3 The New York Years (1931–1953)

A Modern Unified Theory of Evolution 209
differentiated at the level of subspecies or species, in some cases even as monotypic
genera. His examples are sufficient to show that geographical variation and
isolation of populations lead to the formation of new species taxa (geographical or
allopatric speciation). In view of his taxonomic approach the species definition he
proposed was that of a polytypic species taxon:
“A species consists of a group of populations which replace each other geographically
or ecologically and of which the neighboring ones intergrade or hybridize
wherever they are in contact or which are potentially capable of doing so (with
one or more of the populations) in those cases where contact is prevented by
geographical or ecological barriers” (1940c: 256).
Hence, interpretation was required of all the factual information to decide
whether allopatric populations are “potentially capable” of interbreeding, that is,
whether to name them subspecies or species. The differences among representative
island populations were not only quantitative and continuous but often qualitative
and discrete as observed among congeneric species. For this reason he replaced the
typological concept by a concept of species taxa as aggregates of geographically
variable populations. In this article he did not (yet) discuss the general species
concept based on reproductive isolation between populations but he did so during
the following year.
The central topics of Mayr’s Jesup lectures in March 1941 were the biological
species concept and the origin of new species taxa. Only four months later, on 31
July 1941, he spoke at the Marine Biological Laboratory (Woods Hole, Cape Cod,
Massachusetts), on his main research topic, “The origin of gaps between species”
(Mayr 1941i): Evolution is a continuous process, he stated, but the units produced
by evolution are discontinuous, a problem that the geneticists had left open. Two
different classes of gaps between species need to be distinguished:
(a) Absolute “bridgeless” gaps between sympatric species at a particular locality.
The main phenomenon is reproductive isolation between species. These have
objective reality, e.g., the five similar species of thrushes of the genus Hylocichla
of which up to three inhabit the same woods in northeastern North America
withouttheslightestintergradation,and
(b) Relative (gradational) gaps between allopatric representatives. In widespread
species taxa geographically differentiated continuous populations interbreed
and merge into each other. Toward the periphery of the species range there are
often representative forms which are geographically separated by a barrier. In
such situations the taxonomist must use his judgment to infer the taxonomic
status of these latter forms. Obviously, such species taxa cannot be delimited in
an objective way. Geographical variation coupled with geographical separation
completely blurs the borderline.
Geographically separated populations are of the greatest importance for speciation.
Mutation, difference of selective factors and differences of random gene loss
will, through time, produce an increasing divergence. Eventually, isolated populations
will become distinct species and may come into contact with one another