“Are We Still Evolving?” by Gabrielle Walker, in Prospect (July 2004), Prospect Publishing, 2 Bloomsbury Pl., London WC1A 2QA, England.

As humans continue to advance, their evolution may be grinding to a halt. Natural selection works by picking and choosing among millions of random mutations that occur in each generation, favoring those individuals who bear traits conducive to survival and punishing those with less desirable traits. But we have molded our environments to such an extent that natural selection may have nothing left to work with, observes Walker, a British science writer.

All that’s necessary to get everyone’s genes on a level playing field is for people to be able to grow up and reproduce, claims geneticist Steve Jones, of University College, London. And modern technical and cultural developments have assured precisely that. In Britain, a baby who reaches six months of age today has a nearly 100 percent chance of surviving to adulthood. Only 150 years ago, about half the babies born in London died before they reached puberty.

Nature has lost its power to select, Jones argues, and even if certain diseases or conditions, such as obesity, cut a few years off the end of our lives, “evolution won’t notice,” because we’re already past childbearing age. Some in his camp worry that, without the ability to weed out problem mutations, we won’t merely cease to evolve, we’ll start accumulating defective genes that will eventually weaken the species. But it’s also likely that modern medicine is preserving useful genes that would otherwise perish.

Other scientists don’t subscribe to the theory that evolution has reached an impasse. One reason is that—as experts on both sides of the fence agree—cultural changes can affect evolution. A past example of that is the “grandmother effect,” which explains why women don’t die off soon after their child-bearing years, as other female primates do. The speculation is that, as Earth’s climate turned colder and drier and plants grew tougher and more deeply rooted 1.8 million years ago, having Grandma around to manage the increasingly hard work of foraging while Mom tended to the brood became essential to survival.

Proponents of ongoing evolution point
to the continuing role of such cultural changes. Malaria, for example, wasn’t a particularly widespread disease before early humans began clearing tropical forests to establish settlements, thereby creating an ideal environment for malarial mosquitoes. The first human genetic modifications designed to fight the disease appeared after that, about 5,000 years ago. Researchers cite other factors that may still shape the human gene pool, such as drugs that adversely affect people with certain genetic susceptibilities and the rise of “super-resistant” disease organisms bred by the overuse of antibiotics. At least one gene related to human heart disease shows signs of continuing evolution. And in the developing world, which faces plagues of infectious diseases such as malaria and HIV with very little access to modern medicine, “evolution is definitely not over.”

In the end, even leading advocates of the theory that evolution is on hold say there’s no guarantee it will remain there. “We’re on the edge of a cliff,” says Jones, “on the simple grounds that we’re far more abundant in number than we ought to be.” A single deadly epidemic on a global scale might bring back natural selection with a vengeance—unless human ingenuity once again finds a way to stop it.