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 model is one of initial differentiation of allopatric populations, with further differentiation taking place at the time that sympatry is established. Differences that evolve in allopatry are reinforced by a regime of divergent selection on the taxa in sympatry arising from ecological pressures and not from reproductive (genetic) incompatibility. A low level of interbreeding (hybridization) at the secondary contact phase and subsequently may occur with little or no fitness loss. Introgressive hybridization has the potential to play a creative role in evolution, facilitating further divergence by enhancing genetic variation and relaxing genetic constraints on particular directions of evolutionary change under natural selection. Hybridization potential may last for many millions of years after two taxa diverge, implying that post-zygotic isolation evolves slowly. The main alternative model of speciation in island birds is the peripatric model. It emphasizes major genetic changes taking place in the founding of a new population by a small number of individuals. There is no direct evidence that would make it preferable to the standard allopatric model for islands.