Ornithology, Evolution, and Philosophy 123

A Modern Unified Theory of Evolution 225
(not published until 1941k) and, second, at a symposium on speciation organized
by Dobzhansky (Mayr 1940c).
Mayr (1954e) asked the question whether speciation in marine organisms differed
from speciation in land animals (birds, mammals, and butterflies). At the
time he was working with West Indian Cerion snails (see p. 331) and therefore
selected the West Indian shallow water echinoids for study. Each genus consists of
one or several groups of allopatric species, some of which should be considered as
subspecies. Allopatric populations show every grade of distinctness, ranging from
slight difference through subspecific to specific rank. This pattern corresponds to
that of land animals. This is also suggested for deep sea organisms (crinoids) and
pelagic organisms (Scyphomedusae). Dispersal barriers which caused speciation
in warm water echinoids are the eastern Pacific, the Isthmus of Panama and the
cold waters of southwestern Africa. There was no evidence suggesting any other
mode of speciation in sexually reproducing marine organisms than geographic
speciation. Palumbi and Lessios (2005) largely confirmed Mayr’s interpretation of
geographic speciation in sea-urchins using DNA sequence data. They also showed
that the origin of reproductive isolation depends not only on genome-wide steady
accumulation of substitutions but also on the rate of evolution of gamete recognition
proteins.
Sibling species were found among the colorful birds of southeastern Asia (p. 161)
and the polymorphic Californian king snake proved to be one biological species
only (p. 226). Particularly interesting cases of geographical speciation in birds
include the Black Bulbul (Hypsipetes madagascariensis; p. 160), trillers (Lalage),
fantails (Rhipidura), Reef Heron (Demigretta sacra), drongos (Dicruridae, p. 158),
Halcyon kingfishers, Australian tree runners (Neositta; p. 179), wheatears (Oenanthe,
p. 226), paradise magpies (Astrapia, p. 155), and honeyeaters (Melidectes).
When experimentation with birds turned out as logistically too difficult, Mayr
studied isolating mechanisms in Drosophila (p. 228). He also applied the principles
of new systematics to paleanthropology (p. 330). Numerous additional examples of
geographical variation and speciation from peripheral isolates may be found in his
major books on species and evolution (1942e, 1963b, Mayr and Diamond 2001g).
The number of generic and family revisions, faunistic papers or check-list entries
requiring decisions on the species status of geographically isolated populations
totals about 60.
Mayr (1946a) estimated the number of bird species in the world at 8,616 species
and Bock and Farrand (1980) at 9,021. This increase in the number of species
taxa was caused in part by the discovery of genuine new species and in part
by reinterpreting the rank of numerous allopatric taxa as species rather than
subspecies. In a study of a local flora, Mayr (1992a) tested the validity of the
biological species concept for plants concluding that the great majority of taxa
(93.5%) form biological species; only 6.44% pose difficulties by apomixis and by
autopolyploidy; Whittemore (1993) published a critical discussion of these results
from a botanical point of view.