Bio 221a: Biogeographic Evidence for Evolution

Biogeographic Evidence for Evolution

• Biogeography is the study of organisms' geographic distributions, with both historical and ecological components.
• Certain distributions are the consequence of long-term evolutionary history, while others are the result of current ecological factors.

Patterns of Distribution

• Biogeographic realms recognized by Wallace include Palearctic, Ethiopian, Oriental, Australian, Nearctic, and Neotropical.
• Examples of taxa endemic to the Neotropical biogeographic realm exist.
• Disjunct distributions occur, where related species are found in different geographic locations.

Historical Explanations of Geographic Distributions

• Historical processes affecting the distribution of a taxon include 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 to vicariance, but dispersal seems to be the more common cause.

Phylogeography

• Phylogeography is the description and analysis of the processes that govern the geographic distribution of lineages of genes, especially within species and among closely related species.
• Genetic patterns of geographic variation within species can provide information on historical changes in a species' distribution.

Geographic Range Limits: Ecology and Evolution

• The local distribution of species is affected by ecological factors, including abiotic aspects of the environment and biotic features such as competitors and predators.
• Why species do not enlarge their ranges indefinitely is a major question in evolutionary biology.
• Competition tends to prevent ecologically similar species from becoming sympatric.

Geographic Patterns of Diversity

• The latitudinal diversity gradient shows that the numbers of species (and of higher taxa such as genera and families) decline with increasing latitude, both on land and in the ocean.
• Three models of species accumulation have been proposed to account for the latitudinal diversity gradient.
• 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.