X chromosome drive

Jaenike, J,  Current Biology,  18:R508-R511. 2008.

In the past 10 years, the world record for the men’s 100 meter dash has declined from 9.79 to 9.74 seconds, the detection of such small differences being made possible by sophisticated electronic timing devices. If someone were to run the 100 meters in 9.73999 seconds in the 2008 Olympics, would the timers be sensitive enough to show him to be the world’s fastest human? Natural selection could, as differences in fitness of that magnitude (10?6) can be detected in species with large effective population sizes. In this way, natural selection can bring about the evolution of exquisitely well-adapted creatures.; Getting back to the race, imagine that one of the runners somehow manages to get a 50-meter head start. With such an advantage, even an overweight, out of shape, or injured athlete could win the gold. An analogous situation applies in evolution. Mendelian segregation, in which the two alleles carried by a heterozygous individual are passed to equal numbers of gametes, ensures that alleles compete fairly and that they succeed (or fail) on the basis of their effects on survival and fertility. Meiotic drive — the process by which alleles are not represented equally in an individual’s gametes — subverts the entire process. In the best documented examples of drive, one allele may be passed on to ?100% of an organism’s gametes, equivalent to a runner getting a 50-meter head start in the 100 meter dash. The overrepresentation of such alleles in gametes can more than make up for any associated deficiencies in survival and fertility. As a result, these alleles — and those closely linked to them — can rapidly spread through a population, and thus actually cause a decline in the adaptation of a species to its environment. If such an allele drives all the way to fixation, the species may end up a little less well adapted, but there would be no evidence that meiotic drive was the cause, as drive would no longer be expressed.