SPECIATION AND HYBRIDIZATION IN ISLAND BIRDS

The process of speciation in birds can be inferred from the pattern of diversification on islands, especially in archipelagos. The basic mod el is one of initial differentiation of allopatric populations, with f urther differentiation taking place at the time that sympatry is estab lished. Differences that evolve in allopatry are reinforced by a regim e of divergent selection on the taxa in sympatry arising from ecologic al pressures and not from reproductive (genetic) incompatibility. A lo w level of interbreeding (hybridization) at the secondary contact phas e and subsequently may occur with little or no fitness loss. Introgres sive hybridization has the potential to play a creative role in evolut ion, facilitating further divergence by enhancing genetic variation an d relaxing genetic constraints on particular directions of evolutionar y change under natural selection. Hybridization potential may last for many millions of years after two taxa diverge, implying that post-zyg otic isolation evolves slowly. The main alternative model of speciatio n in island birds is the peripatric model. It emphasizes major genetic changes taking place in the founding of a new population by a small n umber of individuals. There is no direct evidence that would make it p referable to the standard allopatric model for islands.