There is an epistatic interaction between alleles such that hybrids have reduced fitness. Selection may act directly on the trait that causes isolation, or there may be pleiotropic effects. The same model fits here, except that the change in allele frequency is due to drift rather than selection. The role of genetic drift Drift of neutral alleles is not likely to be a mechanism of speciation. Very difficult to show, however, because random chance is the null hypothesis.
Reproductive isolation is a by product of adaptive selection during allopatry.
Mate choice may be partly based on beak size. FW pops are always smaller. Lake populations have repeatedly diverged into open water forms and shallow water forms, which differ in size. This parallel speciation is evidence that there is something adaptive about being small in the shallow, and large in the open lake. Sexual selection and speciation Variation in reproductive success due to variation in the ability to acquire mates.