Tóm tắt
Birds, especially those on islands, have contributed disproportionately to the development of theories that explain the origin and maintenance of organic diversity. This essay surveys the major ideas of how bird species are thought to evolve, speciate and multiply in adaptive radiations on islands. I begin with the nineteenth century view that speciation is the result of natural selection slowly causing divergence of geographically separated populations. I then trace the course of discovery and deeper understanding during the twentieth century, from the influence of Mendelian genetics at the beginning to the modern, molecular era at the end. The dominant and pivotal evolutionary question has been how new species are formed. An initial, nearly exclusive, attention to adaptive processes has been replaced by a search for additional reasons for allopatric divergence, coupled with the conditions that permit coexistence in sympatry after allopatric divergence has occurred. Recent studies have shown that pre‐mating barriers to gene exchange caused by natural selection and sexual selection often arise before post‐mating genetic incompatibilities have evolved. Therefore behavioural and ecological factors play a predominant role in this stage of speciation. Imprinting and imprinting‐like processes of early learning contribute to pre‐mating barriers by constraining mate choice. Ecological factors determine coexistence potential and the fates of hybrids. Genetical factors at all stages, from the founding of a new population to the evolution of genetic incompatibilities, are still poorly known. But molecular genetics is now helping us to reconstruct evolutionary history, thereby transforming ideas about systematic relationships, colonization routes and both the tempo and mode of evolution. As patterns of evolution within radiations become better known, more attention needs to be given to the task of explaining the sequential build‐up of avian communities; by evolutionary radiation and additional immigration, offset to some extent by extinction. The new and substantial challenge is to understand evolutionary radiations in relation to changing environments. Until that challenge is met we cannot claim that the problem of explaining adaptive radiations of birds or any other group of organisms is solved.
