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dobeUniversity of Adelaide researchers have developed a computer
simulation model that supports the theory that sexual reproduc-
tion evolved because of the presence of disease-causing microbes
and the need to constantly adapt to resist these co-evolving
pathogens.
“Asexual reproduction, such as laying unfertilised eggs or budding
off a piece of yourself, is a much simpler way of reproducing,” says
Dr Jack da Silva. “It doesn’t require finding a mate, and the time
and energy involved in that, nor the intricate and complicated
genetics that come into play with sexual reproduction. It’s hard
to understand why sex evolved at all.”
According to the Hill-Robertson Interference theory, sex evolved
because it allows the recombination of DNA between mating pairs
so that individuals are produced that carry more than one beneficial
mutation. Otherwise beneficial mutations compete with each other
so that no single mutation is selected over another. However, da Silva
says that this “elegant theory” doesn’t explain why sexual reproduc-
tion would be maintained in a stable, well-adapted population.
“It is hard to imagine why this sort of natural selection should
be ongoing, which would be required for sex to be favoured,” he
says. “Most mutations in an adapted population will be bad. For
a mutation to be good for you, the environment needs to bechanging fairly rapidly. There would need to be some strong ongoing
selective force for sex to be favoured over asexual reproduction.”
An answer lay in bringing another evolutionary theory into the
equation. The Red Queen theory says that bacteria, viruses and
parasites are continuously adapting to us, so we are constantly
having to evolve to become resistant to them. This provides the
opportunity for new mutations to be beneficial, and maintains a
strong selective force.
“These two theories have been pushed around and analysed
independently, but we’ve brought them together,” da Silva says.
“Either on their own can’t explain sex, but looking at them together
we’ve shown that the Red Queen dynamics of co-evolving pathogens
produces that changing environment that makes sex advantageous
through the simple genetic mechanism of the Hill-Robertson
theory.”
The new combined theory, published in theJournal of Evolu-
tionary Biology, was developed through computer simulations that
accurately reproduced the rapid evolutionary increase in the amount
of sex seen in nematode worms coevolving with a highly pathogenic
bacterium. “This is not a definitive test but it shows our model is
consistent with the best experimental evidence that exists,” da
Silva says.Microbes Drove the Evolution of Sex
Male Snakes Worn Down by Mating Frenzy
A frenzied mating season is causing male snakes to die earlier and in
worse condition than their female counterparts, who prioritise body
maintenance over short-term reproductive success.
A University of Sydney-led study of red-sided garter snakes in
Canada, published in Proceedings of the Royal Society B, found that:- males undertake energetic mating for 2–4 weeks per year;
- males don’t eat and must compete with thousands of other males
during the mating season; - females stayed at the orgy site for as little as 1 day, compared with
up to 21 days for males; and - snakes hibernate underground for 8 months in their communal
dens and emerge en masse in spring to form large aggregations
where males scramble to locate and mate with females.
Senior author Dr Christopher Friesen explained that the males
made good use of the relatively short amount of time to procreate,
having only 4 months a year to breed, feed and have babies.
“Although we believe that all females mate every year, they only stay
at the den sites for a short period of 1–3 days, much less than males,
who remain at least a week and up to 21 days,” Friesen said.
“Females reproduce every other year, which depends on their
stored fat/energy reserves. Our previous research has shown that
females can store sperm for up to 15 months or more before she uses
the sperm to fertilise her eggs.”
The paper reported: “The relationship between body condition
and age differed strikingly between sexes, with females maintaining
their body condition with age, while condition decreased with age in
males.”
In addition to prioritising self-preservation over sex, the female
garter snakes did not waste energy on looking after their babies
postnatally, which reflects the parenting approach of other snakes.