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212 5 Biological Species and Speciation—Mayr’s First Synthesis
for the polytypic species a new “multidimensional species concept” (see also Mayr
1957f: 16, 1963b: 19). When he had clarified for himself the distinction between the
theoretical species concept and the species taxon during the early 1960s, “it became
evident that the polytypic species is merely a special kind of species taxon but does
not require any change in the concept of the biological species category” (Mayr
1982d: 290). Therefore in his writings of the late 1960s and following decades he no
longer mentioned a “multidimensional species concept” but stated: “The species
concept is meaningful only in the nondimensional situation: multidimensional
considerations are important in the delimitation of species taxa but not in the
development of the conceptual yardstick” (1982d: 272; see also Hey (2006: 448) for
a summary of Mayr’s changing categorization of species concepts).
Beurton (2002) presented a detailed historical review of Mayr’s struggle with
the biological species concept during more than 50 years. It would have been easier
for the reader to follow Beurton’s arguments if he had clarified from the start the
early terminological confusion regarding (a) the biological species concept per se
and (b) the polytypic species taxon and if he had applied Mayr’s later clarification
to his early writings (rather than presenting the early confusion again in detail
without applying Mayr’s later clarification). Mayr (1940c) discussed species taxa
and that is what his definition referred to (see above). In his Jesup lectures he talked
about the general “species problem” referring both to the absolute gaps between
sympatric species and to the relative gaps between allopatric taxa, as is obvious
from Mayr (1941i), an article Beurton (l.c.) did not consult. Mayr (2002a: 100)
himself commented: “It is a basic weakness of Beurton’s account that the confusion
between species concept and species taxon is not clarified. […] Those authors who
fail to make the distinction, provide a confused and misleading analysis.” Mayr
(1942e) then presented the distinction between the biological species concept
(absolute gaps) and the polytypic species taxon (relative gaps), as in 1941.
The theoretical concept and definition of biological species (Mayr 1942e and
later) found entrance into all textbooks of biology. Bock (1986, 1992a, 1994, 1995b,
2004b) emphasized that genetic isolation was meant when Dobzhansky (1937) and
Mayr (1942e) spoke of the “common gene pool” and the “harmonious genotype
of a species.” Mayr (1968i: 164) stated: “Possession of a shared genetic program
is the common tie uniting individuals derived from the gene pool of a given
species.” Therefore Bock (l.c.) emended the definition of biological species to
read: “A species is a group of actually or potentially interbreeding populations
of organisms which are genetically isolated in nature from other such groups.”
Bock’s emendation appears useful also in view of the discovery in recent years that
several geographically representative taxa, especially of insects, hybridize freely
along the contact zone for lack of premating isolating mechanisms, but such hybrids
are infertile due to fully developed postmating isolating mechanisms. In
addition, Bock (l.c.) made a distinction between the sets of isolating mechanisms
for genetic isolation and reproductive isolation which Mayr failed to do. Some
bird species which meet along “zones of overlap and hybridization” (Short 1969)
may also represent taxa which are genetically isolated but not fully isolated reproductively.
These biospecies would be considered conspecific taxa under Paterson’s