WAITING FOR SPECIATION - THE EFFECT OF POPULATION SUBDIVISION ON THE TIME TO SPECIATION

We study the time required for speciation in a species that is divided into small versus large populations. Following Dobzhansky and Muller, we assume that hybrid sterility or inviability is caused by ''complem entary genes,'' that is, by the accumulation of genes that cause steri lity or inviability when brought together in hybrids but that have no deleterious effect on their normal species genetic background. When di vergence between populations is caused by genetic drift, we show that the time to speciation is independent of population subdivision: speci ation occurs just as quickly in a species split into a few large popul ations as into many small populations. When divergence is driven by na tural selection, however, the time to speciation is very sensitive to population subdivision and speciation occurs most rapidly when a speci es is split into two large populations. These results contradict sever al popular intuitions about the effect of population size on speciatio n.