Unselfish Genes
Female flies that pick mates “fitter” than themselves have very little chance of passing that fitness on to their daughters, and the same goes for males that mate with women fitter than themselves: sons born from such a union are actually less fit than sons born to low-fitness ladies. In the genetic war between the sexes, genes that are good for one sex aren’t necessarily good for the opposite-sex children who inherit them. Alison Pischedda and Adam K. Chippindale discovered this by forcing fruit flies to have sex in various combinations of fit and unfit. Fitness was measured by how many offspring a fly could have.
Conventional wisdom holds that sexual selection is usually good for a species: it creates babies that are stronger, prettier, fitter, since sexual creatures tend to be attracted to mates who are fit in one way or another.
But Pischedda and Chippindale found that seeking out the perfect mate can be detrimental to offspring.
It turns out that certain fitness genes shared by male and female flies on the X chromosome express themselves differently depending on sex. So a gene on a male’s X chromosome might make him an incredibly prolific father, but that same gene expressed in his daughter would prevent her from reproducing in large numbers. Because males only pass along their Y chromosome to male babies, they never pass along their beneficial X genes to sons either.
Pischedda and Chippindale speculate that these genes are acting selflessly. They’re keeping the population diverse. Imagine if fit parents bred only fit children. If their children inherited the fitness gene, they would also spawn lots of children, and so on.
By cutting off fitness after one generation, we’re guaranteed a population whose genes come from a wide variety of sources. If Pischedda and Chippindale are right, sometimes genes work for the good of the species rather than the good of individuals.
Interestingly, the fittest fruit flies come from parents who are not very fit themselves.