Mayr stated repeatedly that geographic speciation prevails among animals but never ruled out that non-geographic speciation may occur.
"Isolating mechanisms are, for example, distinct and nonoverlapping breeding seasons, or in the cases of parasitic or monophagous species strict host specificity with the mating taking place on the host. Enough such cases have been described to make me believe that sympatric speciation is of common occurrence in certain animal groups with very specific ecological requirements, but almost completely absent in all other animal groups" (1941i: 142).
In his 1942 book he concluded "that bona fide evidence for sympatric speciation is very scanty indeed. [.] There is some indirect evidence for the importance of 'host races' for speciation:" Monophagous and oligophagous genera of butterflies and moths are much richer in species than the polyphagous ones (Nepticula, 140 Palearctic species; Lithocolletis, 100 sp., Coleophora, 140 sp.). "Certainty as to the relative importance of sympatric speciation in animal evolution cannotbe expected until a much greater body of facts is available than at present" (p. 209, 215). In 1947(e) Mayr pointed out that the existence of species-rich genera of host specialists by no means proves sympatric speciation. It may merely mean that they have more niches available than generalists. Genetic considerations made it also increasingly difficult to visualize the occurrence of sympatric speciation:
"The realization of the genetically highly complex nature of the isolating mechanisms and, more broadly speaking, of the intricate integration of the total gene pool of a population, makes it exceedingly difficult to conceive of a mechanism that would permit the building up of genetic isolating mechanisms within a physically undivided gene pool" (1959a: 226).
In his 1963(b) book he carefully evaluated the available evidence and concluded: "Host races [of parasitic insects] constitute the only case indicating the possible occurrence of incipient speciation" (p. 460) but assumed that complete stabilization on a new host cannot occur without geographical isolation. "The possibility is not yet entirely ruled out that forms with exceedingly specialized ecological requirements may diverge genetically without benefit of geographical isolation" (p. 480; see also 1976m: 144; 1982d: 605; 1988e: 376). Mayr did not rule out sym-patric speciation categorically but merely pointed out that those cases mentioned in the literature did not prove it. Sympatric speciation requires the simultaneous acquisition of mate preference and niche preference, something Mayr considered improbable in 1942. However, he accepted it when it was demonstrated in cichlid fishes (see Mayr 1984e, 1999k: XXX; 2001f: 100,180; see also Barluenga et al. 2006; Pennisi 2006; Meyer 2007). The females of these fishes have a definite preference for a particular part of the environment, the pelagic or the benthic part, and simultaneously for the particular males that occur in this particular environmental niche. This joint preference could quickly produce a new sympatric species. That both of these aspects could be selected at the same time is something that had not occurred to Mayr, but it has been proven. However, in many other groups spe-ciation is exclusively allopatric, e.g., mammals, birds, butterflies (except in some highly host specific forms). Mayr (2004a: 108) summarized his latest views stating: "After 1942 allopatric speciation was more or less victorious for some 25 years, but then so many well-analyzed cases of sympatric speciation were found, particularly among fishes and insects, that there is no longer any doubt about the frequency of sympatric speciation."
In recent years, several authors have claimed that if natural selection drives speciation, then gene flow will not affect diversification in areas (like, e.g., Amazonia) that are much larger than the per-generation dispersal range of individuals. In view of sufficient genetic variation and ecological diversity in extensive tropical forest regions, species may originate through "isolation by distance." Actual splitting of populations, as required under the model of allopatric speciation, supposedly is unnecessary and splitting of populations in space maybe the result rather than the cause of genetic differentiation during "adaptive speciation" (Schilthuizen 2001; Knapp and Mallet 2003; Tautz 2003; Dieckmann et al. 2004; critical discussion by Gavrilets 2005). Allopatric speciation is well supported and uncontroversial, whereas speciation in the face of gene flow is less well supported and more controversial. Comparative work suggests that it is far less frequent than allopatric speciation (Coyne and Orr 2004: 7).
Work on speciation over the past several decades mostly focused on the geography, ecology, and timing of speciation, in the tradition of Mayr's 1942 book. Molecular studies confirmed that Pleistocene events caused substantial intraspe-cific differentiation and the origin of several closely related pairs of North American bird species. Many other birds split from extant relatives at earlier times (Lovette 2005). Topics such as the role of sexual selection and the frequency of sympatric speciation are now also being addressed in several avian systems (Edwards et al. 2005). Rapid speciation may be frequent in birds in view of the importance of prezygotic isolating mechanisms including song, whereas the slow development of intrinsic postzygotic isolation will facilitate continuing hybridization. In recent years the search for speciation genes has been accelerated which eventually may permit understanding the genetics of avian speciation processes. Such genes seem to have very rapid rates of adaptive amino acid replacement (Orr 2005).
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