Chapter 2 Multi-locus theory of asexual populations

Abstract We calculate the fixation probability of a beneficial allele that arises as the result of a unique mutation in an asexual population that is subject to recurrent deleterious mutation at rate U. Our analysis is an extension of previous works, which make a biologically restrictive assumption that selection against deleterious alleles is stronger than that on the beneficial allele of interest. We show that when selection against deleterious alleles is weak, beneficial alleles that confer a selective advantage that is small relative to U have greatly reduced probabilities of fixation. We discuss the consequences of this effect for the distribution of effects of alleles fixed during adaptation. We show that a selective sweep will increase the fixation probabilities of other beneficial mutations arising during some short interval afterward. We use the calculated fixation probabilities to estimate the expected rate of fitness improvement in an asexual population when beneficial alleles arise continually at some low rate proportional to U. We estimate the rate of mutation that is optimal in the sense that it maximizes this rate of fitness improvement. Again, this analysis relaxes the assumption made previously that selection against deleterious alleles is stronger than on beneficial alleles.

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Effects of intra-gene fitness interactions on the benefit of sexual recombination

Biochemical Society Transactions ◽

10.1042/bst0340560 ◽

2006 ◽

Vol 34 (4) ◽

pp. 560-561 ◽

Cited By ~ 6

Author(s):

R.A. Watson ◽

D.M. Weinreich ◽

J. Wakeley

Keyword(s):

Nucleotide Sequence ◽

Point Mutation ◽

Sequence Space ◽

Fitness Effect ◽

Epistatic Interactions ◽

Genetic Sequence ◽

High Fitness ◽

Sexual Recombination ◽

Evolutionary Trajectories ◽

Asexual Populations

Whereas spontaneous point mutation operates on nucleotides individually, sexual recombination manipulates the set of nucleotides within an allele as an essentially particulate unit. In principle, these two different scales of variation enable selection to follow fitness gradients in two different spaces: in nucleotide sequence space and allele sequence space respectively. Epistasis for fitness at these two scales, between nucleotides and between genes, may be qualitatively different and may significantly influence the advantage of mutation-based and recombination-based evolutionary trajectories respectively. We examine scenarios where the genetic sequence within a gene strongly influences the fitness effect of a mutation in that gene, whereas epistatic interactions between sites in different genes are weak or absent. We find that, in cases where beneficial alleles of a gene differ from one another at several nucleotide sites, sexual populations can exhibit enormous benefit compared with asexual populations: not only discovering fit genotypes faster than asexual populations, but also discovering high-fitness genotypes that are effectively not evolvable in asexual populations.

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Accumulation of mutations in sexual and asexual populations

Genetics Research ◽

10.1017/s0016672300026938 ◽

1987 ◽

Vol 49 (2) ◽

pp. 135-146 ◽

Cited By ~ 98

Author(s):

Pekka Pamilo ◽

Masatoshi Nei ◽

Wen-Hsiung Li

Keyword(s):

Weak Selection ◽

Population Variance ◽

Asexual Population ◽

Term Evolution ◽

A Genome ◽

Analytical Formulae ◽

The Mean ◽

Asexual Populations ◽

Mutant Genes

SummaryThe accumulation of beneficial and harmful mutations in a genome is studied by using analytical methods as well as computer simulation for different modes of reproduction. The modes of reproduction examined are biparental (bisexual, hermaphroditic), uniparental (selfing, automictic, asexual) and mixed (partial selfing, mixture of hermaphroditism and parthenogenesis). It is shown that the rates of accumulation of both beneficial and harmful mutations with weak selection depend on the within-population variance of the number of mutant genes per genome. Analytical formulae for this variance are derived for neutral mutant genes for hermaphroditic, selfing and asexual populations; the neutral variance is largest in a selfing population and smallest in an asexual population. Directional selection reduces the population variance in most cases, whereas recombination partially restores the reduced variance. Therefore, biparental organisms accumulate beneficial mutations at the highest rate and harmful mutations at the lowest rate. Selfing organisms are intermediate between biparental and asexual organisms. Even a limited amount of outcrossing in largely selfing and parthenogenetic organisms markedly affects the accumulation rates. The accumulation of mutations is likely to affect the mean population fitness only in long-term evolution.

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