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Males, sexual selection help keep populations genetically healthy

A pair of mating seed beetles attempt to disengage. Photo by Mareike Koppik
A pair of mating seed beetles attempt to disengage. Photo by Mareike Koppik

June 28 (UPI) -- Just a few males can fertilize the eggs of dozens of females. Thus, males have little impact on population growth.

However, new research -- published this week in the journal Evolution Letters -- suggests males help populations remain genetically healthy.

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Even though the numbers are in their favor, males must still compete for access to mates. Over the long-term -- in theory, anyways -- the most fit males will gain the most mating opportunities.

This competition, called sexual selection, helps ensure deleterious genetic mutations are purged, while healthy genes are passed on.

"When deleterious mutations are purged from a population through rigorous selection in males, resulting in fewer males reproducing, the process can take place with little or no effect on population growth," lead study author Karl Grieshop said in a news release.

"This is because relatively few males suffice to fertilize all the females in a population, hence, whether those females are fertilized by few males or many males makes little or no difference to the number of offspring those females can produce, especially in species where the male doesn't look after its own offspring," said Grieshop, an evolutionary biologist at Canada's University of Toronto.

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"By contrast, such rigorous selection in females would result in fewer females reproducing, hence fewer offspring produced, which could lead to a massive population decline or even extinction," Grieshop added.

To better understand how sex selection influences population-wide genetic health, researchers monitored the movement of gene mutations among 16 genetic strains of seed beetles, Callosobruchus maculatus.

After inbreeding and crossbreeding different seed beetle strains, researchers were able use models to accurately predict the mutations unique to each strain.

When researchers compared inbred strains to the crossbred strains, they found mutations negatively affected males and females equally.

However, when scientists look only at crossbred strains, which more closely reflect real world sexual selection dynamics, researchers found mutations were more likely to negatively affect males.

"This indicates that although these mutations do have a detrimental effect on females' reproduction, they are more effectively removed from the population by selection acting on male carriers than female carriers," Grieshop said.

"Previous research from our group and others has succeeded in showing this effect by artificially inducing mutations, but this is the first direct evidence that it ensues for naturally occurring variants of genes," Grieshop said.

In addition to highlighting the importance of male sexual selection to a population's genetic health, the new research could also help explain why so many multicellular organisms reproduce sexually, as opposed to asexually.

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"Production of males causes a decrease in the reproductive capacity of a species, since males themselves contribute less than females to the production of offspring," said study co-author David Berger.

"The question, then, is why a species evolves to reproduce sexually, instead of just producing females through asexual reproduction," said Berger, an ecologist and research scientist at Uppsala University in Sweden

"Our study shows that production of males, which may engage in intense competition for the chance to mate, enables faster purging of deleterious mutations from the population, which could thereby enable a healthier set of genes and higher reproductive capacity relative to asexual reproduction," Berger said.

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