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. Usually preference is for brighter, louder, etc.
Females better perceive more intense stimulus against background noise.
Taxa with strong sexual selection tend to have high species richness. Also, many of the birds can freely hybridize, but don't. Thus, sexual selection may increase the rate of speciation. Natural selection cannot strengthen postzygotic isolation, because that would require the expansion of alleles that reduce fertility or survival. A pattern whereby characters that reduce mating between populations differ more where the two taxa are sympatric than where they are allopatric.