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Now British scientists have an explanation: Males are
required for a process known as "sexual selection" which helps
species to ward off disease and avoid extinction.
A system where all offspring are produced without sex -- as in
all-female asexual populations -- would be far more efficient at
reproducing greater numbers of offspring, the scientists said.
But in research published in the journal Nature on Monday, they
found that sexual selection, in which males compete to be chose
by females for reproduction, improves the gene pool and boosts
population health, helping explain why males are important.
An absence of selection -- when there is no sex, or no need to
compete for it -- leaves populations weaker genetically, making
them more vulnerable to dying out.
"Competition among males for reproduction provides a really
important benefit, because it improves the genetic health of
populations," said professor Matt Gage, who led the work at
Britain's University of East Anglia.
"Sexual selection achieves this by acting as a filter to remove
harmful genetic mutations, helping populations to flourish and
avoid extinction in the long-term."
Almost all multi-cellular species reproduce using sex, but its
existence is not easy to explain biologically, Gage said,
because sex has big downsides -- including that only half of the
offspring, the daughters, will produce offspring themselves.
"Why should any species waste all that effort on sons?" he said.
In their study, Gage's team evolved Tribolium flour beetles over
10 years under controlled laboratory conditions, where the only
difference between populations was the intensity of sexual
selection during each adult reproductive stage.
The strength of sexual selection ranged from intense competition
-- where 90 males competed for only 10 females -- through to the
complete absence of sexual selection, with monogamous pairings
in which females had no choice and males no competition.
After seven years of reproduction, representing about 50
generations, the scientists found that populations where there
had been strong sexual selection were fitter and more resilient
to extinction in the face of inbreeding.
But populations with weak or non-existent sexual selection
showed more rapid declines in health under inbreeding, and all
went extinct by the tenth generation.
(Reporting by Kate Kelland; editing by Andrew Roche)
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