Ornithology, Evolution, and Philosophy 123

188 5 Biological Species and Speciation—Mayr’s First Synthesis
When Dobzhansky visited New York a year later, in October–November 1936,
to lecture on Genetics and the Origin of Species Mayr naturally attended these
talks which for him were like an “intellectual honeymoon,” learning an enormous
amount of evolutionary genetics and being ever more enthusiastic about speciation.
Dobzhansky’s book with the same title as his lectures appeared in late 1937 2
and Mayr delved into it immediately. He appreciated Dobzhansky’s emphasis on
the importance of morphological taxonomic work and the still imperfect understanding
of many aspects of evolution. Dobzhansky pointed out that the majority
of mutations are very small and frequently beneficial (rather than large and lethal
as most German taxonomists still believed). Geographical variation and its adaptive
nature are discussed at length, including Mayr’s information on the Golden
Whistler (Pachycephala pectoralis). Dobzhansky did mention the difference between
processes of speciation (called “cladogenesis” later) and changes in a single
gene pool over time (“anagenesis”), but–despite the title of the book–there is no
separate chapter on the origin of new species. In the treatment of isolating mechanisms
(a most useful term coined by Dobzhansky [1935, 1937]) he defined species
as reproductive communities. Among the isolating mechanisms he recognized
extrinsic ones (like geographical barriers) and intrinsic properties of organisms.
Mayr (1942e) later provided an improved classification and strong arguments in
favor of geographical speciation which Dobzhansky had treated rather ambiguously.
Mayr disagreed strongly with Dobzhansky’s definitions of subspecies (races)
and species as “stages in the evolutionary process.” Rather, Mayr (1942e) argued,
they are populations or sets of populations.
They exchanged many letters until, in late 1939, Dobzhansky joined the Department
of Zoology at Columbia University, New York. Ever since they had established
contact in 1935, they were striving fervently for a synthesis of genetic and taxonomic
dataand research without realizinghowclosethey weretothat goal.Through
their discussions, presentations and publications they actively forged the evolutionary
synthesis 3 . In December 1939, Mayr spoke on “Speciation phenomena in
birds” stating in the introduction:
“Evolution is a very complicated and many-sided process. Every single branch of
biology contributes its share of new ideas and new evidence, but no single discipline
can hope to find all the answers or is justified to make sweeping generalizations
that are based only on the evidence of its particular restricted field. This is true for
cytology and genetics, for ecology and biogeography, for paleontology and taxonomy.
All these branches must cooperate. […] It is obvious that the taxonomist will
2 On the spine of Dobzhansky’s book (1937) appears an artistic diagram of a cell dividing
which, however, has the chromosomes in the division figure backward. Ernst Mayr
delighted in pointing out this error to Dobzhansky who, so far, had not noticed it: “The
bird taxonomist telling the working cytologist that the cell division pictured on the spine
of his book is ridicuously in error” (Provine 1994: 111).
3 RegardingMayr’sandDobzhansky’srolesinthedevelopmentofevolutionarystudies
in the United States from 1936–1947, in particular the foundation of the Society for the
Study of Evolution and its journal Evolution, see Jepsen (1949), Cain (1993, 1994) and
Smocovitis (1994a, 1994b).