Rensch (1928,1929,1934) and Stresemann (1939: 360) had considered closely related bird species which exclude each other geographically on continents and in island groups because of competition owing to equal or similar ecological requirements. These observations were the basis for Rensch's (l. c.) Artenkreis concept (superspecies of Mayr 1931b). In the section on "The biology of speciation" Mayr (1942e) discussed the influence of ecological and behavioral factors. Speciation may happen rather rapidly or extremely slowly in different groups. The geographical ranges of species may be small or very extensive and geographical barriers may be very effective for some species but ineffective for others, because their dispersal capacity varies greatly. Speciation is balanced by extinction which is frequent on small islands. Where isolating mechanisms between species fail, hybridization occurs.
Stresemann (1943) discussed the evolutionary role of ecological differences among local populations of birds giving many examples. Lack (1944,1947,1949) based his discussion on the ecological differences between species as an indispensable prerequisite for coexistence and Mayr (1944n) was so enthusiastic about Lack's (1944) first article on these topics that he immediately reviewed it in an American ornithological journal: "Lack makes the very important point that reproductive isolation alone is not enough for two species to coexist. They must have also developed certain ecological differences—dissimilar habitat or food preferences, for example—that prevent competition with each other. In many cases there is considerable overlap, but it is never complete" (Mayr 1944n).
Ecological competition may prevent two species from invading each other's ranges, as shown by many members of superspecies which exclude each other geographically along sharply defined contact zones. Lack (1947) showed how competition determined which ecological niche various species of Geospiza occupy on a given Galapagos Island and how this affects the size of the bill. It was Lack who brought ecology into the evolutionary synthesis, Mayr (1947e) remarked in his article on "Ecological factors in speciation."
As to the origin of higher taxa Mayr (1942e: 298) concluded that this process "is nothing but an extrapolation of speciation. All the processes and phenomena of macroevolution and of the origin of the higher [taxa] can be traced back to intraspecific variation, even though the first steps of such processes are usually very minute."
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