By Tina Hesman Saey, Science News
African elephants have new distant cousins—other African elephants.
A genetic analysis of elephants and their extinct relatives, woolly mammoths and mastodons, shows that forest-dwelling African elephants are a separate species from Africa’s savanna elephants. The research, which appeared December 21 in PLoS Biology, “does a very thorough job of nailing shut the coffin on some of the more heretical theories” about elephant evolution, says Stephen O’Brien, a geneticist who studies genomic diversity at the National Cancer Institute in Frederick, Md., and who was not involved in the research.
Forest elephants make up only about a quarter of 500,000 or so of the elephants living in Africa today. Poaching and habitat destruction have caused already endangered populations to dwindle, but the new finding could spur conservation efforts to protect the animals.
Forest and savanna elephants evolved into different species from a common ancestor between 2.6 million and 5.6 million years ago, the new analysis reveals. That’s about the same time as Asian elephants and woolly mammoths came to a fork in their family trees. Asian elephants and mammoths’ many differences mean that some not only consider the animals distinct species but different genera—another level of taxonomic hierarchy.
“If you believe that the mammoth and the Asian elephant are different species, then it’s very difficult to argue that the forest and savanna elephants aren’t separate species,” says David Reich, an evolutionary geneticist at Harvard Medical School in Boston and coauthor of the study.
Nevertheless, people have been debating whether the big savanna elephants and smaller forest elephants belong to one or two species for a very long time. “This has been an ongoing debate since before genetics began,” says Alfred Roca, a study coauthor and conservation geneticist at the University of Illinois at Urbana-Champaign. The two pachyderms look different but sometimes come together and breed, producing hybrids. Hybrid males are sterile, but females can breed.
DNA evidence has also been controversial. Previously, researchers have examined elephant DNA from mitochondria, the cell’s energy-producing organelles that contain a small circle of DNA. Mitochondria are inherited from the mother. Those studies seemed to indicate that forest and savanna elephants have interbred, suggesting that they are not separate species.
But mitochondrial DNA gives clues only about female ancestry. To get the entire picture of genetic history, researchers needed to examine DNA from the cell nucleus, where the vast majority of genes are stored.
In the new study, researchers compared nuclear DNA from living elephants as well as from a 43,000-year-old woolly mammoth bone from Siberia and from a 50,000- to 130,000-year-old North American mastodon tooth. The mastodon DNA allowed researchers to infer the genetic makeup of the common ancestor of all the species and to plot how the genes have changed over time, says Roca. The analysis emphasized the genetic divide between forest and savanna elephants. The forest and savanna groups are at least as different as Asian elephants and mammoths, the researchers say.
“I’ve always argued that they are very different, but that level of difference surprised me,” says Roca.
Forest elephants had the greatest amount of genetic diversity of all the species studied, and savanna elephants were the least genetically diverse. That discrepancy could mean that the two species have different social structures, says Roca.
Only the biggest, strongest savanna males get to mate with females, Roca says. "So you lose the genetic diversity in the other males." Forest elephants' high diversity could mean that males aren't as competitive with each other and more males get access to females, he says.
Mammoths also had relatively low genetic diversity, suggesting that mammoth males were also highly competitive, Roca adds.
Asian elephants may have been responsible for keeping the forest and savanna elephants apart long enough to become separate species. All elephants originated in Africa, and some Asian elephants migrated out of the continent at about the same time as humans did, roughly 200,000 years. The remaining Asian elephants in Africa were wiped out about 35,000 years ago under still mysterious circumstances, Roca says.