Ornithology, Evolution, and Philosophy 123

Zoogeography 167
of Rennell Island [1931e] and especially those of Polynesia [1940i] and of the Sunda
Islands [1944e]).
From the references 1931e and 1942e, 1942h, it is shown that Mayr emphasized
that no fauna can be fully understood until it is segregated into its components
(“elements”) and until one has succeeded in explaining the separate histories of
each of these different species groups (Mayr 1965q). The dispersal ability of each
species and the geological histories of the areas involved need to be studied in detail
to determine the zoogeographical history of a group of animals as a dynamic and
continuing process. This endeavor implies the proper evaluation of (a) the relative
age of the various taxa depending on the degree of their respective differentiation,
(b) the determination of the dispersal capacity of the various taxa, and (c) the
distribution of related taxa. Some bird species disperse readily across water gaps
and others do not. A constraint limiting the range expansion of readily dispersing
species is the occurrence of competing close relatives beyond the range limits. In
these cases ecological competition prevents further range expansion, as shown by
the mosaic distribution patterns of the members of superspecies on continents and
in island regions. The basic zoogeographical processes underlying Mayr’s views
are summarized in Table 4.1.
Range disjunctions of populations or taxa may be primary or secondary (Hofsten
1916) depending on whether they originated actively (through jump dispersal
across a preexisting barrier) or passively (through a disruption of a previously
continuous range). A variety of geological processes led to such range disjunctions
(vicariance) and subsequent rejoining of populations. Climatic-vegetational cycles
on the continents of India, Africa and Australia split the ranges of forest birds in two
or more widely separated portions leading to the differentiation of isolated populations
(1942e: 231, 1950b); Pleistocene sea-level fluctuations caused the separation
Causes of
disjunction
Colonizing ability of
animals across
barriers
Climaticvegetational
cycles,
sea-level
fluctuations,
tectonic
uplift/subsidence,
continental drift
Table 4.1. Concepts underlying Ernst Mayr’s zoogeographical views
Zoogeography Geographical
(allopatric)
speciation
Pattern Process
Primary disjunction Jump dispersal Peripatric (founder
model)
Secondary
disjunction
Vicariance Dichopatric
(dumbbell model)