HIV: The Ultimate Evolver (2 of 3)

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2. Why are some people resistant to HIV?
HIV is by no means the first plague that human populations have weathered. Many pathogens have deeply affected our evolutionary history. In fact, the human genome is littered with the remnants of our past battles with pathogens—and one of these remnants, a mutation to a gene called CCR5, may lead researchers to a new treatment for HIV.

The mutant CCR5 allele probably began to spread in northern Europe during the past 700 years when the population was ravaged by a plague. (It may have been bubonic plague or some other pathogen; research on this topic continues.) The mutant CCR5 probably made its bearers resistant to the disease, and so its frequency increased.

In some parts of Europe today, up to 20% of the population carry at least one copy of the protective allele. However, the populations of Asia and Africa were not exposed to the same epidemics; very few Asians and Africans now carry the allele (see map above). Thus, CCR5 is fairly common in northern Europe but its frequency diminishes as one moves south, and the mutation is rare in the rest of the world.

We now know that the mutant CCR5 allele has an unexpected side effect: it confers resistance to HIV. Scientists hope that studying this by-product of past selection will help them develop new treatments for the HIV epidemic ravaging human populations today.

• Map of CCR5 frequency, adapted from Evolution: The Triumph of an Idea, by Carl Zimmer, © 2001, Harper Collins Publishers. Used with permission.