Why does evolution allow males to exist?

Chuck Bednar for redOrbit.com – @BednarChuck

Evolutionary selection is all about efficiency, so why does it permit the continued existence of males and the reliance of sex as the dominant mechanism for reproduction? The authors of new research published Monday in the journal Nature believe they have found the answer.

Lead investigator Matt Gage, a professor in the University of East Anglia’s School of Biological Sciences, explains that sexual selection – when males compete to be selected by a female as her mate – improves the overall health of populations while also protecting them against extinction, even in the presence of genetic stress due to high levels of inbreeding.

The problems with sex

Gage explained in a statement that the existence of two different sexes encourages the processes of sexual selection, which ultimately dictates which members of a species get to reproduce their genes into the next generation. He added that while this is a “widespread” and “powerful” force, its existence has been difficult to explain because of the inherent problems with the practice.

“Sex carries significant constraints when compared to the obvious alternative which we know can exist – asexual reproduction,” he told redOrbit via email. “Sex requires that: (a) half your offspring (sons) do not contribute to offspring production, (b) only half of the genes in your offspring are yours, (c) you have to waste time and effort and suffer known costs of finding a mate and mating with them, and (d) if you do carry perfectly adapted gene complexes, these are then more likely to get broken up and/or diluted by sex.”

“These four consequences of sex carry serious constraints compared to an asexual route of offspring production where all–daughter progeny just produce reproductive ‘females.’” added Gage. “Why, therefore, are sex and the production of males so widely evolved as the dominant mode of reproduction?”

Using Tribolium flour beetles as a sexual selection model

As part of their research, the study authors evolved several generations of Tribolium flour beetles under controlled laboratory conditions over a 10-year span, with the only differences between the different populations being the intensity of sexual selection during the adult reproductive stage.

Why choose the Tribolium flour beetle? Gage told redOrbit it was because it is “an excellent model for understanding reproductive evolution in action” and it allowed them to “replicate very large numbers of experimental trials so it allows real research rigor and statistical power.” Since it has a generation time of about one month, it allowed them to perform experimental evolution and see how the lineages and populations of the beetles change under controlled circumstances.

He added that Tribolium flour beetles typically reproduce through a promiscuous mating pattern in which males provide no direct care to their offspring. This is common behavior in nature, and using these creatures as test subjects allowed them to generalize their findings.

Males play a key role in preserving genome health

In the experiments, sexual selection strength ranged from intense competition (a 9-to-1 male-to-female ratio) to the complete absence of competition (males in females in monogamous parings). After seven years and nearly 50 generations of beetles, Gage’s team used experimental inbreeding and found that populations in which there had been strong sexual selection were more resilient to the threat of extinction from inbreeding than those that did not face such competition.

In fact, some of the populations that experienced stronger sexual selection survived even after 20 generations of inbreeding, while those that experienced weak or non-existent sexual selection all went extinct by the 10th generation. Gage explained that the findings demonstrate that competition among males improves the overall genetic health of a population.

He elaborated on this concept in an email to redOrbit, explaining that sex and males persist for one of two main reasons (or perhaps for both): That sex purges bad genomes from a population and/or that sex helps spread good genomes through a species.

The UEA professor added that his team’s research provided an experimental advance to explain how populations tolerate the persistence of sex and the continued existence of males, since they provide an effective way to purge negative mutations from the species. The remaining questions are: How far these observations can be applied across other species, how important is it to get rid of these mutations, and how much genomes vary due to prevalence of sexual selection.


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