How do variation in the modes of reproduction benefit organisms in general
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The genetic variation in a partially asexual organism is investigated by two models suited for different time scales. Only selectively neutral variation is considered. Model 1 shows, by the use of a coalescence argument, that three sexually derived individuals per generation are sufficient to give a population the same pattern of allelic variation as found in fully sexually reproducing organisms. With less than one sexual event every third generation, the characteristic pattern expected for asexual organisms appear, with strong allelic divergence between the gene copies in individuals. At intermediary levels of sexuality, a complex situation reigns. The pair‐wise allelic divergence under partial sexuality exceeds, however, always the corresponding value under full sexuality. These results apply to large populations with stable reproductive systems. In a more general framework, Model 2 shows that a small number of sexual individuals per generation is sufficient to make an apparently asexual population highly genotypically variable. The time scale in terms of generations needed to produce this effect is given by the population size and the inverse of the rate of sexuality.
Many higher organisms have both sexual and asexual means of letting themselves be represented in future generations. In plants partial asexuality is normally due to a mixture of sexual reproduction by seed and asexual reproduction by specialized structures such as runners or bulbils, whereas in animals partial asexuality often follows from the failure of cyclical parthenogens to go through the sexual stage successfully.
The present article considers the pattern of selectively neutral genetic variation expected in organisms with a stable mixture of sexual and asexual reproduction. Asexuality has classically been regarded as a factor that reduces genetic variation. As a general notion, this is, of course, not correct. Asexual species often harbour a wealth of variation (Ellstrand & Roose, 1987; Hebert, 1987; Suomalainen et al., 1987; Asker & Jerling, 1992) coming from new mutations as well as remnant sexuality and/or multiple origins. But how genetically variable do we expect a predominantly asexual organism to be?
From models of infinitely large populations is known (Marshal & Weir, 1979), that asexuality as such does not affect the equilibrium genotype frequencies of neutral alleles in organisms practicing at least some outbreeding sexuality. The only role asexuality plays in such reproductively mixed conditions is to slow down the rates at which the multilocus equilibrium values are attained (Marshal & Weir, 1979). Asexuality differs in this respect from, say, inbreeding, in that it does not change the genetic structure of a population in any specific, unidirectional way.
genotypic variation. With the advent of methods for multilocus genotyping the genotypic, ‘clonal’, variation in populations of organisms with different degrees of asexuality has become easy to investigate empirically. It has then been found that apparently asexual organisms often harbour considerable genotypic variability. the expected level of neutral genotypic variation in a limited population with partial asexuality must be known for comparison. This question is investigated in Model 2, which is less restricted than Model 1 with respect to the underlying assumptions made. Numerical examples are used to describe the most important results, many of which are already directly or indirectly known.
Earlier theoretical analyses of fully or partially asexual organisms (see, e.g. Lokki, 1976a,b; Pamilo, 1987; Brookfield, 1992) have dealt primarily with the degree of variation expected for particular genetic markers. My aim is to generalize the type of situations considered and to clarify the interactions between the evolutionary processes involved. The approximations used in the first model apply to very long time scales in which an equilibrium in the population is established between mutation and drift, whereas the second model deals with shorter time scales for which reorganization by recombination of the variation already available is of greater importance. The results are of relevance for the question of what method to use for estimating past and current levels of sexuality/asexuality from population data. However, this is a complex topic (see various approaches in Marshall & Brown, 1974; Stoddard & Taylor, 1988; Maynard Smith & Smith, 1998; Mes, 1998; Maynard Smith, 1999; Ceplitis, 2000) that needs further developments. The aim of the present paper is limited to a description of the qualitative and quantitative interactions between the major parameters that govern neutral evolution in partially asexual populations.