The existence of men has long baffled some scientists. Sexual reproduction is not efficient but the vast majority of species use this method of reproduction. So why do men exist? Researchers from the University of East Anglia (UAE) believe they may have cracked it, postulating that the force known as ‘sexual selection’ carries out a primary role in improving health and protecting a species against extinction.

The study was published in the journal Nature and focussed on Tribolium flour beetles to determine why multicellular organisms rely on physical sex to reproduce. Using controlled conditions, researchers looked at generations of beetles and studied the impact of sexual selection. Sexual selection is the process where males compete to reproduce and females choose which male to reproduce with.

Almost all multicellular species on earth reproduce using sex, but its existence isn’t easy to explain because sex carries big burdens, the most obvious of which is that only half of your offspring—daughters—will actually produce offspring. Why should any species waste all that effort on sons? We wanted to understand how Darwinian selection can allow this widespread and seemingly wasteful reproductive system to persist, when a system where all individuals produce offspring without sex—as in all-female asexual populations—would be a far more effective route to reproduce greater numbers of offspring.

The research found when sexual selection was taken away and beetles were paired up without choice into couples, the population’s health declined quickly and the beetles were gone out by the 10th generation. Wheras beetles that had influence on sexual selection, where intense competition were more resilient to extinction.

“To be good at out-competing rivals and attracting partners in the struggle to reproduce, an individual has to be good at most things, so sexual selection provides an important and effective filter to maintain and improve population genetic health,” said Gage. “Our findings provide direct support for the idea that sex persists as a dominant mode of reproduction because it allows sexual selection to provide these important genetic benefits.”

This study indicates that sexual selection plays a primary role in weeding out dangerous genetic mutations, as the competition means females are unlikely to choose a genetically inferior mate. Even following 20 generations of in-breeding, this study found that the beetles that were strongly focussed towards sexual selection had a higher fitness.