Ornithology, Evolution, and Philosophy 123

A Modern Unified Theory of Evolution 227
melanogaster and D. simulans, were surely something entirely different from the
conspicuous mutations (white-eye, yellow body, crumpled wings, etc.) in these
species. To them environmentally induced continuous geographical variation was
far more important in evolution than such mutants observed by geneticists in
the laboratory and not relevant to the mutation-selection theory (Mayr 1992i:
2, 23). They believed that the evolutionary significance of natural selection was
rather limited explaining, e.g., the similarity (mimicry) in egg color between the
brood-parasitic European cuckoo and its host species. Since the early 1930s Mayr
was persuaded through modern genetic publications and conversations with his
colleague J. Chapin at the AMNH that indeed natural selection was the crucial factor
explaining adaptation and the differentiation of populations, particularly since the
modern geneticists had shown that natural selection works on minute mutations
and recombinations as the raw material of evolution. Interest of evolutionists in
the study of sexual selection increased only during the 1970s (Mayr 1972g; see
below, p. 279); this factor was not mentioned in the textbooks and monographs of
the 1930s and 1940s, including Mayr’s book of 1942(e).
Ecological Factors
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