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. There are many theoretical reasons that prezygotic isolation may be unlikely.
Recombination is an important force countering reinforcement.
Speciation due to a shift, initiated by genetic drift and followed by natural selection, between adaptive peaks. Sampling error will cause the allele frequencies to be different from parent population, which can alter fitness of different genotypes due to epistasis. This theory anticipated the punctuated equilibrium theory. All such theories involve the small population going through an adaptive trough before climbing a new adaptive peak. Other, more specific cases do show strong homozygosity and thus evidence of bottlenecks.