Biogeography

Abstract

This chapter focuses on the biogeography, the study of the patterns of geographic distribution of organisms and the factors that determine those patterns. This discipline plays a critical role in understanding of marine mammal evolution and adaptation. Although marine mammals are very mobile and there is an apparent lack of physical barriers in the world ocean, only Orcinus orca, Physeter macrocephalus, and perhaps some of the balaenopterids could arguably be considered to have cosmopolitan distributions. Other species have restricted distributions (e.g., coastal South America, Indo-West Pacific), reflecting their ecological requirements, and their geographic centers of origin. Because related species tend to have similar ecological requirements and dispersal abilities, the distribution of higher taxa can also show distinct tendencies and restrictions, which reflect the cumulative distributions of their included species. For example, while delphinids, river dolphins, and sirenians have their highest diversity in tropical latitudes, most pinniped, ziphiid, and phocoenid species occur in temperate and Polar regions. From a geographic perspective, specific regions can thus be characterized as centers of diversity for these higher taxa, and past global changes in the environment will have influenced their evolutionary history. For example, cooling of the world climates during the Tertiary may have contributed to the radiation of the cold-water- adapted pinnipeds and mysticetes.