Evolution__3rd_Edition

..
Dispersal is enabled by various
physical factors ...
. . . which may be accidental
CHAPTER 17 / Evolutionary Biogeography 497
average to the south. In practice, though, individuals would move southwards as well
and extend the species range as they did so. If a species originated in one area and
subsequently dispersed to fill out its existing distribution, the place where it originated
is called its center of origin.
Various dispersal routes might have been followed in the biogeographic history of a
species. Simpson distinguished dispersal by means of corridors, filter bridges, and sweepstakes.
Two places are joined by a corridor if they are part of the same land mass a
Georgia and Texas, for example. Animals can move easily along a corridor and any two
places joined by a corridor will have a high degree of faunal similarity. A filter bridge is
a more selective connection between two places, and only some kinds of animals will
manage to pass over it. For instance, when the Bering Strait was above water, mammals
moved from North America to Asia and vice versa, but no South American mammals
moved to Asia and no Asian species moved to South America. The reason is presumably
that the land bridges at Alaska and Panama were so far apart, so narrow, and so different
in ecology that no species managed to disperse across them. Finally, sweepstakes
routes are hazardous or accidental dispersal mechanisms by which animals move from
place to place. The standard examples are island hopping and natural rafts. Many
land vertebrates live in the Caribbean Islands, and (if their biogeography is correctly
explained by dispersal) they might have moved from one island to other, perhaps being
carried on a log or some other sort of raft.
There is good evidence for the power of dispersal. In 1883, for example, a volcanic
eruption covered the small Indonesian island of Krakatau with ash and killed all the
plants and animals. Biologists then recorded the recolonization of the island, particularly
for birds and plants. The recolonization was astonishingly rapid. Fifty years later,
the island was already recovered with tropical forest, which supported 271 plant species
and 31 bird species. Invertebrate animals, such as insects, had come too, though their
numbers were less closely monitored. The immigrants mainly came from the neighboring
islands of Java (25 miles (40 km) away) and Sumatra (50 miles (80 km) away);
the birds would have dispersed by active flight and the plants would have been carried
as seeds. Dispersal, therefore, in the right circumstances can have a clear effect on the
ranges of species.
17.4 Geographic distributions are influenced by climate,
such as in the ice ages
The current geological age is called the Quaternary, which began 2.5 million years
ago (see Section 18.2, p. 525, for geological time). During the Quaternary, the climate
has mainly been cooler than in the preceding Tertiary, and the temperature has cycled
up and down. Many of the cooler times have been glacial periods, and the warmer
times interglacials. These climatic changes have happened recently enough for the fossil
record in some cases to be revealingly complete. When the weather turns cool, the
ranges of animal and plant species tend to contract and move south (in the northern
hemisphere). At any one site, the local ecology changes to one characteristic of the
cooler climate. A change from a temperate to a tundra-type ecosystem, for example,