Everything you ever wanted to know about the evolution of sex (But were too afraid to ask)
Just to warn you, this episode contains discussion of and references to sexual reproduction in humans and other species. And, knowing me, probably some pretty bad sex puns too. So bearing that in mind, let’s get it on…
Sex is everywhere. Not only do birds, bees and educated fleas do it - in the words of Noel Coward - 99% of all multicellular creatures on earth reproduce sexually. From the hayfever-inducing pollination of plants to the mass spawning of salmon, insects, mammals, birds and pretty much everything else is getting busy - finding mates, swapping cells and making the next generation. And yes, that does include giant pandas - whose legendary lack of sex drive is actually more myth than reality.
Even single celled organisms like yeast which normally reproduce asexually by budding can have sex. There are two mating types in yeast - ‘a’ and ‘alpha’ - and when an a and alpha get in close enough proximity, they both form an elongated shape known as a shmoo, named after the strange characters in a 1940s cartoon. Then they fuse together, mixing up their genes and then dividing back into two cells. It’s not exactly sexy, but it certainly is sex.
And although bacteria only reproduce by splitting in two, they can still swap bits of DNA called plasmids through a primitive kind of sex known as conjugation, where one bug sticks a little tube into another and shoves through some genes.
But while we can describe all the weird and wonderful ways in which the great diversity of life makes its babies, we still don’t really know why.
So, let’s get down and dirty - in a purely biological sense, of course - and find out.
This question of why species have sex baffled the great evolutionary biologist Charles Darwin, who wrote: "We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements. The whole subject is as yet hidden in darkness."
However, Darwin and his wife Emma Wedgwood did manage to knock out ten kids, so at least some of the technicalities can’t have been a complete mystery.
Compared with asexual reproduction - quickly budding off identical versions of yourself like yeast or bacteria, or sending out runners like strawberries - sex is a risky business. Not only do you have to find an appropriate member of your species to mate with, there can also be a significant cost.
Organisms will go to a colossal amount of effort to attract a mate - think of the huge tails of male peacocks or the beautifully decorated boudoirs of bower birds, exquisitely evolved flowers and the bright blue face and matching genital combo sported by many species of monkeys. Or the lengths that members of our own species will go to in order to attract someone to get frisky with.
And in many species, one of the pair - usually the female - is left holding the baby, or at least investing more of her resources into her offspring, while the male contributes little to the family. One particularly notable exception is seahorses, where the female deposits all her fertilised eggs in a pouch on the male’s belly and then buggers off, leaving dad to raise all the sea-kids.
So given all the time, trouble and effort, why bother having sex at all?
It must be worth it, given that sex is still here more than 2 billion years after it first evolved, and most species on earth do prefer to reproduce that way.
German evolutionary biologist August Weismann put forward one explanation in the 19th century, suggesting that sexual reproduction reshuffles the genetic deck of cards between generations, allowing organisms to pool and divide their genetic resources and provide differences upon which evolution can act.
To put it simply, sex creates variation, which is the fundamental fuel for natural selection. And for a long time, that’s been one of the main accepted reasons for the evolution of sexytimes.
But this might be too simplistic a view. Researchers have now shown that this genetic shuffling isn’t always preferable compared with asexual reproduction, and that it’s easier under normal conditions not to go to the trouble of mixing your gametes with someone else.
But that changes when times get tough. Under changing or challenging conditions, small populations or other stresses, it becomes advantageous to mix it up as much as possible, rolling the genetic dice with every generation in the hope that at least some of your offspring make it through to have little ones of their own.
Proof of this idea comes from studies of asexually reproducing single-celled species, which will turn into sexual beings if put under enough pressure.
Building on this idea, there are hints that maybe even cancer cells can have sex. While researching my next book about cancer genetics and evolution, I discovered that cancer cells can fuse together under the stress of chemotherapy and produce little cells that are resistant to drugs. It’s a bit like the romantic sub-plot of a disaster movie: what do you do when you think your world is ending? Have sex.
One more unusual hypothesis comes from Fred Thomas and his colleagues, who recently suggested that sex evolved as a defense against transmissible cancers, which are caused by cancer cells moving from one organism to another, right back at the very start of multicellular life. Today, contagious cancers are very rare, such as the facial tumours that affect Tasmanian devils and none are known in humans, but they can and do arise in nature.
Mixing up the host’s genome with every generation means that the offspring are more likely to be genetically distinct from any cancer cells that they pick up, giving a better chance of spotting and destroying any foreign, rogue cells before they can take root and grow into potentially lethal tumours.
It’s a wild idea, but I love the thought that all the joy and pain of our human sex lives - not to mention sexually transmitted diseases - might have originally started as a way of our ancient ancestors protecting themselves from catching cancer off each other a billion years ago.
As well pondering why sex evolved in the first place, the other big question is why most species come in only two genetic sexes, male and female.
Males are biologically defined as the ones that make lots of small, mobile gametes like sperm or pollen, while females make fewer, larger eggs or ovules. This doesn’t necessarily correlate with chromosomes though, as genetically female mammals have two of the same sex chromosomes - XX - while males are XY. Birds do it the other way round, with males having two W chromosomes and females being WZ.
Things can get even more complicated in other branches of the tree of life, with the sex of some species of reptiles being determined by the temperature at which their eggs are incubated. Insects have the largest number of different sex determination mechanisms, ranging from simple X and Y chromosomes in fruit flies to strange jumping sex genes in house flies, and many other flavours in between.
Certain types of fish can even change sex during their lifetime. Clownfish, for example, will switch from male to female if there’s a lack of lady fish - a fact that might make you view the film Finding Nemo in a whole new light.
And while single-celled yeast have their two mating types, a and alpha, the simple slime mould Physarum polycephalum has more than 500 different options, depending on the particular combination of three genes that come in 13 possible variations. Each individual needs to find a mate with different variants of all three genes, making me wonder what kind of chaos the slime mould equivalent of Tinder might be like.
So, that’s the evolutionary imperative and the genetic bits. But you might have noticed that there’s another reason to get down to it - and that’s because sex feels good. And it’s not just humans that have noticed.
An intriguing paper published in 2018 showed that male fruit flies appear to experience pleasure from sex. A team of Israeli researchers used genetic engineering techniques to modify nerve cells in the brains of male flies that caused them to ejaculate in the presence of red light.
These adjusted insects kept coming back to their literal red light district, suggesting that they were enjoying the experience. Fascinatingly, the team also found that the modified flies that don’t manage to get to the red light will drown their sorrows by choosing to drink alcohol if they can’t get their rocks off.
However, while it’s a fun story, there’s also a chance that the modified nerve cells were actually firing off other pleasure systems in the flies’ brains, with ejaculation being an unexpected sexy side effect.
Still, I can definitely sympathise with the need to knock back a couple of stiff drinks after being turned down. So, errrr… fancy a snog?
References and further reading
Ejaculation Induced by the Activation of Crz Neurons Is Rewarding to Drosophila Males. Shir Zer-Krispil et al (2018) Current Biology VOLUME 28, ISSUE 9, P1445-1452 https://doi.org/10.1016/j.cub.2018.03.039
Transmissible cancer and the evolution of sex. Frédéric Thomas et al. PLoS Biology https://doi.org/10.1371/journal.pbio.3000275