by Greg Mayer
Although far from the longest chapter in WEIT, I find the chapter on biogeography the single most persuasive one for showing why evolution is true. I think Jerry finds it compelling as well. This might seem surprising since he’s a geneticist: one might think he would find some of the genetic evidence most compelling. But I don’t think it is surprising, given that it was the biogeographic evidence, that, as the great zoogeographer P.J. Darlington put it, showed Darwin evolution.
Tapirs provide a nice example of the use of multiple lines of evidence in solving a biogeographic puzzle (a puzzle noted by an alert reader in the comments on my first tapir post). Tapirs are usually thought of as South American (where they are most widespread and species rich), with one species in Malaya.
The first thing you might think needs explication is the disjunct distribution. But before tackling this, a mis-impression must be corrected: although we tend to think of tapirs as typically South American, from a historical perspective, they are recent interlopers. Along with many other animals we consider typically South American (jaguars, llamas, peccaries), they entered South America from the north about 3 million years ago when the Panamanian portal became the Panamanian isthmus during the Great American Interchange.
What, then about the disjunction: how did they get from Central America to Malaya? They didn’t. Tapirs are a northern group. They and their relatives date back to the lower Eocene (ca. 50 mya). The modern genus, Tapirus, dates back to the Oligocene (ca. 30 mya), and was found in Europe, Asia, and North America. They have gone extinct in Europe, most of Asia, and most of North America. Tapirs thus have a relict distribution, being still found at two endpoints of their historical distribution. Geology, paleontology, and systematics thus combine to give a most satisfying account.