Ornithology, Evolution, and Philosophy 123

216 5 Biological Species and Speciation—Mayr’s First Synthesis
maintain the integrity of species, protecting the harmonious gene pools, are isolating
mechanisms, a term proposed by Dobzhansky (1935, 1937: 228–258) who,
unfortunately, did not distinguish between external geographical/ecological barriers
operating in the allopatric phase of speciation and intrinsic mechanisms. Mayr
(1963b: 89–109) stressed the enormous diversity of these devices, e.g., sterility
genes, chromosomal incompatibilities, ecological exclusion, behavioral properties
especially in higher animals. Species usually occupy ecological niches that are sufficiently
different to permit the sympatric occurrence of these species. However,
member species of a superspecies exclude one another allo- or parapatrically because
of ecological competition. The Biological Species Concept is not applicable
to asexual organisms (agamospecies) which form clones, not interbreeding populations,
as defined by the biologist. Their gene pools do not require protection
by isolating mechanisms, because they usually are not subject to genetic recombination.
A list of Mayr’s publications dealing with the species problem totals
94.
Allopatric Speciation
Evolutionists understood since the “Fisherian synthesis” (p. 183) how and why
populations of species change over time, yet species are distinct–they do not hybridize
with or blend into one another. How can one separate species arise from
another (parental) species or one species split into two? Some geneticists (“saltationists”
like W. Bateson, H. de Vries, R. Goldschmidt) believed that new species
arise instantaneously by large mutations, sudden steps in which either a single
character or a whole set of characters together become changed. By contrast, the
naturalists were “gradualists” and believed that speciation is a populational process,
a gradual accumulation of small changes often by natural selection. To solve
the apparent contradiction between sympatric species of a local fauna separated
by bridgeless gaps on one hand and the idea of gradual speciation on the other
hand Mayr presented many intermediate “borderline cases” of geographic variation
from his work on the birds of Oceania pointing to a gradual origin of new
species from isolated (peripheral) groups of populations of an ancestral species
(1940c, 1942e, 1951l, 1963b). The differences he observed among representative
island populations were not only quantitative and continuous but often qualitative
and discrete as observed among congeneric species. For these reasons he replaced
the typological concept of species by a concept of species taxa as aggregates of
geographically variable populations.
“What I did, basing my conclusions on a long tradition of European systematics,
was to introduce the horizontal (geographical) dimension, and show that the
process of geographic speciation is the method by which a gradual evolution of
new species is possible, in spite of the gaps in the non-dimensional situation”
(Mayr 1992i: 7).
As he stated in this quote and in other historical reviews (p. 230), the principles
of the biological species concept and of allopatric speciation had long been