3 The New York Years (1931–1953)

214 5 Biological Species and Speciation—Mayr’s First Synthesis
Stresemann) during the 1920s, even though many of such allopatric forms actually
were too distinct to be treated as subspecies.
The units of a regional fauna are zoogeographical species (Bock 1956, Mayr
and Short 1970f) composed of two elements, (a) taxonomically isolated species
(isospecies; that are not members of superspecies) and (b) superspecies (Amadon
1966; Bock and Farrand 1980; Amadon and Short 1992). Much earlier Mayr
had already emphasized that one must count zoogeographical species rather than
individual species in a particular region of the world, in order to avoid misleading
implications. In his letter to Stresemann dated 29 September 1936 he wrote:
“A renewed count of the birds of New Guinea resulted in 513 breeding zoogeographical
species (reckoning [the superspecies] Astrapia and Parotia as one species
each).” However, it was not until 1970 that Mayr (l.c.) introduced the term “zoogeographical
species” formally (actually already used in Bock 1956). He himself
credited it later to B. Rensch (Mayr 1989k: 156) without a specific reference. However,
Rensch (1934) had used the term species geographicum only in the sense of
polytypic species (not in the sense of superspecies plus taxonomically isolated
species).
In their analysis of zoogeographic species Mayr and Short (1970f) studied
the avian species taxa of a well-defined region (North America) to answer the
question of how often the application of the biological species concept leads to
difficulties, controversies, or ambiguities presenting in full their data on which
their conclusions were based. Most of the tabulation and accompanying text was
written by L. Short and evaluated in discussions with Mayr. They showed that
invariably the application of the biological species concept helped to clarify difficult
situations. In a number of peripheral isolates one could not be sure whether or
not they had already reached species level, but this created as much difficulty for
a morphological as for the biological species concept. Only in Mexican populations
of towhees, Pipilo, the same two forms behave in one area as two biospecies and
in another area as one because of extensive hybridization. Several other cases
of “interspecific hybridization between largely allopatric members of the same
superspecies” are not clear-cut either, especially those where there are large zones
of geographical range overlap and extensive hybridization.
The theoretical concept of biological species is nondimensional and refers most
clearly to genetic-reproductively isolated populations at a particular locality (Mayr
1941i, 1942e, 1946l, 1953b, 1963b). Because a fully differentiated biospecies represents
a genetic unit, a reproductive unit and an ecological unit, Mayr (1951l: 92,
1982d: 273) specified as one aspect of biological species a specific niche in nature,
i.e., ecological isolation permitting sympatry with competitors. This theoretical
notion of biological species must be distinguished from the multidimensional
species taxon. If several differentiated groups of populations are in contact and
intergrade, they belong to the same species taxon (Fig. 5.4). Allopatric, i.e., geographically
separated, representative taxa are assigned subspecies or species status
on the basis of inference (Mayr 1948c, Mayr et al. 1953a: 104, 1969b: 197, Mayr
and Ashlock 1991i: 104–105). Auxiliary criteria used for that purpose include: