref name=fisher/> [[J.B.S. Haldane]] (who introduced the concept of the 'cost' of natural selection),<ref>Haldane JBS (1932) ''[[The Causes of Evolution]]''; Haldane JBS (1957) The cost of natural selection. ''[[Journal of Genetics|J Genet]]'' '''55''':511-24([http://www.blackwellpublishing.com/ridley/classictexts/haldane2.pdf].</ref> [[Sewall Wright]] (who elucidated the nature of selection and adaptation),<ref>Wright S (1932) [http://www.blackwellpublishing.com/ridley/classictexts/wright.asp The roles of mutation, inbreeding, crossbreeding and selection in evolution] ''Proc 6th Int Cong Genet'' 1:356–66</ref> [[Theodosius Dobzhansky]] (who established the idea that [[Genetics and the Origin of Species|mutation, by creating genetic diversity, supplied the raw material for natural selection]]),<ref>Dobzhansky Th (1937) ''[[Genetics and the Origin of Species]]'' Columbia University Press, New York. (2nd ed., 1941; 3rd edn., 1951)</ref> [[W. D. Hamilton|William Hamilton]] (who conceived of kin selection), [[Ernst Mayr]] (who recognised the key importance of [[Systematics and the Origin of Species|reproductive isolation for speciation]])<ref>Mayr E (1942) ''[[Systematics and the Origin of Species]]'' Columbia University Press, New York. ISBN 0-674-86250-3</ref> and many others formed the [[modern evolutionary synthesis]]. This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today. ==Impact of the idea== [[Charles Darwin|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&mdash;and provoked the most profound opposition. The radicalism of natural selection, according to [[Stephen Jay Gould]],<ref>[http://www.nybooks.com/articles/1151 The New York Review of Books: Darwinian Fundamentalism] (accessed May 6, 2006)</ref> lay in its power to "dethrone some of the deepest and most traditional comforts of Western thought". In particular, it challenged long-standing beliefs in such concepts as a special and exalted place for humans in the natural world and a benevolent creator whose intentions were reflected in nature's order and design. ===Social and psychological theory=== The social implications of the theory of evolution by natural selection also became the source of continuing controversy. [[Friedrich Engels]], a German [[political philosopher]] and co-originator of the ideology of [[communism]], wrote in 1872 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".<ref>[[Engels]] F (1873-86) ''Dialectics of Nature'' 3d ed. Moscow: Progress, 1964 [http://www.marxists.org/archive/marx/works/1883/don/index.htm]</ref> Interpretation of natural selection as necessarily 'progressive', leading to increasing 'advances' in intelligence and civilisation, was used as a justification for [[colonialism]] and policies of [[eugenics]], as well as broader sociopolitical positions now described as [[Social Darwinism]]. [[Konrad Lorenz]] won the [[Nobel Prize in Physiology or Medicine]] 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."<ref> Quoted in translation in Eisenberg L (2005) Which image for Lorenz? ''Am J Psychiatry'' 162:1760 [http://ajp.psychiatryonline.org/cgi/content/full/162/9/1760]</ref> Others have developed ideas that human societies and culture [[evolution of societies|evolve]] by mechanisms that are analogous to those that apply to evolution of species.<ref>e.g. Wilson, DS (2002) ''Darwin's Cathedral: Evolution, Religion, and the Nature of Society''. University of Chicago Press, ISBN 0-226-90134-3 </ref> More recently, work among anthropologists and psychologists has led to the development of [[sociobiology]] and later [[evolutionary psychology]], a field that attempts to explain features of [[human psychology]] in terms of adaptation to the ancestral environment. The most prominent such example, notably advanced in the early work of [[Noam Chomsky]] and later by [[Steven Pinker]], is the hypothesis that the human brain is adapted to [[language acquisition|acquire]] the [[grammar|grammatical]] rules of [[natural language]].<ref name="Pinker">Pinker S. [1994] (1995). ''The Language Instinct: How the Mind Creates Language.'' HarperCollins: New York, NY, USA. ISBN 0-06-097651-9</ref> Other aspects of human behavior and social structures, from specific cultural norms such as [[Westermarck effect#Westermarck effect|incest avoidance]] to broader patterns such as [[gender role]]s, have been hypothesized 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 of [[meme]]s - "units of cultural transmission", or culture's equivalents of genes undergoing selection and recombination - has arisen, first described in this form by [[Richard Dawkins]]<ref name="Dawkins">Dawkins R. [1976] (1989). ''The Selfish Gene.'' Oxford University Press: New York, NY, USA, p.192. ISBN 0-19-286092-5</ref> and subsequently expanded upon by philosophers such as [[Daniel Dennett]] as explanations for complex cultural activities, including human [[consciousness]].<ref name="Dennett">Dennett DC. (1991). ''Consciousness Explained.'' Little, Brown, and Co: New York, NY, USA. ISBN 0-316-18066-1</ref> Extensions of the theory of natural selection to such a wide range of cultural phenomena have been distinctly controversial and are not widely accepted.<ref name="Rose">For example, see Rose H, Rose SPR, Jencks C. (2000). ''Alas, Poor Darwin: Arguments Against Evolutionary Psychology.'' Harmony Books. ISBN 0609605135</ref> ===Information and systems theory=== In 1922, [[Alfred J. Lotka|Alfred Lotka]] proposed that natural selection might be understood as a physical principle which could be energetically quantified,<ref>Lotka AJ (1922a) [http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1085052&blobtype=pdf Contribution to the energetics of evolution] [PDF] ''Proc Natl Acad Sci USA'' 8:147–51<br>Lotka AJ (1922b) [http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1085053&blobtype=pdf Natural selection as a physical principle] [PDF] ''Proc Natl Acad Sci USA'' 8:151–4</ref> a concept that was later developed by [[Howard T. Odum|Howard Odum]] as the [[maximum power principle]] whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation. Such concepts are sometimes relevant in the study of applied [[thermodynamics]]. 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 specified [[fitness function]].<ref> [[Stuart Kauffman|Kauffman SA]] (1993) ''The Origin of order. Self-organization and selection in evolution.'' New York: Oxford University Press ISBN 0-19-507951-5 </ref> For example, a class of [[heuristic]] [[optimization (mathematics)|optimization]] [[algorithm]]s known as [[genetic algorithm]]s, pioneered by [[John Henry Holland|John Holland]] in the 1970s and expanded upon by [[David Goldberg]],<ref name="Goldberg">Goldberg DE. (1989). Genetic Algorithms in Search, Optimization and Machine Learning. Addison-Wesley: Boston, MA, USA</ref> identify optimal solutions by simulated reproduction and mutation of a population of solutions defined by an initial [[probability distribution]].<ref name="Mitchell">Mitchell, Melanie, (1996), An Introduction to Genetic Algorithms, MIT Press, Cambridge, MA.</ref> Such algorithms are particularly useful when applied to problems whose [[energy landscape|solution landscape]] is very rough or has many local minima. ==See also== * [[Artificial selection]] * [[Co-evolution]] * ''[[Darwin Among the Machines]]'': Evolution of machine consciousness by ''natural'' selection * [[Gene-centered view of evolution]] * [[Genetic algorithm]] * [[Koinophilia]] * [[Negative selection (natural selection)|Negative selection]] * [[Peppered moth evolution]] * [[Ring species]] * [[Unit of selection]] ==References== {{reflist|2}} ==Further reading== * For technical audiences ** {{cite book| author=[[Stephen Jay Gould|Gould, Stephen Jay]]| year=2002|title=The Structure of Evolutionary Theory| publisher=Harvard University Press| id=ISBN 0-674-00613-5}} ** {{cite book| author=[[John Maynard Smith|Maynard Smith, John]]| year=1993|title=[[The Theory of Evolution]]: Canto Edition| publisher=Cambridge University Press| id=ISBN 0-521-45128-0}} ** [[Karl Popper|Popper, Karl]] (1978) ''Natural selection and the emergence of mind.'' Dialectica 32:339-55. See [http://www.geocities.com/criticalrationalist/popperevolution.htm] ** [[Elliott Sober|Sober, Elliott]] (1984) ''The Nature of Selection: Evolutionary Theory in Philosophical Focus.'' University of Chicago Press. ** [[George C. Williams|Williams, George C.]] (1966) ''[[Adaptation and Natural Selection|Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought]].'' Oxford University Press. ** [[George C. Williams|Williams George C.]] (1992) ''Natural Selection: Domains, Levels and Challenges.'' Oxford University Press. * For general audiences ** [[Richard Dawkins|Dawkins, Richard]] (1996) ''[[Climbing Mount Improbable]].'' Penguin Books, ISBN 0-670-85018-7. ** [[Daniel Dennett|Dennett, Daniel]] (1995) ''[[Darwin's Dangerous Idea|Darwin's Dangerous Idea: Evolution and the Meanings of Life]].'' Simon & Schuster ISBN 0-684-82471-X. ** [[Stephen Jay Gould|Gould, Stephen Jay]] (1997) ''Ever Since Darwin: Reflections in Natural History.'' Norton, ISBN 0-393-06425-5. ** [[Steve Jones (biologist)|Jones, Steve]] (2001) ''Darwin's Ghost: The Origin of Species Updated.'' Ballantine Books ISBN 0-345-42277-5. Also published in Britain under the title ''Almost like a whale: the origin of species updated.'' Doubleday. ISBN 1-86230-025-9. **[[Richard Lewontin|Lewontin, Richard]] (1978) ''Adaptation.'' Scientific American 239:212-30 **[[Jonathan Weiner|Weiner, Jonathan]] (1994) ''[[The Beak of the Finch]]: A Story of Evolution in Our Time.'' Vintage Books, ISBN 0-679-73337-X. * Historical ** Zirkle C (1941). Natural Selection before the "Origin of Species", ''Proceedings of the American Philosophical Society'' '''84''' (1), p. 71-123. ** Kohm M (2004) ''A Reason for Everything: Natural Selection and the English Imagination.'' London: Faber and Faber. ISBN 0-571-22392-3. For review, see [http://human-nature.com/nibbs/05/wyhe.html] van Wyhe J (2005) ''Human Nature Review'' 5:1-4 ==External links== * [http://www.literature.org/authors/darwin-charles/the-origin-of-species/chapter-04.html ''The Origin of Species'' by Charles Darwin] - Chapter 4,Natural Selection * [http://www.wcer.wisc.edu/ncisla/muse/naturalselection/index.html Natural Selection]- Modeling for Understanding in Science Education, University of Wisconsin * [http://evolution.berkeley.edu/evolibrary/search/topicbrowse2.php?topic_id=53 Natural Selection] from University of Berkeley education website {{evolution}} {{popgen}} [[Category:Selection]] [[Category:Evolution]] [[Category:Evolutionary biology]] [[Category:Ecological processes]] [[af:Natuurlike seleksie]] [[ar:اصطفاء طبيعي]] [[zh-min-nan:Chū-jiân sóan-te̍k]] [[bg:Естествен отбор]] [[ca:Selecció natural]] [[cy:Detholiad naturiol]] [[da:Naturlig selektion]] [[de:Selektion (Evolution)]] [[et:Looduslik valik]] [[el:Φυσική επιλογή]] [[es:Selección natural]] [[eo:Natura selektado]] [[eu:Hautespen natural]] [[fr:Sélection naturelle]] [[gl:Selección natural]] [[zh-classical:天擇]] [[ko:자연선택]] [[id:Seleksi alam]] [[is:Náttúruval]] [[it:Selezione naturale]] [[he:ברירה טבעית]] [[lt:Natūrali atranka]] [[hu:Természetes szelekció]] [[nl:Natuurlijke selectie]] [[ja:自然選択説]] [[no:Naturlig seleksjon]] [[nn:Naturleg utval]] [[uz:Tabiiy tanlanish]] [[pl:Dobór naturalny]] [[pt:Seleção natural]] [[ro:Selecţie naturală]] [[ru:Естественный отбор]] [[sq:Seleksionimi natyror]] [[simple:Natural selection]] [[sr:Природна селекција]] [[fi:Luonnonvalinta]] [[sv:Naturligt urval]] [[tr:Doğal seçilim.]] [[uk:Природний відбір]] [[yi:נאטירלעכע סעלעקציע]] [[zh:自然选择]]