3 The New York Years (1931–1953)

224 5 Biological Species and Speciation—Mayr’s First Synthesis
highly host specific forms). Mayr (2004a: 108) summarized his latest views stating:
“After 1942 allopatric speciation was more or less victorious for some 25 years, but
then so many well-analyzed cases of sympatric speciation were found, particularly
among fishes and insects, that there is no longer any doubt about the frequency of
sympatric speciation.”
In recent years, several authors have claimed that if natural selection drives speciation,
then gene flow will not affect diversification in areas (like, e.g., Amazonia)
that are much larger than the per-generation dispersal range of individuals. In view
of sufficient genetic variation and ecological diversity in extensive tropical forest
regions, species may originate through “isolation by distance.” Actual splitting of
populations, as required under the model of allopatric speciation, supposedly is
unnecessary and splitting of populations in space may be the result rather than the
cause of genetic differentiation during “adaptive speciation” (Schilthuizen 2001;
Knapp and Mallet 2003; Tautz 2003; Dieckmann et al. 2004; critical discussion
by Gavrilets 2005). Allopatric speciation is well supported and uncontroversial,
whereas speciation in the face of gene flow is less well supported and more controversial.
Comparative work suggests that it is far less frequent than allopatric
speciation (Coyne and Orr 2004: 7).
Work on speciation over the past several decades mostly focused on the geography,
ecology, and timing of speciation, in the tradition of Mayr’s 1942 book.
Molecular studies confirmed that Pleistocene events caused substantial intraspecific
differentiation and the origin of several closely related pairs of North American
bird species. Many other birds split from extant relatives at earlier times (Lovette
2005). Topics such as the role of sexual selection and the frequency of sympatric
speciation are now also being addressed in several avian systems (Edwards et al.
2005). Rapid speciation may be frequent in birds in view of the importance of
prezygotic isolating mechanisms including song, whereas the slow development
of intrinsic postzygotic isolation will facilitate continuing hybridization. In recent
years the search for speciation genes has been accelerated which eventually may
permit understanding the genetics of avian speciation processes. Such genes seem
to have very rapid rates of adaptive amino acid replacement (Orr 2005).
The Species Problem in Mayr’s Empirical Research
Mayr’s theoretical conclusions on the biological species concept and the geographical
(allopatric) origin of species were based on the results of numerous case studies
which comprised not only birds but also snakes, fossil man, and marine organisms
as well as many other examples reported in the literature.
In his early paper on snow finches (1927f), he discussed geographical variation
and geographic speciation in a group of finches (p. 45). His studies in Pacific island
birds convinced Th. Dobzhansky, when Mayr showed him his material in New
York (p. 133). In August and December 1939, Mayr presented many examples of
speciation from isolated populations among the birds of Oceania, first at the 6th
Pacific Science Congress held in California emphasizing zoogeographical aspects