Why Males Exist

Sex really doesn’t seem like the most efficient way to reproduce, so why do so many species do it?

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May 18 2015, 3:50pm

Image: Justin R/Flickr

Males: who needs them? It's a valid question—the existence of the male sex is a real scientific inquiry.

On a survival-of-the-species level, males are basically only needed to provide sperm. In light of that, sex seems like a pretty inefficient way of reproducing in comparison to asexual reproduction. Half the offspring produced are also incapable of directly bearing future generations, and parents have to dilute their genes for the pleasure. Not to mention the hassle of finding a suitable mate in the first place.

So why has evolution allowed two-parent reproduction to persist when it seems so inefficient? Why is sex still a thing? Why don't we just have females who can reproduce by themselves?

In a paper in Nature, biologists suggest a solution to the mystery: the existence of males and of sex allows for sexual selection, which helps keep a species healthy so it doesn't go extinct. They tested this hypothesis in populations of beetles.

Sexual selection helps to purge out 'bad genes' from a population.

Sexual selection is a type of natural selection whereby members of a species (usually male) compete to reproduce with others (female)—think peacocks showing off. The imbalance there is borne broadly from the fact that females usually need to invest more effort into reproducing and are more limited in terms of how often they can try—though the details naturally differ among species.

In the new paper, the researchers write that this kind of competition creates "filters for reproductive success, so that only a subset of males gains paternity." The idea behind sexual selection is that the ones who do reproduce are in the best genetic health. In an email, author Matthew Gage from the University of East Anglia explained it "helps to purge out 'bad genes' from a population."

Small mutations in DNA can add up to have negative effects on a species; if the males given most opportunity to reproduce are those with lower mutation loads, the future of the species will be genetically fitter.

The researchers tested this by raising populations of flour beetles (the kind that often infest store cupboards) in environments that differed only in terms of sexual selection. In some cases, the ratio of male to female bugs was nine to one, meaning competition was intense, whereas in others the inverse was true.

Tribolium castaneum, the flour beetle used in the experiment. Image: Public Domain/Wikimedia

The beetles were kept in their different situations for seven years, going through about 50 generations. To see how healthy the resulting beetles were, the researchers inbred them—as this helps reveal any mutations by keeping them in the line.

"What we found was the populations with histories of high sexual selection carried less mutation load because when we inbred them they maintained fitness and resisted extinction," Gage wrote. "By contrast, populations with histories of low sexual selection rapidly lost fitness under the same inbreeding and went extinct within a few generations."

Of course, flour beetles are but one species, and there could be considerable differences in others. Gage suggested that species where males invest more into their offspring, such as by caring for them, could represent an exception—while sexual selection could still be at play, it wouldn't have as clear an effect as in the beetles, which reproduce a lot with different partners.

But the report nevertheless offers an interesting experimental insight into why the world bothers with males; this kind of sexual selection can only exist if sexual reproduction as we know it exists, and that needs males. A female-only species might lose out on the gene mutation-filtering benefits of being choosy with a mate.