Cultural SelectionThe evolution of evolution.

In the beginning, there was creationism, which assumed we had never evolved.

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Then came the theory of evolution through random mutation and natural selection.

Then came a new hypothesis: Once our ancestors had developed agriculture and stable societies and no longer lived at nature's mercy, human evolution had ceased.

Now we're in the midst of the next mutation in evolutionary theory: Human evolution didn't slow as we advanced from nature to culture. It accelerated and changed. Culture, born of natural selection, became natural selection's driving force.

This is the message of a new study of the human genome. If true, it radically complicates the debate between nature and nurture. The question is no longer simply whether our genes are the source of civilization, but whether they're also its product.

The study, published this week in Proceedings of the National Academy of Sciences and based on DNA samples from around the world, concludes that human evolution has accelerated in the last 40,000 years and particularly in the last 10,000. One reason is that population growth has increased the temporal rate of mutations and selections. But in the case of humans, the authors note, "Rapid population growth has been coupled with vast changes in cultures and ecology … creating new opportunities for adaptation." Such "rapid cultural evolution" has "created vastly more opportunities for further genetic change, not fewer, as new avenues emerged for communication, social interactions, and creativity."

Conceptually, the argument is straightforward. Organisms evolve in response to changing environments. This can lead, paradoxically, to the evolution of traits that change the environment. Once that happens, the process becomes dialectical, and its speed increases, because culture changes more rapidly than nature does.

The authors offer a few simple examples. Dairy cultivation made the ability to drink milk in adulthood advantageous, which in turn led to the genetic spread of lactose tolerance. Settlement elevated the threat of diseases such as malaria and cholera, which in turn caused the dissemination of genes for resisting such diseases.* And the transition from hunting and gathering to growing corn produced new dietary threats such as diabetes, to which our DNA is still adapting.

Many of these genetic trends, while influenced by culture, still fit what we think of as natural selection. In epidemics and dietary diseases, it's nature that does the killing. But the study points out that cultural evolution transforms social systems as well as diets. And new social systems can create reproductive dynamics in which nature plays only a pro forma part.

In particular, the authors cite a previous study, co-authored by two of them, that argued that Jewish IQs rose in medieval Europe due to literacy, inbreeding, and confinement to cognitively demanding jobs. They titled that study "Natural History of Ashkenazi Intelligence." In it, they repeatedly attributed the rise of Jewish intelligence to "natural selection." But the factors they identified—values, religion, and discrimination—were far more cultural than natural. At best, what had happened to Jews was, as the authors put it, "natural selection, stemming from their occupation of an unusual social niche."

You can accept or reject these particular evolutionary explanations as you like. But the underlying message is worth taking home: Much of what now passes for "natural selection" isn't exactly natural. It's social. As such, it deserves no presumptive respect as a validator or promulgator of objective fitness. Nor does the discovery of a genetic basis for this or that trait prove it's more than a social construct. In the era of cultural selection, many genes are a social construct. Which makes them no less real.

All of which poses a problem for anyone who equates genes with human nature, or who expects evolution to take God's place as judge and perfecter of humankind. It may be true that today's God a human creation. But so, in a way, is today's evolution.

Correction, Dec. 14, 2007: The article originally referred to malaria and cholera as viral diseases. They are microbial but not viral. (Return to the corrected sentence.)

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William Saletan is Slate's national correspondent and author of Bearing Right: How Conservatives Won the Abortion War.

Photograph of a monkey on Slate's home page by Peter Macdiarmid/Getty Images.

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