- Gene-centered view of evolution
The gene-centered view of evolution, gene selection theory or selfish gene theory holds that
natural selectionacts through differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation. According to this theory, adaptations 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 Darwinand Alfred Russel Wallace, who proposed the theoryof evolutionby natural selection(Darwin & Wallace, 1858) as opposed to the Lamarckian idea of evolution via acquired changes. According to this theory, a populationof reproductive individuals is subject to natural selection if the following are present: (1) variationin the reproductive performance of individuals within the population; (2) heredity, meaning "like begets like"; and (3) competitionfor 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
theoryof evolutionby natural selectionwas initially based on a vague concept of heredity. Darwin endorsed the blending inheritance hypothesisdue to the absence, at that time, of a rigorous theory of heredity. Subsequently, significant discoveries about both the mechanisms of inheritance and those of development have revolutionised this area of biology.
Discoveries in heredity
In the mid-19th century, the Czech
Augustinianmonk Gregor Mendelproposed 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 Weismannproposed the continuity of the germ plasm, where phenotypic changes environmentally caused in the 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 generations of rats, they will always produce tailed offspring. Similarly puppiesof breeds of dogs which consistently over generations have had their tails or ears docked are born with tails and ears.
This principle was reflected at molecular level by
Francis Crickwhen he formulated the central dogma of molecular biologyin 1958: information flows only from nucleic acidto nucleic acid or protein, and never from proteinto nucleic acid or protein.
Acquired characteristics are not inherited
These discoveries made it clear that the
inheritance of acquired characterswas not an evolutionary factor and identified genes as lasting entities that survive through many generations. Maynard Smith summarized the issue:
The rejection of the inheritance of acquired characters combined with the classical
mathematical evolutionary biologydeveloped by Ronald Fisher(particularly in his 1930 book, " The Genetical Theory of Natural Selection"), J. B. S. Haldaneand 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 selectionwas 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 Pittendrighin his 1958 article, "Adaptation, natural selection, and behavior", and in the classic papers about altruism of 1963 and 1964 by William Hamilton.
According to Williams' 1966 book:
Williams argued that "The natural selection of
phenotypes 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
DNAis the main physical substrate to genetic information, and it is capable of high fidelity 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
genomeis the unique combination of father's and mother's chromosomes produced at the moment of fertilization. It is generally destroyed with its organism, because " meiosisand recombinationdestroy genotypes as surely as death." (Williams, 1966) Only half of it is transmitted to each descendant due to the 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 economistscall a utility function, meaning "that which is maximized", Dawkins 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 ecosystemor a speciesas 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.
The selfish gene theory of natural selection can be restated as follows:
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
adaptations 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.
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 "survival machines" 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.
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:
A gene in a
somaticcell 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.
kin selectiontheory 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 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 effects 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.
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 conflictand 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 meiosisor gametogenesisand ends up in more than half of the functional gametes. 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"
Those prominent among the opponents of this gene-centric view of evolution include
paleontologist Stephen Jay Gould, biologist and anthropologist David Sloan Wilsonand 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 "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 teleological principle. [ [http://www.nybooks.com/articles/1151 Darwinian Fundamentalism - The New York Review of Books ] ]
The gene-centered view of evolution is a synthesis of the theory of
evolutionby natural selection, the particulate inheritance theoryand 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
Richard Dawkinsand George C. Williams, other biologists and philosophers have expanded and refined the selfish gene theory, such as John Maynard Smith, Robert Trivers, David Haig, Helena Cronin, David Hull, Philip Kitcherand Daniel C. Dennett.
* Crick, F. (1970) [http://www.euchromatin.org/Crick01.htm Central dogma of molecular biology] "Nature" 227 (August 8): 561-563.
* Cronin, H. (1991) "The Ant and the Peacock." Cambridge University Press, Cambridge. ISBN 0-521-32937-X
* Darwin, C. & 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.
* Dawkins, R. (1976) "
The Selfish Gene." Oxford University Press, Oxford. ISBN 0-19-286092-5
* Dawkins, R. (1982) "
The Extended Phenotype." Oxford University Press, Oxford. ISBN 0-19-288051-9
* 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.
* Fisher, R. A. (1930) "The Genetical Theory of Natural Selection." Oxford University Press, Oxford. ISBN 0-19-850440-3.
* Haig, D. (1997) "The Social Gene." In J. R. Krebs and N. B. Davies, eds., "Behavioural Ecology", Oxford: Blackwell Scientific, pp. 284-304.
* Hamilton, W. D. (1963) The evolution of altruistic behavior. "The American Naturalist" 97 (896): 354-356.
* Hamilton, W. D. (1964) The genetical evolution of social behaviour. "Journal of Theoretical Biology" 7: 1-52.
* Leigh, E. (1971) "Adaptation and Diversity." Cooper, San Francisco.
* Maynard Smith, J. (1998) "Evolutionary Genetics: 2nd Edition." Oxford University Press, Oxford.
* 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.
* Pittendrigh, C. (1958) Adaptation, natural selection, and behavior. In A. Roe and G. G. Simpson, eds., "Behavior and Evolution", New Haven: Yale University Press, pp 390-416.
* Williams, G. C. (1966) "Adaptation and Natural Selection". Princeton University Press, Princeton. ISBN 0-691-02615-7
* Williams, G. C. (1985) "A defense of reductionism in evolutionary biology." Oxford Surveys in Evolutionary Biology, 2: 1-27.
* Williams, G. C. (1992) "Natural Selection: Domains, Levels and Challenges." Oxford University Press, Oxford. ISBN 0-19-506932-3
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