24 Jan 2001
John Knox mentioned a recurring problem in science writing about species, and illustrated it with an example from Science:
...after examining 37 pairs of closely related species, one from each side of the arid Tehuantepec barrier, a team led by ornithologist A. Townsend Peterson of the Natural History Museum of the University of Kansas, Lawrence, found that members of each pair had similar ecological niches. In every case it appears that the geographic barrier, rather than any difference in ecology, was the critical factor in speciation, isolating the original populations so that they accumulated genetic differences and eventually became unable to interbreed, says Peterson.

(SPECIATION: Mexican Pairs Show Geography's Role ( Bernice Wuethrich, Volume 285, Number 5431, Issue of 20 Aug 1999, p. 1190)

Whether the science writer is quoting Peterson accurately is (at the moment, and without looking at what Peterson says in his publications) unknown, but John suspects that she is not. If we look at what Ernst Mayr actually says in Animal Species and Evolution (1963, pp. 14 ff), it's clear that interbreeding is not an effective criterion for separation of species:
As early as 1760 Koelreuther had stated that all those individuals belong to a species that are able to produce fertile offspring. With increasing frequency since then, interbreeding has been considered a decisive criterion in species definitions. Unfortunately, this criterion has often been narrowed down to a single aspect of successful interbreeding, that of fertility. Cross-fertility was accepted as the decisive species criterion in much of the genetic and botanical literature until naturalists pointed out that fully cross-fertile species of animals may live side-by-side without interbreeding because their reproductive isolation is maintained by isolating mechanisms other than the sterility barrier. Authors like myself, who have consistently advocated that the noninterbreeding of natural populations rather than the sterility of individuals be taken as the decisive species criterion, have nevertheless been accused by hasty readers of having a species concept "based on cross-sterility." (15)

...It is evident that the word "species" has meant and still means different things to different people... (15)

...I defined species (Mayr 1940) as "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups," and Dobzhansky (1950) defined the species as "the largest and most inclusive... reproductive community of sexual and cross-fertilizing individuals which share a common gene pool" (19)