An Elephant's DNA Never Forgets
The African elephant has long been considered a single species, but a new, comprehensive map of elephant history has recently altered that view. Comparing the DNA of modern African and Asian elephants with their extinct woolly mammoth and American mastodon forebears, a new study shows that the African elephant actually comes in two varieties. They may look fairly similar, but members of the forest (Loxodonta cyclotis) and savanna (Loxodonta africana) groups are as genetically distinct from one another as lions are from tigers or as humans are from chimpanzees. The two African elephant species diverged from a common ancestor somewhere between 2.5 and 5 million years ago (about the same time that lions and tigers and humans and chimps split).
What defines a species, anyway? Scientists use a rubric called the biological species concept, which -- as with much that motivates change in the natural world -- mostly has to do with sex. The concept defines a given species as a collection of populations that freely interbreed; species diverge when they stop "mingling genes" (as intercourse is referred to in polite biologist circles).
Although many previous studies have explored elephant evolution, this time researchers were able to obtain usable DNA samples from both well-preserved mastodon and woolly mammoth fossils. In what the study’s authors believe is the first-ever genetic analysis of an American mastodon -- the most ancient of the five species -- they gained a starting point from which they could chart the divergence of the other four elephant groups.
Alfred Roca, an animal sciences professor at the University of Illinois and a co-author of the study, says he and his colleagues expected the resulting data to prove that the Asian elephant is most closely related to the mammoth, which it did. Surprisingly, though, it also showed that the forest and savanna elephants had an equal depth of genetic deviation from each other, meaning they stopped mingling genes around the same time their Asian cousin and the mammoth did.
As glaciers have advanced and retreated over the past 4 million years, Africa’s forests have pulled back from the savanna, creating a stark boundary between the two environments that caused the elephants living in each region to adapt differently. Forest elephants are considerably smaller than their savanna counterparts, with rounder ears and thinner, straighter tusks. Their tropical habitat provides them with more fruit, and their tribes are more genetically diverse -- likely a result of having developed less restrictive social and mating behaviors.
The International Union for the Conservation of Nature and Natural Resources considers African elephants vulnerable to poaching and habitat loss. Scientists hope that treating the two branches of Loxodonta independently could help focus and improve efforts to preserve them. "We found good evidence for the two groups being separate species, therefore there need to be separate conservation plans," Roca says. "In particular, the forest elephant is the more highly endangered of the two, so forest elephants need to be a priority for conservation."