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Adaptation of asexual populations under Muller’s ratchet
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Resumo(s)
We study the population genetics of adaptation in nonequilibrium haploid asexual populations. We find
that the accumulation of deleterious mutations, due to the operation of Muller’s ratchet, can considerably reduce the
rate of fixation of advantageous alleles. Such reduction can be approximated reasonably well by a reduction in the
effective population size. In the absence of Muller’s ratchet, a beneficial mutation can only become fixed if it creates
the best possible genotype; if Muller’s ratchet operates, however, mutations initially arising in a nonoptimal genotype
can also become fixed in the population, since the loss of the least-loaded class implies that an initially nonoptimal
background can become optimal. We show that, while the rate at which adaptive mutations become fixed is reduced,
the rate of fixation of deleterious mutations due to the ratchet is not changed by the presence of beneficial mutations
as long as the rate of their occurrence is low and the deleterious effects of mutations (sd) are higher than the beneficial
effects (sa). When sa . sd, the advantage of a beneficial mutation can outweigh the deleterious effects of associated
mutations. Under these conditions, a beneficial allele can drag to fixation deleterious mutations initially associated
with it at a higher rate than in the absence of advantageous alleles. We propose analytical approximations for the
rates of accumulation of deleterious and beneficial mutations. Furthermore, when allowing for the possible occurrence
of interference between beneficial alleles, we find that the presence of deleterious mutations of either very weak or
very strong effect can marginally increase the rate of accumulation of beneficial mutations over that observed in the
absence of such deleterious mutations.
Descrição
Palavras-chave
Adaptation, Physiological Evolution Genetics, Population Models, Biological Background selection Genetic hitchhiking Muller’s ratchet Rate of adaptation
Contexto Educativo
Citação
Bachtrog, D. and I. Gordo (2004) Adaptation of asexual populations under Muller’s ratchet. Evolution 58(7) 1403-13