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220 5 Biological Species and Speciation—Mayr’s First Synthesis
factor becomes because it no longer involves a very strong selection for just one
kind of thing. By pure chance a lot of genes and gene combinations simply drop
out and so in small populations, improbable combinations are quite frequent and
they may be the starting point of something very interesting which in a large
population would never happen. Among the classical population geneticists, the
only one who saw that was Sewall Wright. Fisher and Haldane never saw it and they
would say that small and large populations are one and the same thing. In fact,
Fisher said something that was completely and utterly wrong, he said ‘the larger
a population, the faster it will evolve.’ We now know that the truth is exactly the
opposite. The larger a population, the more inert it becomes because it is difficult
for any change to penetrate through a large population. That is why Wright had his
shifting balance theory by which a little piece of a population went outside, in the
spirit of my 1954 paper. Then as a unit, it goes back inside again and spreads into
the large population. The critics of Wright, in particular J. Coyne [et al. 1997], claim
that this whole thing of Wright’s is quite impossible, it just can’t happen for many
reasons. But you have to give Wright credit, that at least he saw there was a genuine
problem there, which Fisher and Haldane didn’t see” (see Wilkins 2002: 966). 11
Current evidence provides little support for founder effect speciation and it
appears that selection is more important for speciation than genetic drift, which
also plays little part in morphological evolution (Coyne and Orr 2004). One recent
case study of founder effects in silvereyes (Zosterops lateralis) on Pacific islands
appears to be inconclusive (Grant 2002b).
In his review of different patterns and theories of speciation, Mayr (1982l) refuted
the theory of stasipatric speciation and discussed the weakness of the theories
of parapatric and sympatric speciation. Chromosomal changes and geographical
isolation occur simultaneously. Similarly, Mayr and O’Hara (1986a) refuted an
earlier claim that parapatric speciation had occurred in West African birds.
Mayr summarized the various speciation models in the following table (1987a:
311–312): New species originate:
(A) Through a speciation event
(a) Instantaneous (e.g., polyploidy, stabilized hybrid) 12
(b) Very rapid (peripatric speciation, conceivably sympatric speciation)
11 Provine (2005) criticized (1) Mayr’s concept of genetic revolution as being “devoid of
genetic content” and (2) his usage of terms like gene pool and homeostasis of gene
pools as being “biological nonsense.” However, Futuyma (2006) rejected these criticisms
pointing out that (ad 1) “given what Mayr had learned from his population geneticist
colleagues, his hypothesis […] had as much genetic content, I believe, as Wright’s shifting
balance theory” and (ad 2) that “Mayr was taught genetic homeostasis in one of the major
schools of evolutionary genetics of his day.” Moreover, unlike Darwin, Mayr has shown
“that species of sexually reproducing organisms are real and that they exist by virtue of
reproductive isolation rather than of phenotypic distinctiveness” and often originate in
allopatry.
12 New evidence indicates that hybrid speciation occurs not only in plants but also in
animals and is more frequent than previously thought (O. Seehausen, Trends in Ecology
and Evolution 19: 198–207, 2004 and J. Mallet, Nature 446: 279–283, 2007).