Evolution__3rd_Edition

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Figure 17.1
The natural distribution of
three species of toucans in the
genus Ramphastos in South
America: R. vitellinus and R.
culminatus have endemic
distributions, whereas R.
ariel’s distribution is disjunct.
There is an extensive hybrid
zone between the species.
Modified, by permission of the
publisher, from Haffer (1974).
Geographic distributions of species
may be endemic, cosmopolitan, or
disjunct
Ramphastos
culminatus
CHAPTER 17 / Evolutionary Biogeography 493
Ramphastos
vitellinus
Hybrids
17.1 Species have defined geographic distributions
Ramphastos
ariel
The geographic distributions of species can be of a number of types. Consider Figure 17.1,
which shows the distribution of three species of toucans in the genus Ramphastos, in
South America. Two of the species, R. vitellinus and R. culminatus, have endemic distributions:
they are limited to a particular area. Endemic distributions can be more or
less widespread, and the extreme case of species that are found on all continents of the
globe are called cosmopolitan. The pigeon, for example, is found on all continents except
Antarctica; on a strict definition, the pigeon might not be allowed to be cosmopolitan,
but the term is usually intended less strictly a and the pigeon is called a cosmopolitan
species. Other species, like R. ariel in Figure 17.1, are not confined to a single area, but
are distributed in more than one region with a gap between them: these are called
disjunct distributions.
Maps like the one for species in Figure 17.1 can be drawn for a taxonomic group at
any Linnaean level: just as species have geographic distributions, so too do genera, families,
and orders. Biogeography aims to explain the distributions of the higher taxa too,
in addition to those of species, and different explanatory processes are often appropriate
at different levels. Short-term movements of individuals influence the distributions
of populations and species, whereas slower acting geological processes may control the