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What’s Driving Human Evolution Now?

Sterling Hayden as Brig. Gen. Jack D. Ripper (left), with Peter Sellers as Group Capt. Lionel Mandrake in Dr. Strangelove (Photo from Wikimedia Commons)

Last year, when Sir David Attenborough, the British naturalist and broadcaster, speculated that human beings had ended natural selection through cultural and technological innovations, he got a deserved amount of blowback. The criticism started with his toy-model view of evolution, and went from there. But beneath it all, there was one lingering question: Was he getting at, in any way, a kernel of truth?

If there’s a specter hanging over my recent feature in The Chronicle Review, it’s that big question: Have people, through the rise of modern medicine and technology, changed our evolutionary path? Even more controversially, have we caused a decline in our overall genetic health? And, if either is true, is that a problem?

It’s a question that’s been batted around at faculty dinners for decades, usually after a few drinks. Invariably it invokes the failed eugenics movement, which saw a generation of scientists seeking “rational” control of human reproduction to prevent “genetic degeneration.” Hermann J. Muller, a Nobel Prize-winning biologist, gave a defining warning in a 1949 speech. Weakened selection could allow an increasing burden of harmful mutations—increasing humanity’s “genetic load,” to use a term he coined. In time, he speculated, mankind would recognize “the necessity of exercising some guidance over his reproduction.”

Genomics still cannot answer that big question. We’re only beginning to get at the legacy of our extreme population growth over the past few thousand years; to detect signatures of declining selection in the past few hundred, we’ll need far more samples. Those are likely to come over the next decade. Until then, however, some scientists have speculated (in a more-informed way than Muller) about what humanity has in store.

First, the caveats: We can say definitely that evolution itself hasn’t “stopped.” When it comes to evolution, natural selection is not the only game in town. For all life, random mutations that succeed without selective pressures—a process called genetic drift—play an important role in evolution. (There’s a decades-old debate about the relative importance of selection and drift.) Also, many of the world’s seven billion people don’t yet have the full benefits of modern medicine; if we’re altering selection, we’re not doing it the same everywhere.

Medicine is not the only way we could alter our evolution. Social changes work, too. For example, in wealthy countries, especially, men are having children at later stages in life. That means while archaic human beings had a generation time of 18 years, modern generations last up to 30 years, on average. It’s a dynamic best summed up by the title of a 2013 paper describing it: “The Human Mutation Rate Is Increasing, Even as It Slows.”

All that said, there’s actually little debate among researchers that we’ve altered natural selection to some extent. It’s an unavoidable consequence of medicine; saving lives means preserving new mutations that may otherwise have dropped out of the population. (I, for one, will always take a saved life over a new mutation.) The question is more one of degree: How much has natural selection relaxed?

You can count Michael Lynch, a biologist at Indiana University at Bloomington, among those concerned about our genetic load. In 2009 he published a paper in the Proceedings of the National Academy of Sciences arguing that Muller’s theories had “gained a level of quantitative credence.” Comparing it to the multigenerational challenge posed by climate change, Lynch warned that the accumulation of harmful mutations could some day lead to an epidemic of significant neurobiological and other handicaps.

Unlike Muller, Lynch does not endorse some form of positive eugenics. Given that most human traits depend on a dizzying array of genes, often of small influence, genetic counseling could do little to alleviate the problem.

Lynch’s paper has garnered a muted response, but it has been widely cited by psychiatric geneticists, where it has begun to look as if new mutations may play an important role in several mental disorders, including autism and schizophrenia. Scientists have begun gauging the additional risk older fathers may have of passing on such disorders to their children, which seems real, even if they may have only marginally increased the absolute risk. And it remains to be shown that those risks do indeed stem from new mutations.

Peter D. Keightley, an evolutionary biologist at the University of Edinburgh, is one researcher who has directly addressed Lynch, in a 2012 paper in Genetics. It’s not exactly a stinging rebuttal. He cites the uncertainty over selection’s relative importance, compared with drift, and the differing models that could drive very different conclusions. (This is what happens without concrete data; it all gets a bit hand-wavy.) Also, sexual selection for factors like wealth or intelligence is alive and well in people. All genetic load is relative: No one is born without any new mutations, so there’s no “perfect” model for comparison. Given certain circumstances, he writes, “it might not matter if everyone becomes 5 percent less sexually attractive.”

Again, before you panic, even if our collective genetic load becomes a problem, it won’t be one for centuries, everyone seems to agree. We’re not plummeting into some sort of Idiocracy. Indeed, one large study out of Scotland, published last month, found zero evidence of a tie between cognitive ability and number of rare genetic mutations. Even writing about the topic begins to feel strange, as it so often seems older white men are most concerned about this hypothetical problem. It reminds me of Brig. Gen. Jack D. Ripper, from Dr. Strangelove, who worried about “precious bodily fluids.”

There is more data coming soon, so stay tuned. Until then, it’s helpful to go back to Muller’s speech, which is more measured than our cartoons of the past might imagine. Whatever the genetic trajectory of the human race, he said, we’re in it together:

“Thus, the ailments and infirmities caused by mutations, although they are perhaps not so often a ‘direct cause of disease,’ or at least of the once rampant infectious diseases that are now rapidly being overcome, are nevertheless of vital importance to all of us. None of us can cast stones, for we are all fellow mutants together.”

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