3 The New York Years (1931–1953)

A Modern Unified Theory of Evolution 207
the chances for a mutation to prevail.” He reemphasized (1931: 644) that speciation
occurs only through spatial separation of populations (either through jump dispersal
or disruption of the continuous range of the parental species) and pointed
out the particularly favorable conditions for speciation on island archipelagoes
like the Hawaii and Galapagos Islands. Speciation occurs in isolated populations
through small mutations and natural selection. During the 1920s he had studied
polymorphism in birds and had emphasized repeatedly that conspicuous morphs
(“mutations”) have nothing to do with the origin of new species. Regarding ecological
segregation in speciating populations, he concluded (1939: 360): “The correspondence
of two forms may have been reduced to a correspondence of only their
ecological requirements with, at the same time, divergent differentiation of their
sexual activities. Two forms at that stage of the differentiation compete with each
other for space and where they meet by range expansion, they abut sharply against
each other without forming hybrids. […] Examples of such situations are probably
much more common than currently known” (Haffer et al. 2000). He concluded
that hybridization, after the removal of a geographical barrier between incipient
species invariably leads to a secondary intergradation of these populations, and
not to speciation.
However, Stresemann never prepared a general treatment of species and speciation,
mainly because his main interests during the late 1920s had shifted to
functional anatomy and physiology 8 and because he may have considered B. Rensch’s
book of 1929 as having covered the subject sufficiently well.
Bernhard Rensch (1928) introduced the concept of Artenkreis (superspecies
Mayr 1931). In his book, The Principle of Polytypic Species and the Problem of
Speciation (1929) Rensch showed that in many groups of animals numerous geographical
taxa may be combined as subspecies of polytypic species. He also
discussed numerous borderline cases between subspecies and species which document
that in the majority of cases, species originate from geographically isolated
populations. Conspicuous phenomena of geographical variation are described in
Bergmann’s, Allen’s, and Gloger’s Rules. At that time, he interpreted these rules by
a direct influence of the environment. His book was the first manifesto of “the new
systematics” and Rensch the first “new systematist.” In further publications he
explained the geographic principle, the geographical replacement of conspecific
taxa, individual and geographical variation, superspecies, and speciation in what
he called Instructions for zoological-systematic Studies (1934). He also demonstrated
the adaptive nature of some subspecific differences in birds. No equivalent
title for Rensch’s booklet of 1934 existed in the English literature until Mayr (1942e)
published his volume which, in the first chapters, includes an introduction to taxonomic
procedures. On June 6, 1941 Mayr wrote to Stresemann: “I am presently
busy preparing my book manuscript on Systematics and the Origin of Species.One
cannot deal with this topic without noticing all the time, how much the solution
or at least the clear exposition of these problems owes to our friend Rensch.”
8 “My real interests have shifted [from taxonomy and zoogeography] to very different fields,
in particular functional anatomy and physiology” (letter to Mayr dated 14 October 1929
and see p. 141, footnote).