Evolution__3rd_Edition

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The Great American Interchange is one of the most dramatic case studies in historic
biogeography. The mammalian faunas of North and South America have only been
connected, by a narrow isthmus, for less than 3 million years. Yet 50% of the mammalian
genera in the south are now of northern origin, and such wonderful animals
as that rhinoceros-sized rodent, the giant ground sloth, and the saber-toothed
borhyaenid were somehow involved in the general destruction of species during the
Interchange. That the events of the Interchange were at least partly due to competition
is very plausible, but to demonstrate it is a harder task.
17.9 Conclusion
CHAPTER 17 / Evolutionary Biogeography 517
Evolutionary biogeographers have been particularly interested in the historic processes
that have shaped the geographic distributions of species a though they by no means
rule out the well documented influence of modern ecology. They have mainly studied
two sorts of historic process: movement and range splitting. Species undoubtedly do
move by dispersal, and when a new corridor appears on Earth allowing a new
encounter of faunas, it can precipitate dramatic evolutionary events. The Great
American Interchange is a famous example. It is not easy to disentangle the exact causes
that were at work, but the data allow a plausible inference that the faunal changes were
substantially influenced by both the weight of numbers and competition.
Biogeography is one area of evolutionary biology that is particularly benefitting from
the expansion of molecular phylogentic research. New molecular markers can be used
to study the phylogeny of populations within a species, and of groups of related species,
in relation to space. We have seen how the history of European species such as hedgehogs
is written into the geographic distribution of their major clades, each of which is
descended from a different ice age refuge. We also saw how the adaptive radiation of
Caribbean lizards has been studied with molecular phylogenetic techniques. The combination
of biogeography and phylogeny, now often called phylogeography, builds on
older cladistic methods that were developed in the 1960s, 1970s, and 1980s, to study
vicariance biogeography. Now, with the rise of molecular systematics, phylogenetic
biogeography has flowered into a thriving and revealing research program.