African AIDS May Alter Human Evolution

by Robert Sanders

The study, published in the May 31 issue of Nature, shows that AIDS provides a rare example of how diseases can exert selective pressure on the human evolution.

The only other similar example is malaria. Over hundreds of years, more people explosed to the disease survived to reproductive age if they had certain genes. (Unfortunately, the same gene that provided resistance to malaria also made people prone to other blood diseases, primarily sickle cell anemia.)

Charles Darwin coined the term "natural selection" and focused on it as the driving force of evolution, whereby slight variations that increase the number of offspring tend to become more common in successive generations. The same idea is embodied in the phrase "survival of the fittest."

Scientists have speculated that infectious diseases that reached epidemic proportions over the course of history -- from bubonic plague and measles to smallpox -- have affected human evolution, selecting for genes that reduce mortality before or through the reproductive years. Evidence, however, is hard to find, said UC Berkeley mathematical biologist Montgomery Slatkin. Then Slatkin and his colleagues noticed that a United Nations study of AIDS in Africa reported that nearly 40 percent of the population has the disease in some countries.

Today the lifetime chance of a 15-year-old boy dying of AIDS is 65 percent in South Africa and nearly 90 percent in Botswana.

The Berkeley researchers found that over the course of 100 years, the gene conferring greater resistance to AIDS would increase in frequency from 40 percent of the population to more than half. The mutation that makes people more susceptible would decrease from 20 percent of the population to only 10 percent. As a result, in a century, AIDS symptoms will appear a year later than they do now -- at around 8.8 years.

As a result, about one extra child per person will be born, Slatkin said.

Interestingly, a different mutation, called delta-32, in the CCR5 gene is found in northern Europeans, though rarely in Africans. A person with two copies of this mutated CCR5 gene apparently is completely resistant to HIV infection. CCR5 seems to be important in AIDS because the HIV virus locks onto it before entering cells.

Slatkin said that it appears the CCR5 mutation has been common in northern Europe for in the past 700 years, possibly by another epidemic disease with as strong a selective pressure as malaria and AIDS. Some scientists have suggested that bubonic plague, which decimated Europe during the 14th century, may have been the cause.

Slatkin, however, suspects it was measles or some disease that strikes young children, since that would have a greater effect on reproduction than diseases like plague, which affects all ages. Bubonic plague, though devastating, also did not last long enough to exert sufficient selection on resistance genes, he said.