From an evolutionary perspective, sexual reproduction could be seen as a non-starter.
Compared with cloning, it is a major waste of time and energy.
Even without predators, sex and its attendant rituals can be dangerous: When stags butt heads or alpha-male lions fight for mating rights, it does not always end well.
Some animals and plants — starfish and bananas, to name two — reproduce asexually. Even a few birds and bees do it solo, Others, like the Komodo dragon, can work it either way.
In short, without males in the picture the business of reproducing is faster and less fraught.
And yet, sex remains by far the dominant means by which the world’s fauna and flora pass on genes to future generations.
“One of the oldest questions in evolutionary biology is: Why does sex exist?” said Stuart Auld, who is a biologist at the University of Stirling in Scotland.
The process of natural selection proposed by Charles Darwin dictates that doing it the hard way — sex rather than cloning, in this case — confers some major advantages.
Granted, sexual reproduction fuels genetic variation, which boosts the likelihood that offspring in the wild will have the genetic makeup to thrive in an ever-changing environment.
By contrast, clones do not vary, and so if the environment deteriorates, a clonal mother will produce offspring that lack the genes they need to succeed.
However, “sex needs to be over twice as efficient as cloning to outweigh its costs,” Auld told reporters.
“If sex is to be favored by natural selection, a sexual mother needs to either produce twice as many offspring as an asexual mother, or produce offspring that are twice as good.”
Biologists have long agreed that the enhanced ability to fight off disease was a major advantage of the genetic changes that come with sexual reproduction.
However, constructing an experiment to confirm this has always proved difficult: How do you compare the costs and benefits of sexual strategies in different species?
To get around that “apples and oranges” problem, Auld and two colleagues used an organism — the humble waterflea — that can reproduce both ways.
“By comparing clonal and sexual daughters from the same mothers, we found sexually produced offspring get less sick,” Auld said.
The ever-present need to evade disease, it turned out, explains why sex persists in the natural world in spite of the high “costs” that come with it, he said.
According to evolutionary ideas, parasites and their hosts are in a constant tug-of-war, evolving defenses and building means to overcome them.
The findings were published in the British journal Proceedings of the Royal Society B.
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