Species

220 Species

Species are groups of interbreeding natural populations that are reproductively iso-

lated from other such groups.^60

Gone is the phrase “actually or potentially” from the 1942 edition; Mayr now

thinks that it is only in sympatry (“with respect to sympatric and synchronous popu-

lations”) that we can tell for sure that two organisms are distinct species. The defini-

tion is “biological,” he says,

... not because it deals with biological taxa, but because the definition is biological. It

utilizes criteria that are meaningless as far as the inanimate world is concerned.

When difficulties are encountered, it is important to focus on the basic biological

meaning of the species: A species is a protected gene pool. It is a Mendelian popula-

tion that has its own devices (called isolating mechanisms) to protect it from harm-

ful gene flow from other gene pools. Genes of the same gene pool form harmonious

combinations because they become coadapted by natural selection. Mixing the genes

of two different species leads to a high frequency of disharmonious gene combinations;

mechanisms that prevent this are therefore favored by selection.^61

The text emphasized above is surprising. From being the “effect” of a process in
1942, Mayr now treats species as a mechanism of protecting gene pools. Moreover,
selection now plays a role in “protecting” species; previously species were not formed
through selection, and reproductive isolation was a side effect of geographical isola-
tion. Still, isolating mechanisms are still “potentially or actually” active in sympatry
and have to be intrinsic mechanisms of the organisms, and not, for example, “geo-
graphic or any other purely extrinsic isolation.”^62
Mayr’s view of species seemed not to have changed much since the 1970 volume.^63
He repeats it in several places.^64 To summarize his final position, let us consider his
second last paper^65 on the species concept, where he claims that species are concrete
describable objects in nature, that they are reproductively isolated even when there is
“leakage of genes,” and that the biological species concept (now abbreviated as BSC)
is based on the properties of populations. Although Mayr always stressed the popu-
lational nature of species as a result of his insistence on genetic and morphological
polytypy in species, over time there is an increasing emphasis on “populational think-
ing” in opposition to “essentialism” in his works. For instance, in the introduction to
his history of biology,^66 he discussed this, citing Hull and Ghiselin,^67 dividing west-
ern thinking into two phases. The first phase was essentialism deriving from Plato,
and the second, population thinking beginning with Leibniz’s theory of monads but
really taking root with the British animal breeders and Darwin and his contemporary

(^60) Quoted in Mayr 1970, 12.
(^61) Mayr 1970, 13, emphasis added.
(^62) Mayr 1970, 56.
(^63) Apart from flying the “ecological niche” variant mentioned above.
(^64) For example, Mayr 1976, 1985, 1988, 1992, 1996.
(^65) Mayr 1996. He also provided a chapter in Wheeler and Meier 2000 [Mayr 2000].
(^66) Mayr 1982, 45–47.
(^67) Ghiselin 1974, Hull 1976.