Biogeography is the study of how species are distributed. It notes where organisms live, and why they are (or are not) found in a certain geographical area.

Biogeography teaches how animals and plants are adapted to the places they live in, and how similar places often have quite different animals and plants.

Between about 1800 to 1855, natural historians made lists of species in various regions of the world. These lists were published as tables in their books. Charles Darwin and Alfred Russel Wallace published the idea of evolution by natural selection. They travelled to tropical countries, and wrote about the life in those countries. They said that evolution was the key to understanding geographical distribution.

New species are usually formed by speciation  an earlier species splitting into two. These species may travel to new places. but they may be stopped from travelling by mountains and seas, and by climate. This means that two places with similar climate often have different kinds of animals and plants. For example marsupials, which live in Australia, are very different from the fauna in South America. The species on islands (Hawaii, Galapagos) may be very different to species on mainland continents.

"The more distant countries are, the more dissimilar are their animals and plants; and the nearer they are, the more similar are their animals and plants". This is true, but only up to a point. There are many examples of species with huge ranges.

History 

The scientific theory of biogeography grows out of the work of  Alexander von Humboldt (1769–1859), Hewett Cottrell Watson (1804–1881), Alphonse de Candolle (1806–1893), Alfred Russel Wallace (1823–1913), Philip Lutley Sclater (1829–1913), and other biologists and explorers.

Wallace studied the distribution of flora and fauna in the Amazon Basin and the Malay Archipelago in the mid-19th century. Wallace and Sclater saw biogeography as a source of support for the theory of evolution. Key findings, such as the sharp difference in fauna either side of the Wallace Line, can only be understood in this light. Otherwise, the field of biogeography would be a purely descriptive one.

Both Darwin and Wallace gave a great deal of attention to oceanic islands as offering examples of evolution, especially speciation. Darwin visited the Galapagos Islands and studied its fauna. Wallace spent years on the islands of S.E. Asia. This interest was revived by The theory of island biogeography by Robert MacArthur and E.O. Wilson in 1967. They showed that the variation in species in a single area could be predicted if one knew the habitat area, immigration rate, and extinction rate.

It was realised that habitat fragments are like islands. They can be investigated by the same methods. This spurred the development of conservation biology.

Genome analysis allows scientists to test theories about the origin and dispersal of populations, such as island species. It allows biologists to test theories about where the species come from.

Books 
Cox C.B. and Moore P.D. 2010. Biogeography: an ecological and evolutionary approach. 8th ed, Wiley.  (standard text)

Related pages 
Speciation#Geographical isolation
Great American Interchange
Ecology

References 

Branches of biology
Branches of geography