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Zusammenfassung:
Both theory and experiments have demonstrated that sex can facilitate adaptation, potentially yielding a group-level
advantage to sex. However, it is unclear whether this process can help solve the more difficult problem of the maintenance
of sex within populations. Using experimental populations of the facultatively sexual rotifer Brachionus calyciflorus, we show
that rates of sex evolve to higher levels during adaptation but then decline as fitness plateaus. To assess the fitness
consequences of genetic mixing, we directly compare the fitnesses of sexually and asexually derived genotypes that
naturally occur in our experimental populations. Sexually derived genotypes are more fit than asexually derived genotypes
when adaptive pressures are strong, but this pattern reverses as the pace of adaptation slows, matching the pattern of
evolutionary change in the rate of sex. These fitness assays test the net effect of sex but cannot be used to disentangle
whether selection on sex arises because highly sexual lineages become associated with different allele combinations or with
different allele frequencies than less sexual lineages (i.e., ‘‘short-’’ or ‘‘long-term’’ effects, respectively). We infer which of
these mechanisms provides an advantage to sex by performing additional manipulations to obtain fitness distributions of
sexual and asexual progeny arrays from unbiased parents (rather than from naturally occurring, and thereby evolutionarily
biased, parents). We find evidence that sex breaks down adaptive gene combinations, resulting in lower average fitness of
sexual progeny (i.e., a short-term disadvantage to sex). As predicted by theory, the advantage to sex arises because sexually
derived progeny are more variable in fitness, allowing for faster adaptation. This ‘‘long-term advantage’’ builds over multiple
generations, eventually resulting in higher fitness of sexual types.