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 An initial, nearly exclusive, attention to adaptive processes has been replaced by a search for additional reasons for allopatric divergence, coupled with the conditions that permit coexistence in sympatry after allopatric divergence has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. Therefore behavioural and ecological factors play a predominant role in this stage of speciation.
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. 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.