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When it comes to moving evolution forward, men are in charge, a new study suggests.

For species to evolve, changes, or mutations, must occur in genes, and the genes must be passed on to the next generation. New mutations occur during the cell divisions that give rise to sperm or eggs.

Scientists have long suspected that fathers would pass on more mutations than mothers because there are more cell divisions in the ancestry of a sperm than in an egg. For example, sperm produced by a 30-year-old man have arisen through 400 cell divisions, while an egg in a woman of any age has arisen through only 24 cell divisions.

Researchers have tried to test the idea by comparing mutations on the X and Y chromosomes. X chromosomes are passed through mothers (who have two X chromosomes) and fathers (who have an X and a Y), but Y chromosomes are passed through fathers only. After several studies, the question was still unresolved, and no one had confirmed the result in species in which two different sex chromosomes make a female.

In the latest issue of the journal Nature Genetics, scientists from The Swedish University of Agricultural Sciences in Uppsala report studies of mutation rates in the sex chromosomes of birds. In birds, two of the same chromosomes (Z and Z) make a male and two different chromosomes (Z and W) make a female.

The researchers found that the mutation rate in male birds was higher than in females. So, it seems, the male birds are in the evolutionary driver's seat.

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