Ornithology, Evolution, and Philosophy 123

226 5 Biological Species and Speciation—Mayr’s First Synthesis
The Nature of Species in Bisexual Animals
Sexuality leads to genetic recombination and integration of natural populations
into species. Most species taxa are polytypic, that is they are composed of lesser
units, subspecies and local populations, which deviate more or less from one
another. Species are, through their geographical extension, multidimensional systems.
The subspecies is not a biological unit like the local population and the
species, but on continents often an artificially delimited taxonomic unit. Island
populations are more clearly defined but their subspecies status must be inferred.
Taxonomists study clinal variation, especially in species with extensive ranges on
continents, which is correlated with gradients in selective factors of the environment.
Peripheral or otherwise separated forms of a group of relatives are often
the most distinct, e.g., in birds Junco vulcani (Miller 1941), Ptilinopus huttoni
(Ripley and Birckhead 1942), Dicrurus megarhynchus and four other taxa of drongos
(Mayr and Vaurie 1948d), Dicaeum tristrami (Mayr and Amadon 1947d) and
many others. Many of these are borderline cases between subspecies and species
and indicate the course of speciation.
Most morphological characters and numerous physiological species differences
are neutral with respect to the maintenance of reproductive isolation, as they are
merely accidental by-products of the genetic divergence of the populations during
geographic speciation (Mayr 1948c, 1949f). Other characters promote the coexistence
of species and have selective value, like different ecological requirements
which also reduce or prevent the meeting of potential mates functioning as sexual
isolating mechanisms (Dobzhansky 1937). Behavioral barriers like courtship
differences in ducks and Drosophila function similarly. Mayr (1948c) emphasized
that isolating mechanisms are not simple lock and key mechanisms, because often
several different factors and mechanisms are involved. Natural selection will improve
imperfect premating isolating mechanisms between populations when they
come in secondary contact.
Difficulties with the biological species concept may arise, when morphological
variants occur within a single population which may be due to simple Mendelian inheritance
(e.g., in the Californian king snake, Mayr 1944g) or to balanced polymorphism
(e.g., the wheatear genus Oenanthe, Mayr and Stresemann 1950f). Another
problem is the interpretation of the taxonomic status of allopatric populations.
Since isolating mechanisms are usually correlated with a certain amount of morphological
differences rather typical for a given genus, the taxonomist uses this
evidence to work out a yardstick which can be applied to allopatric populations
(pp. 214–215). Mayr’s opinion was that it is preferable to treat doubtful cases as
subspecies, because allopatry indicates inability of coexistence (given the fact of
dispersal) and closest relationship.
Natural and Sexual Selection
As a young systematist Mayr, like many of his colleagues including Stresemann
and Rensch, thought that the differences between animal species, e.g., Drosophila