Stochastic migration and maintenance of genetic variation in sink populations

We explore the effect that demographic parameters and different patterns of migration have on the maintenance of genetic variability in a single sink population that receives migrants from a source population and whose size changes with time. We use, as a measure of genetic variability, the expected number of nucleotide differences between two genes randomly sampled from the sink. We assume that changes in the environment are translated into changes in the number of individuals arriving at the sink. We show that the pattern of migration from source to sink affects the amount of genetic variation maintained within the sink. The level of heterozygosity maintained within the sink is negatively correlated with the migrational variance, being lowest under stochastic migration patterns that lead to long runs of no migration events. If migration is indeed stochastic, a purely deterministic treatment of this process will be highly misleading in particular cases and will generally result in some loss of useful information. We also show that the relative importance of migration, genetic drift, and mutation, as they affect the amount of genetic variation maintained within the sink, is a function of the rate of population decay.