SCIENCE / MEDICINE : Darwin’s Theory Bolstered by Study of Closely Related Rats

For the first time, scientists have found a group of mammals so closely related that their DNA fingerprints--sophisticated maps of their genetic composition--are indistinguishable. The finding, reported today in the prestigious Proceedings of the National Academy of Sciences, helps resolve a key difficulty facing Charles Darwin’s theory of evolution by natural selection.

Neurobiologist Paul W. Sherman and his colleagues at Cornell University in Ithaca, N.Y., have been studying the naked mole-rat, a three-inch-long hairless rodent that lives in elaborate tunnels in the rock-hard soil of Kenya. The mole-rats have a social structure, very similar to that of bees and ants, in which multiple generations live together, cooperating in the care of young, and in which all of a colony’s efforts focus on helping one female (the “queen”) reproduce, to the exclusion of other females in the colony.

Darwinian theory, however, holds that animals will exhibit self-interest behaviors that increase the individual’s chances of surviving and passing on its own genes to progeny.

“Darwin said that if anybody can find any attribute of any species--or even of any individual--that evolved for the exclusive good of a reproductive competitor, it would annihilate his hypothesis,” said graduate student Hudson Kern Reeve, the principal author of the new paper.

The behavior of ants and other social insects was explained in 1964 by University of Oxford biologist William D. Hamilton. He held that the insects’ assistance to nearly identical kin increases the likelihood that the genes responsible for such behavior will be perpetuated, even though the helpers themselves do not reproduce.

Hamilton claimed that a peculiarity in the genetic structure of ants, bees and wasps--that sisters are more closely related, genetically, to each other than to their mother or to their own offspring, if they had any--was the basis for their social behavior. Helpers in those insect societies are raising nearly identical gene copies of themselves.

Biologists had thought that the same situation could not hold for mammals, however, because of genetic variability. The Cornell findings of genetic similarity, which resulted from many generations of inbreeding within colonies in the wild, indicate that the explanation offered for insects can hold true for mammals as well.

Darwin’s theory is in no danger.