Phylogeography is the study of genetics and population distribution as they are influenced by and related to geography. In simple terms, it means figuring out why animals live in the geographic areas and climates that they do, and looking at how the animals' genetics may have evolved in response to the environment. This also subtly includes some animal behaviors, like migration.

John Avise, Distinguished Professor at University of California, Irvine, coined the term 'phylogeography' in 1987. He was influenced strongly by two developments in the 60s. The first being cladistics, or the practice of categorizing creatures of various species by the shared traits they possess, such as standing on four legs, or having opposable thumbs. The second was the theory of plate tectonics, a monumental discovery which showed that the Earth's crust is made up of large plates, which float on an ocean of magma. These plates can push against each other to form mountain ranges and push landmasses up from the sea.

A fascinating aspect which includes these two discoveries is vicariance, or the splitting of a taxon (a grouping of animals of the same species or genus) in half due to geographic changes. A great example of this is the formation of the Isthmus of Panama three million years ago. North and South America used to be disconnected, allowing marine animals to swim between them freely. Over millions of years, tectonic plates under the ocean forced the crust up above sea level, separating the Atlantic from the Pacific. Members of the same species were trapped on either side, and evolved in different ways.

Phylogeography really took off in in the mid-70s, when advancements in genetic research yielded the discovery of the polymerase chain reaction, a method by which strings of DNA could be easily replicated and studied in large amounts. This, along with computing and breakthroughs in laboratory methods, allowed scientists to process millions of DNA sequences easily, and pushed the discipline to new levels. These days, phylogeographers also make use of statistical models in their work.

These breakthroughs and technologies allow scientists to formulate clear pictures of migration patterns and extrapolate the reasons behind them. For instance, scientists can overlay the DNA data they gather on top of periods of climate change discovered by climatologists. This can lead them to see the state of the world millions of years ago, and figure out why, for example, some animals are ubiquitous in Eurasia but absent in the Americas. Understanding how geography affects animal species can also shed light on the origins of human beings.

Phylogeography has been applied to hominids, and has yielded the Out-of-Africa theory, which proposes that modern humans evolved in present-day Ethiopia over 100,000 years ago, and migrated around the globe. There are theories backed by DNA research that modern humans interbred with Neanderthals, close genetic cousins which could have been native to Europe. The reasons why modern humans migrated out of Africa, and why the Neanderthals died out, are still unclear, though climate and geography are thought to have played a critical role.

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