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Human Evolution Isn't What It Used to Be

If you write about genetics and evolution, one of the commonest questions you are likely to be asked at public events is whether human evolution has stopped. It is a surprisingly hard question to answer.

I'm tempted to give a flippant response, borrowed from the biologist Richard Dawkins: Since any human trait that increases the number of babies is likely to gain ground through natural selection, we can say with some confidence that incompetence in the use of contraceptives is probably on the rise (though only if those unintended babies themselves thrive enough to breed in turn).

More seriously, infertility treatment is almost certainly leading to an increase in some kinds of infertility. For example, a procedure called "intra-cytoplasmic sperm injection" allows men with immobile sperm to father children. This is an example of the "relaxation" of selection pressures caused by modern medicine. You can now inherit traits that previously prevented human beings from surviving to adulthood, procreating when they got there or caring for children thereafter. So the genetic diversity of the human genome is undoubtedly increasing.

Or it was until recently. Now, thanks to pre-implantation genetic diagnosis, parents can deliberately choose to implant embryos that lack certain deleterious mutations carried in their families, with the result that genes for Tay-Sachs, Huntington's and other diseases are retreating in frequency. The old and overblown worry of the early eugenicists—that "bad" mutations were progressively accumulating in the species—is beginning to be addressed not by stopping people from breeding, but by allowing them to breed, safe in the knowledge that they won't pass on painful conditions.

Still, recent analyses of the human genome reveal a huge number of rare—and thus probably fairly new—mutations. One study, by John Novembre of the University of California, Los Angeles, and his colleagues, looked at 202 genes in 14,002 people and found one genetic variant in somebody every 17 letters of DNA code, much more than expected. "Our results suggest there are many, many places in the genome where one individual, or a few individuals, have something different," said Dr. Novembre.

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TAGGED: BIOLOGY, EVOLUTION


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