The maintenance of sexual reproduction in a structured population

We present the results of a computer simulation model in which a sexual pop ulation produces an asexual mutant. We estimate the probability that the ne w asexual lineage will go extinct. We find that whenever the asexual lineag e does not go extinct the sexual population is out-competed, and only asexu al individuals remain after a sufficiently long period of time has elapsed. We call this type of outcome an asexual takeover. Our results suggest that , given repeated mutations to asexuality, asexual takeover is likely in an unstructured environment. However, if the environment is subdivided into de mes that are connected by migration, then asexual takeover becomes less lik ely. The probability of asexual takeover declines towards zero as the numbe r of demes increases and as the rate of migration decreases. The reason for this is that asexuality leads to a greater loss of fitness due to mutation and genetic drift, in comparison to what occurs under sexual reproduction. Population subdivision slows the spread of asexual lineages, which allows more time for the genetic degeneration caused by asexuality to take place.