3 The New York Years (1931–1953)

5
Biological Species and Speciation—
Mayr’s First Synthesis
A Modern Unified Theory of Evolution
Ernst Mayr’s early interests in evolution and genetics (pp. 26–29, 45) led to his
decisive contributions to the modern synthesis of the late 1930s and 1940s, when
a largely unified evolutionary theory emerged. Biological evolution comprises two
components–adaptive development of populations during long geological periods
(anagenesis, evolution as such) and multiplication of species during relatively short
periods (speciation, cladogenesis). Evolution as such and the theory of common
descent were accepted by biologists within a few years of the publication of the Origin
in 1859 during the first Darwinian revolution.Asynthesisofmoderngenetics
and evolution as such was accomplished when, by 1932, the mathematical geneticists
R. A. Fisher, J. B. S. Haldane and S. Wright had convincingly shown that small
mutationsandnaturalselectionplaythemainrolesinthegradualprocessofadaptive
evolution of populations through time, thereby solving one of the two major
problems of evolutionary biology, the problem of anagenesis, a historical accomplishment
which Mayr (1999k, 2004a) called the “Fisherian synthesis.” During the
so-called Evolutionary Synthesis of the period 1937–1950 (the second Darwinian
revolution) the other main problems of evolutionary biology were solved or generally
accepted (gradualism, speciation, and natural selection) and the processes of
speciation were combined with those of adaptive evolution (Mayr 1993a). None of
the mathematical geneticists had discussed the phenomenon of speciation or did so
only superficially. In general, the period of the modern synthesis, when a new unified
theory of evolution originated, saw a synthesis (1) between the thinking in three
major biological disciplines–genetics, systematics and paleontology, (2) between
an experimental-reductionist approach (genetics) and an observational-holistic
approach (naturalists-systematists) and (3) between an anglophone tradition with
an emphasis on mathematics and adaptation and a continental European tradition
with an emphasis on populations, species, and higher taxa. The three genetical
aspects (then not new insights) that were firmly and universally adopted during
the evolutionary synthesis were (1) that inheritance is hard, there is no inheritance
of acquired characters, (2) that inheritance is particulate, that is, the genetic
contributions of the parents do not blend but remain separate, to be differently
recombined in future generations, and (3) that most mutations are very small and
evolution therefore is gradual. The evolutionary synthesis also led to a refutation
of the three anti-Darwinian paradigms (a) the typological-saltational, (b) the
teleological-orthogenetic and (c) the transformationist-lamarckian theories.