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Microbes Drove the Evolution of Sex

University of Adelaide researchers have developed a computer simulation model that supports the theory that sexual reproduction 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 reproduction 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 be changing 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 the Journal of Evolutionary 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.