Sex, health and behaviour: Sexual reproduction might have emerged to provide better immunity against pathogens and further evolved to select for behaviour.

Abstract

Understanding 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 Plon, 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.” One big advantage of infectious pathogens compared with their larger and longer lived hosts, including humans, is that they evolve much faster through mutation. The only recourse is a highly diverse immune system capable of quickly adapting to new and mutating pathogens, according to Daniel Davis, Director of Research at the Manchester Collaborative Centre for Inflammation Research in the UK. “The genes that vary the most between individuals are those to do …