Heterosis increases the effective migration rate

Individuals coming from the same subpopulation are more likely to share del eterious mutations at any given locus than hybrids formed between parents f rom different populations. Offspring of migrants therefore may experience h eterosis and have higher fitness than resident individuals. This will, in t urn, result in the immigrant alleles being present in higher frequencies th an predicted from neutral expectations and thus a higher effective migratio n rate. In this paper we derive a formula to calculate the effective migrat ion rate in the presence of heterosis. It is shown that the effect of heter osis on the migration rate can be substantial when fitness reduction within local populations is severe. The effect will be more pronounced in species with relatively short map lengths. Furthermore the heterosis effect will b e highly variable throughout the genome, with the largest effect seen near selected genes and in regions of high gene density.