. Why would one type of DNA paint a picture of strict barriers erected between groups and another reveal a widely intermingling population? According to the scientists, the disagreement could be a matter of timing. Since changes in mitochondrial DNA only have to spread through the maternal side of the genome, it appears to "evolve" more rapidly. Like the difference between an instant Polaroid snapshot and a photograph slowly developing in a darkroom, mitochondrial and nuclear DNA may be suited to capturing evolutionary events--such as the development of discrete subspecies--on different time scales.

In the case of the chimpanzees, "it is likely that the subspecies are old enough for the mitochondrial DNA to have become different between the subspecies, but not old enough for this to have happened to the nuclear genes," said Pbo. Since biologists often single out genetically unique populations like subspecies as a focus of conservation efforts, this distinction could have real-life implications for protecting endangered chimpanzees.

Xq13.3 had yet another surprise up its sleeve when it came to the bond between the chimpanzee and the bonobo. Their evolutionary tree revealed a closer relationship between the two separate species than was inferred from other DNA sequences. In fact, some common chimpanzees were more genetically distant from each other than they were from bonobos, a startling fact given that the two are currently separated at the species level. The results indicate that the chimpanzee and the bonobo may have split off in separate evolutionary directions quite recently.

The researchers believe that this high degree of genetic intermixing among the chimpanzee groups makes it unlikely that any cultural differences between chimpanzee populations are due to genes determining behavior. "Rather, these differences are probably truly the result of cultural evolution, the transmission of learned behaviors from generation to generation," sa
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Contact: Heather Singmaster
hsingmas@aaas.org
202-326-6414
American Association for the Advancement of Science
3-Nov-1999

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