Adaptive Dynamics of Speciation: Sexual Populations

When John Maynard Smith (1966) wrote on sympatric speciation more than 35 years ago, he acknowledged that the argument “whether speciation can occur in a sexually reproducing species without effective geographical isolation” was an old problem and voiced his opinion that the “present distribution of species is equally consistent either with the sympatric or the allopatric theory.” Yet, from the heyday of the Modern Synthesis until relatively recently, the importance of sympatric speciation has been downplayed, and the corresponding hypotheses remained obscure well beyond Maynard Smith’s seminal study. Looking back from today’s perspective, it is astounding that, for such a long period, the research community at large essentially turned a blind eye to sympatric speciation. Given the widely acknowledged difficulties involved in inferring past process from present pattern, one can only feel uneasy about a logic that claims to find evidence for the prevalence of allopatric speciation in the present-day distribution of species. To a large extent it seems to have been the scientific community’s perception of the theory of sympatric speciation that has brought about a profound skepticism toward the broader empirical relevance of this speciation mode. Scientific attempts to overcome this skepticism have come and gone in waves. In the 1960s, luminaries of North American evolutionary biology pulled no punches when assessing the merit of such attempts. Displaying a characteristic hint of restrained intimidation, Ernst Mayr (1963) wrote on sympatric speciation, “One would think that it should no longer be necessary to devote much time to this topic, but past experience permits one to predict that the issue will be raised again at regular intervals. Sympatric speciation is like the Lernaean Hydra which grew two new heads whenever one of its old heads was cut off.” And also Theodosius Dobzhansky’s verdict was categorical when he remarked, in 1966, that “sympatric speciation is like the measles; everyone gets it and we all get over it” (Bush 1998). Sometimes models of sympatric speciation were interpreted to imply that such speciation was only possible under very special and narrow conditions, while at other times the same models were called into question as they allegedly predicted sympatric speciation to happen too easily. In the words of Felsenstein (1981), “one might come away from some of these papers with the disturbing impression that [sympatric speciation] is all but inevitable.” Such concerns were echoed again recently, by Bridle and Jiggins (2000), for example.