Biogeography is the study of the distribution of organisms across the planet, and includes the study of how and when species arrived in various places, and how they move into and out of regions. Some biogeographists work on the historical aspects of species movement, and others on mapping and following current conditions. Biogeography provides data and support for evolutionary theory.
A number of factors control distribution of species. Plate tectonics is the largest and most important underlying factor - how long ago one landmass separated from another, and what species were present when the separation occurred, drive the basis of species distribution for most of the planet. Factors that isolate one area from others, thus limiting the ability of species to immigrate or emigrate, is another; for this reason islands that are varying distances from the nearest landmass are a focal point of biogeographic studies. Mountain ranges, bodies of water, and similar land features are also important barriers to species movement.