On Why We Have Sex

Updated: Aug 22, 2018

By Owen Page


We all, more or less, know what sex is and how it happens. But why do we have sex? Why has this form of reproduction been chosen time and time again evolutionarily over other modes? This is an incredibly intriguing question that has puzzled biologists for some time. In a recent paper, McDonald et al. have given insight to the complex issue. There are two main issues with sex that make the process more energetically involving than asexual reproduction: the cost of having males and the cost of meiosis (the manner in which you get ½ of your genes from each parents). Asexual populations, to put it in perspective, are able to replicate their population two times faster than sexual populations. Males do not give birth, only females, so they are not necessarily required for reproduction. Furthermore, due to the fact that we consist of ½ of our parent’s genes, this means the other ½ isn’t used and is wasted. Which begs the question: why sex? And why are most animals sexual?


There are two prevailing theories. The first theory is known as Mueller’s Rachet: in an asexual population, all individuals are identical and only find variation via mutation. If a detrimental mutation enters a population, like one that would make it harder for them to survive, this mutation would stay in the population from that point on. More mutations therefore group until fitness, or ability to give raise to offspring, completely decreases. Mueller’s argument is that recombination solves this problem by providing variation. The other theory is known as the “Red Queen” hypothesis. The argument here is that the environment that we live in is constantly evolving and changing and we therefore must be able to evolve at a rapid rate. Sexual reproduction enables us to do this. Between humans and diseases, we are often considered to be in an “arms race” for superiority as a disease like malaria continuously evolves so medicine isn’t effective and can infect us. Humans, however, have the benefit of being able to create solutions rather than simply attempt to out-evolve diseases.

Art by Macy Chutoransky

McDonald et al. further cement the importance of recombination in sex. As they mentioned, recombination speeds adaptation. Using S. cervisae, or yeast, and genome sequencing, a manner by which the very genes of DNA or the coding of all life could be surveyed for differences by focusing on single nucleotide polymorphisms, or SNPs. An example of a SNP would be that sickle cell anemia, a disease common in Africa, is a result of a single change in the DNA. Sexual and asexual populations were evolved and fitness was measured.


What was found is very interesting: evidence for Mueller’s Rachet theory was found to be present within the asexual populations, but in the sexual populations, such a fixation—when a gene or trait becomes the only possible outcome, like how everyone has a single head rather than two or three—is not present. According to McDonald et al., this is due to beneficial adaptations made by recombination. Recombination increases the presence of these beneficial mutations, or mutations that provide a new function that will help us survive. Sex, in a sense, is able to sort the bad mutations from the good which makes natural selection more efficient by increasing the rate of adaptation. An example of such selection would be the development of making sex feel good as organisms are more likely to have sex if it felt good than if it was, say, painful.


For humans, this doesn’t really matter all that much as females, unlike female turkeys, cannot impregnate themselves. We require sex to produce offspring. What McDonald et al. show is that sex, biologically, makes sense. So in short, have sex. And use protection. Unless you want to have kids, in which case you will be illustrating precisely the importance and the necessity of sex.


References:


McDonald MJ, Rice DP, Desai MM. 2016. Sex speeds adaptation by altering the dynamics of molecular evolutions. Nature, 531(7593): 233-236.

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