Reconstructing the evolution of birds on islands: 100 years of research

Birds, especially those on islands, have contributed disproportionately to the development of theories that explain the origin and maintenance of orga nic diversity. This essay surveys the major ideas of how bird species are t hought to evolve, speciate and multiply in adaptive radiations on islands. I begin with the nineteenth century view that speciation is the result of n atural selection slowly causing divergence of geographically separated popu lations. I then trace the course of discovery and deeper understanding duri ng the twentieth century, from the influence of Mendelian genetics at the b eginning to the modern, molecular era at the end. The dominant and pivotal evolutionary question has been how new species are formed. An initial, near ly exclusive, attention to adaptive processes has been replaced by a search for additional reasons for allopatric divergence, coupled with the conditi ons that permit coexistence in sympatry after allopatric divergence has occ urred. Recent studies have shown that pre-mating barriers to gene exchange caused by natural selection and sexual selection often arise before post-ma ting genetic incompatibilities have evolved. Therefore behavioural and ecol ogical factors play a predominant role in this stage of speciation. Imprint ing and imprinting like processes of early learning contribute to pre-matin g barriers by constraining mate choice. Ecological factors determine coexis tence potential and the fates of hybrids. Genetical factors at all stages, from the founding of a new population to the evolution of genetic incompati bilities. are still poorly known. But molecular genetics is now helping us to reconstruct evolutionary history, thereby transforming ideas about syste matic relationships, colonization routes and both the tempo and mode of evo lution. As patterns of evolution within radiations become better known, mor e attention needs to be given to the task of explaining the sequential buil d-up of avian communities: by evolutionary radiation and additional immigra tion, offset to some extent by extinction. The new and substantial challeng e is to understand evolutionary radiations in relation to changing environm ents. Until that challenge is met we cannot claim that the problem of expla ining adaptive radiations of birds or any other group of organisms is solve d.