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Stirling University scientists explain existence of sex

Starfish, banana and komodo dragon Image copyright Thinkstock
Image caption No sex for starfish, bananas or (occasionally) komodo dragons - but scientists believe they could be missing out on evolutionary benefits

A team of scientists at the University of Stirling say they have finally worked out why sex exists.

Organisms that clone themselves use much less time and energy - which poses the question of why sex is still the dominant form of reproduction.

The team studied creatures that unusually use both clonal and sexual methods of reproduction.

They concluded that sexual reproduction evolved to help future generations fight infection.

Sex allows genes to mix, allowing populations to evolve quickly and adapt to changing environments.

Image copyright Thinkstock
Image caption Scientists found sex has benefits not just for the individuals involved - but also for the species as a whole

Plants and animals that can reproduce without sex include komodo dragons, starfish and bananas.

Clonal populations should always reproduce faster than sexual ones - so the scientists believed that there had to be large-scale benefits to sex which made a difference to the next generation.

But the theory has been difficult to test as most organisms are either wholly sexual or clonal and cannot be compared easily.

The team at Stirling used waterfleas to test the costs and benefits of sex as they can reproduce both ways. They found that sexually-produced offspring were more than twice as resistant to infectious disease as their clonal sisters.

Image copyright University of Stirling
Image caption Birds do it, bees do it. Even educated waterfleas do it... sometimes

Dr Stuart Auld, of Stirling University's Faculty of Natural Sciences, said pondering why sex exists when it takes up so much time and energy was one of the "oldest questions in evolutionary biology".

"Sex explains the presence of the peacock's tail, the stag's antlers and the male bird of paradise's elaborate dance.

"But if a female of any of these species produced offspring on her own, without sex, her offspring should come to dominate, while the other females watch the redundant males fighting and dancing. So, why are we not surrounded by clonal organisms?

"By comparing clonal and sexual daughters from the same mothers, we found sexually produced offspring get less sick than offspring that were produced clonally. The ever-present need to evade disease can explain why sex persists in the natural world in spite of the costs."

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