Ornithology, Evolution, and Philosophy 123

A Modern Unified Theory of Evolution 223
Sympatric Speciation
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, 140sp.).“Certaintyastothe
relative importance of sympatric speciation in animal evolution cannot be 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 sympatric
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 speciation
is exclusively allopatric, e.g., mammals, birds, butterflies (except in some