Biogeography, the study of organisms’ geographic distributions, has both historical and ecological components. Certain distributions are the consequence of long-term evolutionary history; others are the result of current ecological factors.
The geographic distributions of organisms provided Darwin and Wallace with some of their strongest evidence for the reality of evolution.
The historical processes that affect the distribution of a taxon are extinction, dispersal, and vicariance (fragmentation of a continuous distribution by the emergence of a barrier). These processes may be affected or accompanied by environmental change, adaptation, and speciation.
Histories of dispersal or vicariance can often be inferred from phylogenetic data.
Disjunct distributions are attributable in some instances to vicariance, but dispersal seems to be the more common cause.
Genetic patterns of geographic variation within species can provide information on historical changes in a species’ distribution.
The local distribution of species is affected by ecological factors, including both abiotic aspects of the environment and biotic features such as competitors and predators. Why species do not enlarge their ranges indefinitely, by incrementally adapting to conditions farther and farther away, is a major question in evolutionary biology.
Geographic patterns in the number and diversity of species may stem partly from current ecological factors, but long-term evolutionary history also may explain them.