It's a classic tale of how mediocrity is maintained. Evolutionary biologists in California have discovered that when males shower attractive females with attention, it actually undermines those females' fitness as mothers. That means fit females don't pass their genes on.
Today PLoS Biology published a study of fruitflies, a species where the male flies show a marked preference for mating with larger females because they are more fecund. The problem is that the males show such aggressive preferences that they basically badger the females constantly to mate. What this means is that the females are so harried that they have less time to search for food, which degrades their health. Also, among fruitflies, the mating process is itself damaging to the health of the females - fruitfly sperm is toxic.
As a result, the most-desired females become far less capable of generating healthy offspring. And the smaller, less fit females wind up bearing as many offspring as the fitter ones. In the end, the males' aggressive mating with the fittest females ends up preventing their species from evolving into a much fitter group.
Tristan A. F. Long, one of the authors of the study, said:
These larger females are disproportionately harassed and harmed, by males attempting to obtain matings. When these males are ‘choosy' with their courtship, there may be negative consequences to the species' ability to adaptively evolve.
What's interesting about this study is that it's one of the few to point out how male mate choice affects evolution of a species. Usually female mate choice is emphasized, except in species where females are dominant. Here we can see clearly that male mate choice is having a profound and not very salutary affect on the future of fruitfly fitness. The issue here is obviously not attractiveness, but instead the kind of fitness associated with being larger and more fecund. If larger, "attractive" females are harrassed into reproductive uselessness by the males, then any traits they possess that make them healthier (a trait for metabolic efficiency, for instance) won't be able to spread through the population as quickly as it might if males chose mates randomly.
via PLoS Biology