Sex isn’t quite what it seems – while superficially wasteful in an evolutionary sense (why pass on only one half of your genes, when you can inherit all of them asexually, or why waste resources in mating when you don’t need a mate asexually?), theory and empirical studies have argued for the evolutionary advantages of sex in the context of favoring recombination, efficacy of natural selection in purging or fixing new mutations, and thus evolving faster (see my review of literature and arguments for the evolution of recombination, and thus sex).
My favorite summary on the subject by far has to come from Sarah Otto, in the article on “Sexual Reproduction and the Evolution of Sex”, which decimates some common misconceptions on the evolutionary advantages of sex. You think it’s pleasurable? As Otto puts it,
The first eukaryotes to engage in sex were single-celled protists that appeared approximately 2 billion years ago, over 1.3 billion years before development of the first animals with neurons capable of assessing pleasure…Surely, pleasure was not in a bacterium’s realm of experience.
Otto also argues using a simple example how sex evolves adherent to recombination producing new combinations of genes, maintaining diversity within a population, whereas non-recombining, or asexual populations deleterious mutations in a “Muller’s ratchet”.
Birds do it, and bees do it. Indeed, researchers estimate that over 99.9% of eukaryotes reproduce sexually. What, then, are the true costs and benefits of sex?
However, there has been little empirical evidence that points directly towards the adaptive advantages conferred by sex. McDonald et al. (2016) sought genomic evidence of what we’ve known for a good part of population genetics since the modern synthesis – does sex speed adaptation? Using experimental evolution strains of S. cerevisae across 12 asexual (mitotic, budding) and 6 sexual (meiotic, sporulating) populations for ~1000 generations. Fitness assays of evolved populations were performed against ancestral strains. Genomes of every 90th generation were sequenced across four sexual, and one asexual population.
Key findings from this study include (1) the striking significant increases in fitness of sexually evolving populations, compared to the asexual populations (see above figure), (2) an average of 44 de novo mutations per population, with similar proportions of nonsynonymous, synonymous, and intergenic mutations between sexual and asexual populations (indicating that there was no net difference in the nature of mutations that were segregating or fixed in the asexual versus the sexual populations), (3) but importantly, a significant difference in rates of fixation of de novo mutations – fewer new mutations fix in sexual populations, indicating that sex improves the efficiency of natural selection, disallowing potentially harmful mutations to fix, and (4) the ubiquity of epistasis wherein a new mutation mayhaps be beneficial in one genomic background, but deleterious in another. This also makes it difficult to make an umbrella statement about which new mutations are “harmful” or “beneficial”, since it is bound to depend on the genomic background in which the mutation arose.
Future studies are needed to fully understand the consequences of this interplay between sex and balancing selection, and to investigate how epistasis interacts with recombination to alter the dynamics of sequence evolution. By combining precise control of the sexual cycle with whole-population time-course sequencing, this experimental system offers the potential to understand how these factors affect the rate, molecular outcomes, and repeatability of adaptation.
McDonald, Michael J., Daniel P. Rice, and Michael M. Desai. “Sex speeds adaptation by altering the dynamics of molecular evolution.” Nature 531.7593 (2016): 233-236. DOI: 10.1038/nature17143
Otto, S. “Sexual reproduction and the evolution of sex.” Nature Education 1.1 (2008): 182.