Biogeography was central to Darwin's logic when he summarized his findings from five years of collecting evidence around the world as a passenger on the HMS Beagle. He realized that animal and plant species, though diverse, were more similar to each other on the same continent.
So what is BioGeography? It is the study of the distributions of plants and animals over the surface of the Earth spatially and temporally. The spatial component describes and explains the distributions of one or more species over the world. The temporal component is used to explain the changing distributions of organisms over time, either in the short term or over geological time.
For example Australian species were more similar to each other than they were to South American species. But such geographic diversity also played out on local island groups such as those of the Archipelago Galapagos in the South Pacific. The famous Darwin finches were his prime exhibit in formulating the theory of evolution.
How does this idea work?
Species will change over time in go in a different direction if they are isolated from each other over long periods of time. Now remember that time periods in evolution are very, very long. They are measured in geological time, for example in MYA or million years ago.
Fossil records together with the theory on plate tectonics and continental drift support the idea of speciation, which come from a long lasting period of geographic isolation.
What is speciation? Speciation is the process of evolving two different species from a founder species as the result of an event that caused separation of the founder population into two isolated populations. Consequently, individuals from one population cease to reproduce with individuals from the other population. Their similarities will continue to exist, but their differences will start to become apparent.