ECOLOGICAL FACTORS IN SPECIATION

The past decades have seen the gradual formulation of the theory of geographical speciation, principally based on work in the fields of the systematics of mammals, birds, insects, and mollusks. This theory 'postulates that in bisexual animals a new species can develop only "if a population, which has become geographically isolated from its parental species, acquires during this period of isolation characters which prorhote or guarantee reproductive isolation when 'the external barriers break down" (Mayr, 1942). A number of workers in recent years have considered this statement an oversimplification. The omission of any reference in this description of the speciation process to the ecological factors-so vitally involved in the process of speciation-has also been objected to. Even though the importance of geographical speciation is universally admitted, there are a number of authors, particularly among the ichthyologists and entomologists, who believe that in addition to geographical speciation there is another process of speciation, variously referred to as ecological or sympatric speciation. Thorpe (1945), for example, is inclined to consider that in addition to geographical speciation there is a speciation process which is characterized by the fact "that local differences of habit may be the starting point for the evolution of new species." It appears to me that there is no real conflict between those authors who stress the ecological aspects of speciation, such as Stresemann (1943) and Thorpe (1945), and those who, like myself, have stressed the geographical aspects. This was pointed out by Timofeeff-Ressovsky (1943). The seeming differences are due partly to misunderstandings and partly to a different emphasis placed on various aspects of a single process of speciation. The field has suffered from looseness of thought and vagueness of expression, and it may therefore be ulseful to attempt a fresh analysis and possibly a synthesis of the two viewpoints. The principal obstacle blocking such synthesis up to now has been that those authors who have emphasized the role of ecological factors in speciation generally accept sympatric speciation as an integral part of ecological speciation. To remove this difficulty, a special section (pp. 269-285) has been devoted to a discussion of sympatric speciation. It is more important, however, to determine the respective roles of geographical and ecological factors in speciation, and to find out whether they operate consecutively or concomitantly. The field has suffered particularly from the lack of a clear-cut Fragestellung. The result has been that only few investigations have ever been undertaken which would give decisive answers to the unsolved questions. Most of the published studies sadly miss the crucial points. It is one additional object of this discussion to focus the attention of investigators on those aspects that are in particular need of further study.

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