Ornithology, Evolution, and Philosophy 123

202 5 Biological Species and Speciation—Mayr’s First Synthesis
for the Chermock circle Mayr’s Systematics and the Origin of Species, following
upon Dobzhansky’s book, was the hunter’s vade mecum. From Mayr we learned
how to define species as biological units. With the help of his written word we
pondered the exceptions to be expected and the processes by which races evolved
into species. We acquired a clearer, more logical way to think about classification
by using the phylogenetic method” (Wilson 1994: 110–112; see also Wilson
1998: 4).
Working on a revision of his successful 1942 book, Mayr soon decided to separate
methodological aspects of systematic zoology from the science of species and
evolutionary biology. In this way originated his textbook, Methods and Principles of
Systematic Zoology (1953a, with Linsley and Usinger) which covered methodological
aspects (p. 321) and his masterly monograph on Animal Species and Evolution
(1963b) treating the species problem and aspects of evolutionary biology.
In his article, “Trends in avian systematics” Mayr (1959e) emphasized two
main topics: (1) Population systematics and (2) macrotaxonomy. Subspecies are
unsuitable for describing the population structure of a species. This can be done in
terms of three major population phenomena: (a) Geographical isolates which are
particularly common near the periphery of the species range and which represent
important units of evolution; (b) The population continuum, a continuous series
of populations which make up the main body of the species, and (3) Zones of
secondary intergradation between a former geographical isolate (which did not
yet attain full species rank) and the main body of the species. To make such an
analysis of all species of a family, and to record the relative frequency of the
three elements, is one of the steps leading to “comparative systematics.” Recording
the position of belts of secondary hybridization permits the reconstruction of
formerly existing barriers and of the location of “refuges” caused by drought
conditions in low latitudes during the Pleistocene. With respect to macrotaxonomy,
Mayr proposed to utilize various new sources of information like promising new
character complexes of behavior and biochemistry to determine the phylogenetic
relationships between genera, families and orders.
Behavioral characters are completely equivalent to morphological characters
(Mayr 1958g). If there is a conflict between the evidence provided by morphological
characters and that of behavior, taxonomists are increasingly inclined to
give greater weight to the ethological evidence, e.g., in the case of grasshoppers,
in certain swallows (p. 327), ducks (p. 327), and finches (p. 329). Based on behavioral
characteristics the weaver finches (Estrildidae) and weaverbirds (Ploceidae)
are not as closely related as previously thought. Movements often precede special
morphological features that make them particularly conspicuous. Comparative
studies of behavior permit statements about trends in the evolution of behavior,
as shown, e.g., by the work of Whitman on pigeons, Tinbergen on gulls, Heinroth
and Lorenz on ducks, Meyerriecks on herons, Hinde on finches, Morris on
weaver finches. Other groups studied in this way include grasshoppers, Drosophila,
hymenopterans, and cichlid fishes.
In 1964(n), Mayr again summarized the then “new systematics” mentioning that
certain aspects can be traced back to some authors of the mid-19th century who