Ornithology, Evolution, and Philosophy 123

356 11 History and Philosophy of Biology—Mayr’s Third Synthesis
of evolution, the origin of diversity through the multiplication of species, was
solved during the “Evolutionary Synthesis” between 1937 and 1950 (p. 183). Mayr
summarized the events leading up to the evolutionary synthesis as follows (1982d:
535–570; see Table 8.1).
During the late 19th century, a widening gap had developed between the experimental
biologists including the geneticists and the naturalists (most zoologists,
botanists, and paleontologists) who worked with whole organisms. Both groups
were interested in evolution but had difficulties communicating with each other.
The naturalists dealing with populations studied mainly diversity, its origin and
meaning; the systematists were intrigued by the species problem and the paleontologists
by evolutionary trends and the origin of the higher taxa. By contrast,
the geneticists dealing with genes focused on evolutionary changes within populations,
transformational evolution, disregarding diversity, the origin of taxa.
After 1900 and the rediscovery of Mendel’s rules Bateson and de Vries proposed
typological mutationism (saltationism) as an explanation of the origin of species,
whereas the naturalists were impressed by and emphasized gradual variation and
speciation (either sympatric or allopatric). The naturalists continued to believe
in soft inheritance, but acknowledged natural selection as a major evolutionary
force.
Advances made in both groups during the first decades of the 20th century prepared
an eventual reconciliation of the two opposing camps. According to Mayr
population geneticists (Haldane, R.A. Fisher, Wright, Chetverikov, Timoféeff-
Ressovsky) showed that (1) there is no soft inheritance, (2) recombination and
small mutations are the most important source of genetic variation in populations,
(3) continuous phenotypic variation is not in conflict with particulate inheritance,
and (4) natural selection is an effective evolutionary cause (“Fisherian synthesis”).
Population systematics of the naturalists had been in existence since the early
19th century and new studies of the early 20th century on fishes, moths, and mice
could easily be translated into population genetics. The naturalists studied “series”
as population samples, geographical gradients of populations, analyzed adaptive
variation statistically and studied geographical speciation (Mayr 1963b).
Geneticists and naturalists reconciled their differences during the short period
of the Evolutionary Synthesis (1937–1950), when a small group of evolutionists
in North America and Europe were able to build bridges among different fields
and to remove misunderstandings. These “architects” bridged the gap between
the gene-frequency approach of the reductionist population geneticists and the
population thinking of the naturalists, especially the study of species and their
variation. The main architects were Dobzhansky, Mayr, Simpson, Huxley, Stebbins
in North America and several workers in Germany and Russia (Mayr 1999a).
At an international conference in Princeton, New Jersey, in January 1947 there
was general agreement among the participating geneticists and naturalists on the
gradualness (continuity) of evolution (but with varying rates), the importance of
natural selection, and the populational aspect of the origin of diversity. A true
synthesis had occurred between the two very different research traditions (Jepsen,
Simpson, and Mayr 1949). Some differences that remained at that time included