Genes separate Africa's elephant herds
Genetic work reveals forest and savanna pachyderms as distinct species.
Genes Separate Africa?s Elephant Herds - Science News
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Tina Hesman Saey 21.12.10
- Science News
Africa's forest & savanna elephants are 2 distinct spp, a new genetic study
shows. Forest elephants are smaller than savanna elephants and have rounder
smaller ears & straighter thinner tusks.
African elephants have new distant cousins ‹ other African elephants.
A genetic analysis of elephants & their extinct relatives, woolly mammoths &
mastodons, shows that forest-dwelling African elephants are a separate
species from Africa's savanna elephants. The research (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 (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 & habitat destruction have caused
already endangered populations to dwindle, but the new finding could spur
conservation efforts to protect the animals.
Forest & savanna elephants evolved into different spp from a common ancestor
between 2.6 & 5.6 Ma, the new analysis reveals. That's about the same time
as Asian elephants & woolly mammoths came to a fork in their family trees.
Asian elephants & mammoths' many differences mean that some not only
consider the animals distinct spp but different genera ‹ another level of
taxonomic hierarchy.
David Reich (co-author): "If you believe that the mammoth & the Asian
elephant are different spp, then it's very difficult to argue that the
forest & savanna elephants aren't separate species."
Nevertheless, people have been debating whether the big savanna elephants &
smaller forest elephants belong to 1 or 2 spp for a very long time. Alfred
Roca (co-author: "This has been an ongoing debate since before genetics
began." The 2 pachyderms look different, but sometimes come together &
breed, producing hybrids. Hybrid males are sterile, but females can breed.
DNA evidence has also been controversial. Previously, researchers have
examined elephant mtDNA. Those studies seemed to indicate that forest &
savanna elephants have interbred, suggesting that they are not separate spp.
But mtDNA 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-ka woolly mammoth bone from Siberia & from a 50-to-130-ka
N.American mastodon tooth ... The forest & savanna groups are at least as
different as Asian elephants & mammoths: "I've always argued that they are
very different, but that level of difference surprised me."
Forest elephants had the greatest amount of genetic diversity of all the spp
studied ; savanna elephants were the least genetically diverse. That
discrepancy could mean that the 2 spp have different social structures, says
Roca.
Only the biggest strongest savanna males get to mate with females: "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 rel.low
genetic diversity, suggesting that mammoth males were also highly
competitive.
Asian elephants may have been responsible for keeping the forest & savanna
elephants apart long enough to become separate spp. All elephants
originated in Africa, and some Asian elephants migrated out of the continent
at about the same time as humans did c 200 ka. The remaining Asian
elephants in Africa were wiped out c 35 ka ago under still mysterious
circumstances, Roca says.
Until that time, Asian elephants were the dominant elephant spp in Africa
and may have kept the other 2 groups separated, O'Brien says: "They couldn't
move around because the continent was full of these big Asian elephants that
didn't like them" ...
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Note that savanna animals typically have long narrow ears, arboreal animals typically have more smaller rounded ears. True in elephants and monkeys, etc.