The Evolution of Sex

Abstract The evolution of sexual reproduction has perplexed biologists for decades. The earliest organisms on Earth could replicate by themselves, so why do almost all multicellular species combine their genetic material with other individuals to reproduce? After all, asexual reproduction was very successful for well over a billion years. It remains the only mode of reproduction in all of today’s bacteria and is used by many multicellular organisms at certain times in their lifecycle. Sex is costly. It is characterized by increased time and energy at the cellular and organismal levels, may result in injury or even death, and exposes individuals to predators and parasites. Conversely, it can repair damaged stretches of DNA and create the variability necessary for natural selection. Furthermore, assuming that there is some benefit to reproducing sexually, why are sex partners so different? Given that the objective of both individuals is to optimize the production of high-quality progeny, one would expect all members of a species to be similar. Instead, the two types of individuals we denote as biological males and females often exhibit an array of anatomical and behavioural differences that may influence their approach to sexual encounters. Interactions between the two can be co-operative, but their unique strategies often reveal an underlying conflict. This book explores the possible reasons why sex evolved, examines how the differences between males and females arose, and discusses how those differences affect their reproductive strategies.