The rate of mutation is too small for mutation to serve as a source of variation.
According to Philip Kitcher:
The charge that mutations are rare depends on confusing the mutation rate per locus (on the order of 1 mutation per 100,000 loci) with the rate per zygote (of the order of 1 mutation per zygote) or the rate per population (of the order of 1 billion per population). From an evolutionary perspective, it is the last of these rates that is important. Hence, although [it] is right to claim that mutations are rare (in one sense), [it] is quite wrong to think that this spells trouble for evolutionary theory. Indeed, neo-Darwinian evolutionary theory insists on the rarity of mutation at any individual locus, claiming, for this reason, that natural selection is a more powerful force than mutation (if mutation were extremely frequent, then selection would play a less crucial role). (Kitcher 1982:97)
Although mutation without natural selection is not expected to substantially transform a population (though it certainly can have an impact), natural selection causes beneficial mutations to spread quickly through a population. Moreover, if a mutant allele is only detrimental in homozygous form, it will take many generations to eliminate it from the gene pool, even if the homozygous form is lethal (Ruse 1982:79-84). This, as well as the common phenomenon of heterozygote fitness (cases in which the heterozygous form has an advantage neither homozygous form has, as with the malaria resistance conferred by a single sickle-cell allele), helps to keep populations supplied with a reservoir of mutant genes, some of which will have an advantage over the "normal" alleles in the event of environmental change. Populations do not have to wait around for lucky mutations to occur after the environment changes—the mutant genes, now beneficial, are already in the gene pool.
Kitcher P. 1982. Abusing Science: The Case Against Creationism. Cambridge: MIT Press.
Ruse M. 1982. Darwinism Defended: A Guide to the Evolution Controversies. London: Addison-Wesley.
Last updated: 21 Mar 2008
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