What Happens When Natural And Sexual Selection Collide?

An assassin bug with a male broad-horned flour beetle that made the mistake of over-investing in its mandibles and under-investing in its running-away capacity. Image Credit: University of Exeter

Charles Darwin famously discovered evolution through natural selection but noted this wasn't the only driver of biological change, pointing out sexual selection also plays a role. Other scientists have since added more subtle or unusual factors. Sometimes natural and sexual selection appear to be pushing in opposite directions, but this tension has been relatively understudied experimentally.

Sexual selection occurs when one sex of a species has a preference for certain traits in the other, even if they are not exactly survival enhancing. Extreme cases, such as peacocks' tails, can be a major disadvantage when confronted by predators, at least until humans decided to like and protect them. More commonly, sexual selection takes to attributes useful for other purposes, such as antlers – initially applied to fighting off predators, but also used for mating combats.

In a paper published in Nature CommunicationsProfessor David Hosken of the University of Exeter studied the way natural and sexual selection interact using broad-horned flour beetles. Beetles' short lifespans compared to vertebrates allow for study over many generations. It is also easier to get ethics approval.

Like many mammals, the male beetles have evolved large weapons, in their case mandibles, with which to fight for access to females – an example of sexual selection. Like Pecora, the male beetles have had to develop larger heads and thicker necks to support their mighty mouthparts. Females, meanwhile, have larger abdomens allowing them to carry more eggs.

A comparison of the heads of male and female broad-horned flour beetles, showing the larger male mandibles and head to support them. Image Credit: University of Exeter

Hosken added assassin bugs to this environment. The bugs mostly ate the males with the largest mandibles, probably because they couldn't run away as fast, but showed no preference among females. Natural selection, in this case, goes against sexual selection.

After repeating the process over eight generations, Hosken compared the surviving beetles with a control group protected from predators. He reports that the females in the predated population produced 20 percent more offspring through their lifespan. This finding is not a surprise, in light of past evidence the daughters of large-mandibled males have fewer offspring. Nevertheless, it is one of the first times this sort of interplay has been studied experimentally.

"Males and females of every species share genes, but in some cases - including broad-horned flour beetles - the genes good for one sex aren't always ideal for the other," Hosken said in a statement. In humans, this plays out across our hips. “Optimal hips for women would be broad enough to allow childbirth, while optimal hip width for men is narrower. Humans reach a sort of evolutionary compromise, in which neither males nor females get the body shape that would be optimal for them," Hosken added.

Although Hosken's description of a human pelvic truce has long been accepted, it's not exactly practical to run controlled trials on people, whereas this study proves the principle. "Our findings show that sexual selection favouring large-horned males drags female body shape away from the female optima,” he added. Natural selection, it would seem, rectifies this.

Sometimes it can seem that sex is the most powerful force in the universe, but it turns out there may be a stronger one.


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