Species

218 Species

Darwin thought of individuals when he talked of competition, struggle for existence

among variants, and survival of the fittest in a particular environment.^49 Such a strug-

gle among individuals leads to a gradual change of populations, but not to the origin

of new groups. It is now being realized that species originate in general through the

evolution of entire populations. If one believes in speciation through individuals, one is

by necessity an adherent of sympatric speciation, the two concepts being very closely

connected. However, fewer and fewer situations are interpreted as evidence for sym-

patric speciation, as it is realized more and more clearly that reproductive isolation

is required to make the gap between two incipient species permanent and that such

reproductive isolation can develop only under exceptional circumstances between indi-

viduals of a single interbreeding population.^50

This has a whiff of circularity. Mayr defines species as reproductively isolated
populations formed in allopatry and absolutely distinct in sympatry. He then claims
that sympatric variations cannot form reproductively isolated species because spe-
cies are formed in allopatry since sympatric “species” have to be absolutely isolated.
Of course, there is a lot more to it than this, and Mayr brings in all sorts of impressive
empirical evidence, so the charge of plain circularity, once made, must be dismissed.
However, this means that the strength of the biological species concept rests entirely
on the absence of plausible empirical reasons to believe that reproductive isolation
does not occur in sympatric populations. All it would take to undercut this definition
of species, of course, or at any rate Mayr’s argument in its favor, is to find an unam-
biguous case of sympatric speciation. There is a case—cichlid fishes in various lakes
in Africa, for instance—where Mayr wonders if these “species flocks” are evidence
for “explosive” sympatric speciation.^51 He rejects the idea on the grounds that the
closest relatives of each species are not sympatric.^52
Finally, Mayr considers the factors that cause species, since species are the “effect”
of a process. He divides them into internal factors, such as genetic mechanisms, muta-
tion rates, and the like, and external factors. Then he lists a number of subcategories,
isolating mechanisms: geographical barriers that restrict random dispersal, ecologi-
cal barriers, ethological factors, mechanical factors, and “genetic” and physiological
factors.^53
A case for which Mayr does not insist upon allopatry is what he calls “instanta-
neous sympatric speciation,”^54 such as via polyploidy (the duplication of chromo-
somes and possible subsequent reduction to a diploid form of differing composition

(^49) In fact, Darwin occasionally explicitly talked of selection and the struggle for existence as occurring
between species, as we have seen.
(^50) Op. cit., 190.
(^51) Op. cit., 215.
(^52) Recent work has (unfortunately for Mayr’s views) shown otherwise [Mazeroll and Weiss 1995,
Albertson et al. 1999, Salzburger et al. 2002, Stauffer et al. 2002, Martin et al. 2015]. Schilthuizen
2000 gives an excellent summary.
(^53) Mayr 1942, 237f.
(^54) Op. cit., 190ff.