More Attractive Males Might Pass on Worse Genes

Promiscuous mallards might not be doing the next generation any favors. By Georgi Tsachev/Shutterstock

Prettier males might have more fun in terms of attracting more mates, but they might be doing a disservice to their species in the process. This is according to a new study by researchers at the University College London which found that promiscuity in male birds leads to small genetic faults in the species' genome, reports

In many bird species that exhibit strong sexual dimorphism, or differences in appearance between males and females, promiscuity is the norm. This can be commonly observed in species where one sex is more colorful (usually the male) than the other, such as with mallard ducks or peafowl. The reason for the link between promiscuity and attractiveness is that males with more vibrant feathers tend to attract more mates.

So what UCL researchers essentially found was that the more attractive a male is, the more likely he is to pass on faulty genes to the next generation.

"We've found that promiscuous birds that have to fight others for mating rights have a genome that evolves faster than birds which are monogamous and pair for life. What's interesting is that this evolved genome includes mildly negative mutations. So a male may be attractive to a female and fight hard to mate with her but he doesn't deliver at the genetic level. As a result, his descendants will be less fit," explained study author Judith Mank.

The genetic faults passed on to later generations by attractive males are small, but they could accumulate over the course of a species' evolution, thus making the species less able to adapt to changing environments over time.

The findings represent a classic example of the delicate and tenuous balance that exists between sexual selection and natural selection. That is, though some individuals possess traits that make them better at securing mates than others, these traits are not necessarily adaptive. A peacock's tail makes for a good example. His vibrant tail might be attractive to females, but it is a burden to his survival. Such a large tail makes him easier to spot for predators, and easier to catch.

Though sexual selection is a powerful instrument of evolution, natural selection still gets the last laugh. A species can't survive to the next generation on attractiveness alone. So there's a catch to being pretty, and the UCL study provides evidence that this even extends to the genetic level, in the sense that promiscuity can lead to an unhealthy proliferation of faulty mutations.

"We found a significant association between the turnover of male-biased genes and the extent to which birds use physical ornaments to attract mates. We predicted a link between gene expression evolution across species and the degree of sexual selection, but this is the first statistical evidence for it and shows how powerful sexual selection can be in leading to major changes in how a gene is expressed," explained Mank.