Sex, health and behaviour

U nderstanding the emergence of sexual reproduction has been one of nature’s long-standing mysteries that even now is only partially understood. There is a clear distinction between the reason sex evolved in the first place in early eukaryotes—and even before that in prokaryotes—and why it has been so stubbornly maintained in virtually all higher species. After all, asexual reproduction is superior in a strict Darwinian sense, since it yields more offspring per unit time without having to waste energy on finding mates. On the question of the origin of sex, there is little progress, with plenty of competing theories but scant evidence. But a clearer picture has emerged of why sex has resisted the pull towards asexual reproduction and what has driven its evolution. According to Manfred Milinski, Director of the Max Planck Institute for Evolutionary Biology in Plön, Germany, the bottom line is that the constant threat from parasites and pathogens has maintained the need for sex. “In a world without infectious diseases, asexual reproduction would prevail,” he explained. “However, with asexual reproduction, there is no evolutionary improvement, only genetic degeneration through fatal mutation accumulation. Infectious diseases are indeed drivers of sexual reproduction and thus evolution. So if we removed all infectious diseases, we would return to asexual reproduction and disappear as a species after about 200 generations because of inevitable mutation accumulation. Infectious diseases help us survive.”