Ornithology, Evolution, and Philosophy 123

A Modern Unified Theory of Evolution 221
(B) Without a speciation event (parental species transformed)
(c) Dichopatric speciation
(split by a geographic barrier with gradual divergence)
(d) Gradual phyletic transformation of a single lineage.
The crucial process in allopatric speciation is geographic isolation (not selection).
This statement only refers to the geographic position and separation of the
speciating populations. It still remains completely unknown as to what happens
genetically during speciation. Possibly rather different genetic mechanisms are
involved in the speciation of different kinds of organisms and under different circumstances.Ofcourse,thespeciatingpopulationsremainwelladaptedthrough
naturalselectionwhichisalwayspresent.
Some authors have suggested that Sewall Wright’s writings have influenced
Mayr’s thinking and that in particular, his model of the “adaptive landscape” had
given Mayr the idea of geographic speciation. 13 Nothing could be further from
the truth. Geographic speciation was discussed among naturalists since the 19th
century (p. 230) and was standard thinking in Erwin Stresemann’s department
in Berlin; Mayr described geographical speciation already in 1927(f) in his paper
on the Rosy-Finches (Leucosticte). Moreover, Wright’s model of the “adaptive
landscape” was an abstract model, not one of a three-dimensional landscape; it
hasverylittletodowithspeciation,butrefersmainlyto“theevolutionofthe
species as a whole” (quote in Mayr 1992i: 11). As Mayr (1999k: XXIV) pointed out,
S. Wright was never particularly interested in problems of speciation and, although
he published reviews of all the major works on evolution, the only one he omitted
was Mayr’s Systematics and the Origin of Species (1942e).
Species originate through “splitting” and “budding” (Fig. 5.6). “Budding” occurs
when, e.g., a derivative population of a widespread mainland species reached
species status on a nearby island. This speciation event had no effect on the
parental biospecies (no. 3, Fig. 5.6) on the mainland from which neospecies 4 has
budded off. The mainland species is real in the sense that it represents a biological
unit characterized by close genetic-reproductive and ecological relations among
its component subspecies taxa. The cladistic analyses schematically illustrated in
Fig. 5.6 (if feasible at that intraspecific level) yield relevant phylogenetic (“vertical”)
and biogeographical data on the origin and relationships of the various groups
of taxa. Mayr (e.g., 2000f: 164) feels that most new species originate by budding,
splitting is less common.
One may ask the question why Mayr, in 1942, preferred the dichopatric model
of speciation (the “dumbbell” model), although the island populations of the
Polynesian and Melanesian birds he studied in detail certainly had originated
through jump dispersal (and peripatric speciation). When he studied a particular
aberrant peripheral island population, the question foremost in his mind at that
13 Thus M. Ruse (1999: 118) wrote: “Mayr, who in later years put distance between himself
and Wright, based his original picture of the evolutionary process on the hypothesis”
(the shifting balance hypothesis of S. Wright).