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Males Drive Evolution

THE IDEA that male mammals contribute more to evolution than females has been confirmed by a study of birds in Sweden. Scientists had long suspected that males play a bigger part in evolution than females because sperm cells divide many more times than egg cells as they mature. More divisions means more chances for genes to mutate. Support for this idea came when researchers found that the Y chromosome in mammals, which only males carry, mutates at a higher rate than the other sex chromosome, the X. Others argued that the Y chromosome might be a special case because it carries very few genes. X chromosomes, by contrast, carry many vital genes and may have greater resistance to mutation in both males and females. To resolve the question, Hans Ellegren and Anna-Karin Fridoiffson of the Swedish University of Agricultural Sciences in Uppsala looked at birds, in which the arrangement of sex chromosomes is reversed. Rather than an X and a Y, avian males carry two copies of a sex chromosome called Z. Females have one Z and one W. If greater protection of the X accounts for the more rapid mutation of Y in mammals, then the less important W should have mutated more rapidly during the evolution of birds. But when Ellegren looked at a gene found on both Z and W in sparrows and in finches-related species that diverged tens of millions of years ago-he found that copies of the gene on the Z chromosomes differed between species much more than those on the W chromosomes. He concludes in this month's Nature Genetics (vol 17, p 223) that the mutation rate is four to seven times faster in males than in females. "This suggests that evolution is driven by males," he says. Jonathan Knight