Diversity patterns and speciation processes in a two-island system with continuous migration

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.