the evolution of sex is favoured during adaptation to new environments性是青睐的进化在适应新环境.pdf

The Evolution of Sex Is Favoured During Adaptation to New Environments 1,2 ¤ 1 Lutz Becks * , Aneil F. Agrawal 1 Department of Ecology Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada, 2 Department of General Ecology, Zoological Institute, Center for ¨ Biological Sciences, University of Cologne, Koln, Germany Abstract 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 (