Species

The Synthesis and Species 217

have worn well. But for our purposes, the most important aspect of the book is that
here Mayr popularized the definition of species he had already given in 1940:

A species consists of a group of population which replace each other geographically or

ecologically and of which the neighboring ones intergrade or interbreed wherever they

are in contact or which are potentially capable of doing so (with one or more of the popu-

lations) in those cases where contact is prevented by geographical or ecological barriers.

Or shorter: Species are groups of actually or potentially interbreeding natural pop-

ulations, which are reproductively isolated from other such groups.^42

Mayr contrasted this with the “typological” conceptions of taxonomy at the time,
especially the “morphological species concept” as Mayr called it (for the first time
anyone had, so far as I can tell), as well as the “practical species concept” (a ver-
sion of Regan’s definition), the “genetic species concept” (homozygous populations,
which he attributes to Lotsy), and one based on sterility. He called it the “biologi-
cal species concept”; and so the proliferation of general “species definition” names
began. Mayr included Dobzhansky’s definition under this rubric, but says of it,

[t]his is an excellent description of the process of speciation, but not a species defini-

tion. A species is not a stage of a process, but the result of a process.^43

Hence, he proposed the definition above as a practical compromise, allowing the
systematist the judgment call, since (as is noted later by critics of the concept) one
cannot use reproductive isolation as a test in many cases, not least in allochronic and
allopatric populations. Further,

[t]he application of a biological species definition is possible only in well-studied taxo-

nomic groups, since it is based on a rather exact knowledge of geographical distribu-

tion and on the certainty of the absence of interbreeding with other similar species.^44

Moreover, while it works for “bisexual organisms” (sexual species), it fails, he
notes, for “aberrant cases” like protozoans and plants, which are either unisexual
(asexual) or freely hybridizing.^45 The remainder of the book discusses speciation
processes, and Mayr presents a Wagnerian view that geographical isolation is a pre-
condition for the formation of new species^46 Sympatric species are reproductively
isolated absolutely (“otherwise they would not be good species”^47 ), but the gaps that
separate allopatric species are “often gradual and relative, as they should be, on the
basis of the principle of geographic speciation.”^48 He considers sympatric speciation
as a possibility, but concludes

(^42) Mayr 1940, 120.
(^43) Mayr 1942, 119.
(^44) Op. cit., 121.
(^45) Op. cit., 122.
(^46) Op. cit., 154 –185.
(^47) Op. cit., 149, italics original.
(^48) Loc. cit.