The existence of men has long puzzled scientists. Sexual reproduction is inefficient and costly, yet the majority of multicellular species opt for this method to keep their genes going. So why do men exist? Researchers from the University of East Anglia (UAE) believe they may have finally cracked it, suggesting that the evolutionary force known as ‘sexual selection’ plays a key role in improving population health and protecting us against extinction.

The study, published in the journal Nature, looked at Tribolium flour beetles to understand why most multicellular organisms rely on sex to reproduce. Under controlled laboratory conditions, researchers studied 50 generations of beetles over a ten year period and tested the impact of sexual selection. Famously theorized by Charles Darwin, sexual selection is a process whereby males compete for a chance 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?” lead researcher Professor Matt Gage, from UEA's School of Biological Sciences, said in a statement.

"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," he added.

The researchers found that when sexual selection was removed and beetles were paired up into monogamous couples, the population's health declined rapidly and the bugs were wiped out by the 10th generation. Conversely, beetles that had a strong influence on sexual selection, where intense competition saw 90 males trying to compete to reproduce with only 10 females, 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."

The study suggests that sexual selection plays a crucial role in sifting out harmful genetic mutations, as competition means females are less likely to mate with genetically inferior individuals. Even after 20 generations of inbreeding, the study found that the populations that were strongly influenced by sexual selection had a higher fitness and were able to maintain population health and avoid extinction.