Sex is everywhere.
But just because we see it everywhere doesn’t mean that we can definitively explain why it’s everywhere.
Sexual reproduction is a costly endeavour: You need two parents to reproduce, and the male half of a sexually reproducing species is fundamentally incapable of directly producing offspring.
Given the Darwinistic and cutthroat rules of natural selection and the high evolutionary cost of sexual reproduction, why sex continues to exist, and why so many different organisms reproduce that way, is a fascinating and difficult question.
Before we jump to the research, let’s run through a refresher of the terminology.
Reproduction is the process by which organisms (parents) produce new organisms (offspring.)
There are two types of reproduction: asexual and sexual. With asexual, a given organism can reproduce by itself (without the assistance of another organism). Examples include budding — in which a new organism develops as a growth from the original organism — and cloning. The key thing about asexual reproduction is that the offspring is identical or extremely similar genetically to the parent.
Meanwhile, sexual reproduction requires two organisms, a male and a female. The male fertilizes a female of the same species to create offspring. The key thing here is that the offspring’s genetic composition derives from both parents, meaning it is not identical to either parent. A simple example: A child could have the hair colour of one parent but the eye colour of the other.
So what’s the big problem with sexual reproduction?
Natural selection is ruthless and efficient, but sexual reproduction is not the most efficient way of reproducing. After all, in asexual reproduction, all the organisms can reproduce, while in sexual reproduction, only half of the organisms can reproduce — the females.
In less abstract terms, two hydra can both reproduce asexually, which gives you two offspring. But a human female and a human male together only create one offspring (excluding the possibility of having twins, etc.) since the male cannot directly create offspring.
“This is a huge cost in evolutionary terms, so there must be something very valuable about [sexual reproduction],” Robert Axelrod, a political scientist at the University of Michigan and the mind behind a computer study of the iterated Prisoner’s Dilemma, told the Evolution Institute a few years ago. “The fact that sex is so universal means that it must be something that large animals and plants have in common.”
A look at the reproduction of yeast
So what gives?
As noted above, sexual reproduction leads to genetic variation in the offspring. And so it’s possible that sex, although costly in terms of the rate of reproduction, could help the offspring organism adapt to an environment better than its parents. (Anyone who took biology in high school likely remembers the example of the peppered moth during the Industrial Revolution.) This makes intuitive sense, but the trick is actually getting empirical evidence.
Notably, there are a few organisms that reproduce both asexually and sexually, which makes them the prime candidates to test the benefits of sexual reproduction. One such organism is yeast.
Yeast reproduce asexually when there’s an abundance of food. But if there’s not enough food, then they reproduce sexually.
Back in 2005, a team of researchers led by Matthew Goddard, then a postdoc at Imperial College London, genetically engineered yeast so that it would continue to reproduce asexually in dire circumstances. This, then, allowed them to compare how yeast that can reproduce sexually fare in tough conditions compared to those that could only reproduce asexually.
And here’s where it gets interesting: under normal conditions, both yeasts did equally well; but in the harsher conditions, the yeast that could sexually reproduce adapted more quickly and survived better.
“Our results indicate that sexual reproduction can provide a selective advantage during adaptation to new environments,” the team wrote in Nature. “A challenge now is to understand the nature of the mutations that underlie adaptation and to extend these techniques to larger plants and animals.”
Another stunning idea about sexual reproduction came from a paper produced by William Hamilton, Robert Axelrod, and Reiko Tanese back in 1990, which explored the idea of sexual reproduction as an adaptation helping organisms resist parasites.
“Darwinian theory has yet to explain adequately the fact of sex. If males provide little or no aid to offspring, a high (up to 2-fold) extra average fitness has to emerge as a property of a sexual parentage if sex is to be stable,” the team wrote in the abstract.
“The advantage must presumably come from recombination but has been hard to identify. It may well lie in the necessity to recombine defences to defeat numerous parasites.”
As Axelrod explained at the Evolution Institute:
“Parasites evolved to mimic our cells so that our immune system wouldn’t attack them. As a result, they can evolve around thirty times faster than we can since their generation time is so short. If you were to reproduce asexually it would mean you’d have an offspring that was almost identical to you, so the parasites that are adapted to you would also be adapted to your offspring. However, by reproducing sexually our offspring are quite different from us. Therefore, the parasites have to start all over. [Hamilton’s] idea was that sexual reproduction is an adaptation to resist parasites. It is just a brilliant idea.”
Here, too, offspring appear better set up for survival because of sexual reproduction. However, what’s notable in this theory is that offspring do not evolve just for an advantage to a changing environment — as with the yeast example — but rather just to survive as other rival organisms also change and therefore threaten the survival of the their species.
Why do males still exist?
One final interesting thing to think about is why men continue to exist if their primary contribution to reproduction is sperm. A possible explanation is sexual selection: Males competing for females, and females choosing among males.
In research published in Nature in 2015, a team from the University of East Anglia, led by professor Matthew Gage, evolved replicate populations of flour beetles over several years under controlled conditions. The only difference between populations was the intensity of sexual selection at each adult reproductive stage — which ranged from 90 males competing for 10 females to the “complete absence of sexual selection, with only single males and females in monogamous pairings, where females got no choice and males experienced no competition.”
And after seven years they found that populations with stronger sexual selection were fitter and more likely to survive than those without:
“Lineages from populations that had previously experienced strong sexual selection were resilient to extinction and maintained fitness under inbreeding, with some families continuing to survive after 20 generations of sib × sib mating [Editor’s note: sibling x sibling]. By contrast, lineages derived from populations that experienced weak or non-existent sexual selection showed rapid fitness declines under inbreeding, and all were extinct after generation 10.”
“Multiple mutations across the genome with individually small effects can be difficult to clear, yet sum to a significant fitness load; our findings reveal that sexual selection reduces this load, improving population viability in the face of genetic stress,” they added.
Ultimately, it’s important to note that it would not be appropriate to oversimplify or extrapolate from any of these studies. And, moreover, none of this even begins to touch on how sexual reproduction came about. However, it is still fascinating to consider the evolutionary benefits from sexual reproduction.