Gene-centered view of evolution 1662015 221968969 2008-06-26T22:33:07Z Quadell 57108 Converting to standard <ref>s using [[Project:AutoWikiBrowser|AWB]] [[Image:The Selfish Gene3.jpg|right|200px|thumb|The 1976 book ''[[The Selfish Gene]]'' by [[Richard Dawkins]] was a notable early work of [[popular science]] that focused on the gene-centered view of evolution.]] The '''gene-centered view of evolution''', '''gene selection theory''' or '''selfish gene theory''' holds that [[natural selection]] acts through differential survival of competing [[gene]]s, increasing the frequency of those [[allele]]s whose [[Phenotype|phenotypic]] effects successfully promote their own propagation. According to this theory, [[adaptation]]s are the phenotypic effects through which genes achieve their propagation. ==Evolution by natural selection== The predominant modern scientific explanation for the adaptation of living beings was initially tailored by [[Charles Darwin]] and [[Alfred Russel Wallace]], who proposed the [[theory]] of [[evolution]] by [[natural selection]] (Darwin & Wallace, 1858) as opposed to the Lamarckian idea of evolution via acquired changes. According to this theory, a [[population]] of [[reproduction|reproductive]] individuals is subject to natural selection if the following are present: (1) [[variation]] in the reproductive performance of individuals within the population; (2) [[heredity]], meaning "like begets like"; and (3) [[competition]] for the resources required for reproduction, be it fertile mates or food. So, those characteristics that augment reproductive performance tend to be represented at a greater proportion than their competing alternative. ==Improved theory of heredity== The [[theory]] of [[evolution]] by [[natural selection]] was initially based on a vague concept of [[heredity]]. Darwin endorsed the [[blending inheritance]] [[hypothesis]] due to the absence, at that time, of a rigorous theory of heredity. Subsequently, significant discoveries about both the mechanisms of [[Biological inheritance|inheritance]] and those of [[Morphogenesis|development]] have revolutionised this area of biology. ==Discoveries in heredity== === Gregor Mendel === In the mid-19th century, the [[Czech people|Czech]] [[Augustinian]] monk [[Gregor Mendel]] proposed the particulate inheritance theory, which states that genes are preserved during development and are passed on unchanged (Fisher, 1930). According to this theory, genes can and usually do mix their phenotypic effects in an [[organism]], but themselves are not mixed and are transmitted in an "all-or-nothing" mode to the next generation. === August Weismann === The biologist [[August Weismann]] proposed the continuity of the germ plasm, where phenotypic changes environmentally caused in the [[Somatic|soma]] are not converted into changes in the [[genotype]] (Weismann, 1893). The classic illustration of this principle is that even if you cut off the tails of thousands of [[generation]]s of rats, they will always produce tailed [[offspring]]. Similarly [[puppies]] of breeds of [[dog]]s which consistently over generations have had their [[Docking (dog)|tails or ears docked]] are born with tails and ears. === Francis Crick === This principle was reflected at molecular level by [[Francis Crick]] when he formulated the [[central dogma of molecular biology]] in 1958: information flows only from [[nucleic acid]] to nucleic acid or protein, and never from [[protein]] to nucleic acid or protein. ===Acquired characteristics are not inherited === These discoveries made it clear that the [[inheritance of acquired characters]] was not an evolutionary factor and identified genes as lasting entities that survive through many generations. Maynard Smith summarized the issue: {{cquote|If the central dogma is true, and if it is also true that nucleic acids are the only means whereby information is transmitted between generations, this has crucial implications for evolution. It would imply that all evolutionary novelty requires changes in nucleic acids, and that these changes - mutations - are essentially accidental and non-adaptive in nature. Changes elsewhere - in the egg cytoplasm, in materials transmitted through the placenta, in the mother's milk - might alter the development of the child, but, unless the changes were in nucleic acids, they would have no long-term evolutionary effects. (Maynard Smith, 1998, p.10)}} The rejection of the inheritance of acquired characters combined with the classical [[mathematic]]al evolutionary [[biology]] developed by [[Ronald Fisher]] (particularly in his 1930 book, ''[[The Genetical Theory of Natural Selection]]''), [[J. B. S. Haldane]] and [[Sewall Wright]], they paved the way to the formulation of the '''selfish gene theory'''. For cases when environment can influence heredity see:- [[Epigenetics]]. ==Gene as the unit of selection== The view of the gene as the [[unit of selection]] was developed mainly in the books ''[[Adaptation and Natural Selection]]'' (1966), by [[George C. Williams]], and in ''[[The Selfish Gene]]'' (1976) and ''[[The Extended Phenotype]]'' (1982), both by [[Richard Dawkins]]. It had earlier been proposed by [[Colin Pittendrigh]] in his 1958 article, ''Adaptation, natural selection, and behavior'', and in the classic papers about altruism of 1963 and 1964 by [[W. D. Hamilton|William Hamilton]]. According to Williams' 1966 book: {{cquote|The essence of the genetical theory of natural selection is a statistical bias in the relative rates of survival of alternatives (genes, individuals, etc.). The effectiveness of such bias in producing adaptation is contingent on the maintenance of certain quantitative relationships among the operative factors. One necessary condition is that the selected entity must have '''a high degree of permanence and a low rate of endogenous change''', relative to the degree of bias (differences in selection coefficients). (Williams, 1966, p.22-23)}} Williams argued that "The natural selection of [[phenotype]]s cannot in itself produce cumulative change, because phenotypes are extremely temporary manifestations." (Williams, 1966) Each phenotype is the unique product of the interaction between genome and environment. It doesn't matter how fit and fertile a phenotype is, it will eventually be destroyed and will never be duplicated. Since 1954, it has been known that [[DNA]] is the main physical substrate to genetic information, and it is capable of high fidelity [[DNA replication|replication]] through many generations. So, a particular sequence of DNA can have a high permanence and a low rate of endogenous change. The question that remains is how long the segment must be. In normal sexual reproduction, an entire [[genome]] is the unique combination of father's and mother's chromosomes produced at the moment of fertilization. It is generally destroyed with its organism, because "[[meiosis]] and [[recombination]] destroy genotypes as surely as death." (Williams, 1966) Only half of it is transmitted to each descendant due to the [[Mendelian inheritance#Mendel.27s law of segregation|independent segregation]], and only fragments of it are transmitted because of [[recombination]]. If the gene is defined as "that which segregates and recombines with appreciable frequency", it will generally fulfill the requisite of high degree of permanence and a low rate of endogenous change. The gene as an informational entity persists for an evolutionary significant span of time through a lineage of many physical copies. In his book ''[[River out of Eden]]'', Dawkins coins the phrase ''[[God's utility function]]'' to further expound his view on genes as units of selection. He uses this phrase as a synonym of the "[[meaning of life]]" or the "purpose of life". By rephrasing the word ''purpose'' in terms of what [[economists]] call a [[utility function]], meaning "that which is maximized", Dawkins [[reverse engineering|reverse-engineers]] the purpose in the mind of the Divine Engineer of Nature, or the ''Utility Function of God''. In the end, Dawkins shows that it is a mistake to assume that an [[ecosystem]] or a [[species]] as a whole exists for a purpose. In fact, it is wrong to suppose that individual organisms lead a meaningful life either. In nature, only genes have a utility function – to perpetuate their own existence with indifference to great sufferings inflicted upon the organisms they build, exploit and discard. ==Genic selection== The selfish gene theory of natural selection can be restated as follows: {{cquote|Genes do not present themselves naked to the scrutiny of natural selection, instead they present their phenotypic effects. (...) Differences in genes give rise to difference in these phenotypic differences. Natural selection acts on the phenotypic differences and thereby on genes. Thus genes come to be represented in successive generations in proportion to the selective value of their phenotypic effects. (Cronin, 1991, p.60)}} The result is that "the prevalent genes in a sexual population must be those that, as a mean condition, through a large number of genotypes in a large number of situations, have had the most favourable phenotypic effects for their own replication." (Williams, 1985) In other words, we expect selfish genes, "selfish" meaning that promotes its own survival without necessarily promoting the survival of the organism, group or even species. This theory implicates that [[adaptation]]s are the phenotypic effects of genes to maximize their representation in the future generations. An adaptation is maintained by selection if it promotes genetic survival directly or some subordinate goal that ultimately contributes to successful reproduction. ==Vehicles== As stated above, genes are not naked in the world. They are usually packed together inside a genome, which is itself contained inside an organism. Genes group together into genomes because "genetic replication makes use of energy and substrates that are supplied by the metabolic economy in much greater quantities than would be possible without a genetic division of labour" (Haig, 1997). They build vehicles to promote their mutual interests of jumping into the next generation of vehicles. As Dawkins put it, organisms are the "[[The Selfish Gene#Crude analogy|survival machine]]s" of genes. The phenotypic effect of a particular gene is contingent on its environment, including the fellow genes constituting with it the total genome. A gene never has a fixed effect, so how is it possible to speak of a gene for long legs? It is because of the phenotypic ''differences'' between alleles. One may say that one allele, all other things being equal or varying within certain limits, causes greater legs than its alternative. This difference may be enough to enable the scrutiny of natural selection. "A gene can have multiple phenotypic effects, each of which may be of positive, negative or neutral value. It is the net selective value of a gene's phenotypic effect that determines the fate of the gene" (Cronin, 1991). For instance, a gene can cause its bearer to have greater reproductive success at a young age, but also cause a greater likelihood of death at a later age. If the benefit outweighs the harm, averaged out over the individuals and environments in which the gene happens to occur, then phenotypes containing the gene will generally be positively selected and thus the abundance of that gene in the population will increase. ==Detecting selection of genotypes== ==Individual altruism, genetic egoism== The gene is a unit of hereditary information that exists in many physical copies in the world, and which particular physical copy will be replicated and originate new copies doesn't matter from the gene's point of view. (Williams, 1992) A selfish gene could be favoured by selection by producing altruism among organisms containing it. The idea is summarized as follows: {{quote|If a gene copy confers a benefit ''B'' on another vehicle at cost ''C'' to its own vehicle, its costly action is strategically beneficial if ''pB > C'', where p is the probability that a copy of the gene is present in the vehicle that benefits. Actions with substantial costs therefore require significant values of ''p''. Two kinds of factors ensure high values of ''p'': relatedness (kinship) and recognition (green beards). (Haig, 1997, p. 288)}} A gene in a [[somatic]] cell of an individual may forego replication to promote the transmission of its copies in the germ line cells. It ensures the high value of ''p = 1'' due to their constant contact and their common origin from the [[zygote]]. The [[kin selection]] theory predicts that a gene may promote the recognition of kinship by historical continuity: a mammalian mother learns to identify her own offspring in the act of giving birth; a male preferentially directs resources to the offspring of mothers with whom he has copulated; the other chicks in a nest are siblings; and so on. The expected altruism between kin is calibrated by the value of ''p'', also known as the [[coefficient of relationship|coefficient of relatedness]]. For instance, an individual have a ''p = 1/2'' in relation to his brother, and ''p = 1/8'' to his cousin, so we would expect, ''[[ceteris paribus]]'', greater altruism among brothers than among cousins. [[Green-beard effect]]s gained their name from a thought-experiment of [[Richard Dawkins]] (1976), who considered the possibility of a gene that caused its possessors to develop a green beard and to be nice to other green-bearded individuals. Since then, a 'green beard effect' has come to refer to forms of genetic self-recognition in which a gene in one individual might direct benefits to other individuals that possess the gene. ==Intragenomic conflict== As genes are capable of producing individual altruism, they are capable of producing conflict among genes inside the genome of one individual. This phenomenon was called [[intragenomic conflict]] and arises when one gene promotes its own replication in detriment to other genes in the genome. The classic example is segregation distorter genes that cheats during [[meiosis]] or [[gametogenesis]] and ends up in more than half of the functional [[gamete]]s. These genes persist even resulting in reduced [[fertility]]. Egbert Leigh (1971) compared the genome to "a parliament of genes: each acts in its own self-interest, but if its acts hurt the others, they will combine together to suppress it" to explain the relative low occurrence of intragenomic conflict. ==Challenges to the "Selfish Gene"== {{Confusing|date=March 2008}} Those prominent among the opponents of this gene-centric view of evolution include [[paleontologist]] [[Stephen Jay Gould]] (September 10, 1941 – May 20, 2002), biologist and anthropologist [[David Sloan Wilson]] and [[philosopher]] [[Elliot Sober]], who have disputed the theory's applicability and fruitfulness. Gould has characterized this perspective as confusing book-keeping with [[causality]]. Gould views selection working on many levels, and has called attention for a hierarchical perspective of selection. Gould has also called the position "strict [[adaptationism]]," "ultra-Darwinism," and "[[fundamentalist|Darwinian fundamentalism]]," describing it as "[[reductionist]]." He saw it as leading to a simplistic "algorithmic" theory of evolution, or even to the re-introduction of a [[teleology|teleological principle]].<ref>http://www.nybooks.com/articles/1151</ref> Such challenges may be [[phenomenology|phenomenological]] in character, derived, in part, from [[common-sense]] analysis of the "experience" of evolution. ==Summary== The [[selfish gene theory]] is a synthesis of the theory of [[evolution]] by [[natural selection]], the [[particulate inheritance theory]] and the [[non-transmission of acquired characters]]. It states that those genes whose phenotypic effects successfully promote their own propagation will be favourably selected in detriment to their competitors. This process produces adaptations to the benefit of [[genes]], which promotes the reproductive success of the [[organism]], or of other organisms containing the same gene (kin altruism and green-beard effects), or even only its own propagation in detriment to the other genes of the genome (intragenomic conflict).{{Fact|date=February 2007}} ==Other main figures== Besides [[Richard Dawkins]] and [[George C. Williams]], other [[biologist]]s and [[philosopher]]s have expanded and refined the selfish gene theory, such as [[John Maynard Smith]], [[Robert Trivers]], [[David Haig (biologist)|David Haig]], [[Helena Cronin]], [[David Hull]], [[Philip Kitcher]] and [[Daniel C. Dennett]]. ==Bibliography== {{reflist}} * [[Francis Crick|Crick, F.]] (1970) [http://www.euchromatin.org/Crick01.htm Central dogma of molecular biology] ''Nature'' '''227''' (August 8): 561-563. * [[Helena Cronin|Cronin, H.]] (1991) ''The Ant and the Peacock.'' Cambridge University Press, Cambridge. ISBN 0-521-32937-X * [[Charles Darwin|Darwin, C.]] & [[Alfred Russel Wallace|Wallace, A.]] (1858) [http://www.life.umd.edu/emeritus/reveal/pbio/darwin/darwindex.html On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.] ''Proceedings of Linnean Society'' '''3''' (July): 45-62. * [[Richard Dawkins|Dawkins, R.]] (1976) ''[[The Selfish Gene]].'' Oxford University Press, Oxford. ISBN 0-19-286092-5 * [[Richard Dawkins|Dawkins, R.]] (1982) ''[[The Extended Phenotype]].'' Oxford University Press, Oxford. ISBN 0-19-288051-9 * [[Richard Dawkins|Dawkins, R.]] (1982) [http://www.stephenjaygould.org/library/dawkins_replicators.html "Replicators and Vehicles"] King's College Sociobiology Group, eds., ''Current Problems in Sociobiology'', Cambridge, Cambridge University Press, pp. 45-64. <!-- not cited in article; kept for now in case they are later * [[Richard Dawkins|Dawkins, R.]] (1986) ''[[The Blind Watchmaker]].'' Oxford University Press, Oxford. ISBN 0-393-31570-3 * [[Richard Dawkins|Dawkins, R.]] (1995) ''[[River Out of Eden]].'' Basic Books, New York. ISBN 0-465-06990-8 --> * [[Ronald Fisher|Fisher, R. A.]] (1930) ''The Genetical Theory of Natural Selection.'' Oxford University Press, Oxford. ISBN 0-19-850440-3. * [[David Haig (biologist)|Haig, D.]] (1997) ''The Social Gene.'' In J. R. Krebs and N. B. Davies, eds., ''Behavioural Ecology'', Oxford: Blackwell Scientific, pp. 284-304. * [[W. D. Hamilton|Hamilton, W. D.]] (1963) The evolution of altruistic behavior. ''The American Naturalist'' '''97''' (896): 354-356. * [[W. D. Hamilton|Hamilton, W. D.]] (1964) The genetical evolution of social behaviour. ''Journal of Theoretical Biology'' '''7''': 1-52. * [[Egbert Leigh|Leigh, E.]] (1971) ''Adaptation and Diversity.'' Cooper, San Francisco. * [[John Maynard Smith|Maynard Smith, J.]] (1998) ''Evolutionary Genetics: 2nd Edition.'' Oxford University Press, Oxford. * [[Ernst Mayr|Mayr, E.]] (1997) [http://www.pnas.org/cgi/content/full/94/6/2091 The objects of selection] ''Proc. Natl. Acad. Sci. USA'' '''94''' (March): 2091-2094. * [[Colin Pittendrigh|Pittendrigh, C.]] (1958) Adaptation, natural selection, and behavior. In A. Roe and [[George Gaylord Simpson|G. G. Simpson]], eds., ''Behavior and Evolution'', New Haven: Yale University Press, pp 390-416. * [[George C. Williams|Williams, G. C.]] (1966) ''Adaptation and Natural Selection''. Princeton University Press, Princeton. ISBN 0-691-02615-7 * [[George C. Williams|Williams, G. C.]] (1985) ''A defense of reductionism in evolutionary biology.'' Oxford Surveys in Evolutionary Biology, '''2''': 1-27. * [[George C. Williams|Williams, G. C.]] (1992) ''Natural Selection: Domains, Levels and Challenges.'' Oxford University Press, Oxford. ISBN 0-19-506932-3 {{evolution}} [[Category:Evolution]] [[Category:Evolutionary biology]] [[Category:Selection]] [[fi:Geenikeskeinen evoluutionäkemys]]