Why does nature tolerate males? They produce no offspring of their own, one consequence of which is that populations of plants and animals capable of self-fertilisation, or “selfing”, grow at twice the rate of populations that depend on sexual reproduction.

Yet, there must be evolutionary advantage in sex with a partner. Scientists at Oregon university argue in Nature magazine that their research with roundworms – which can self-fertilise or reproduce sexually – shows selfing populations are more susceptible to harmful genetic mutations and slow to adapt to swiftly changing environmental conditions. “Selfing populations are more likely to become extinct,” said Levi Morran, lead author.

Yet, selfing populations thrive and survive. One explanation, based on a discovery in rice plants which Susan Wessler of the University of Georgia described as “brand new and really stunning”, is that movable genetic material called transposable elements insert themselves at various points in the chromosomes, providing a source of genetic variation in response to changing conditions.