  Geographic isolation is a central mechanism of speciation, but perfect
isolation of populations is rare. Although speciation can be hindered if gene
flow is large, intermediate levels of migration can enhance speciation by
introducing genetic novelty in the semi-isolated populations or founding small
communities of migrants. Here we consider a two island neutral model of
speciation with continuous migration and study diversity patterns as a function
of the migration probability, population size, and number of genes involved in
reproductive isolation (dubbed as genome size). For small genomes, low levels
of migration induce speciation on the islands that otherwise would not occur.
Diversity, however, drops sharply to a single species inhabiting both islands
as the migration probability increases. For large genomes, sympatric speciation
occurs even when the islands are strictly isolated. Then species richness per
island increases with the probability of migration, but the total number of
species decreases as they become cosmopolitan. For each genome size, there is
an optimal migration intensity for each population size that maximizes the
number of species. We discuss the observed modes of speciation induced by
migration and how they increase species richness in the insular system while
promoting asymmetry between the islands and hindering endemism.
