Controversy About Genetic Variation

Not all experts accept the bottleneck theory. A more likely scenario, some say, is that the cheetah population remained abundant but was fragmented into numerous small, isolated subpopulations. Such fragmentation could have taken place toward the end of the Ice Age, as climate fluctuations caused the advance and retreat of ecosystems such as forest and savanna. As a forest extended into a savanna, cutting off one portion from another, the animals on either side of the forest could have been separated and remained so for thousands of years. According to this theory, when one such group died out because of disease, overhunting, or some other reason, its territory was eventually recolonized by a few individuals from a nearby subpopulation. Repeated over and over, this process would eventually lead to a loss of genetic variation.

Population geneticist Philip W. Hedrick of Arizona State University in Tempe is one expert who agrees that the cheetah could have achieved its low genetic variation without its population dropping to the verge of extinction. But Hedrick also points out that several animal species—including beavers in Sweden and northern elephant seals living off the California coast—have low genetic variation, yet their large populations appear to suffer no health or reproductive problems as a result. "We don't really know how much variation is required for a species to be fit and healthy," Hedrick says.