Why do we have sex? If this question keeps you up at night, you either have really loud neighbors, or you have the makings of an evolutionary biologist. Some of the most brilliant minds in the field – William Hamilton, John Maynard Smith and George Williams – have spent much of their careers wondering about the value of sex. This is not a reflection on the quality of their sex lives. Rather, it has more to do with their creative insight and ability to look at the world with fresh eyes.

A billion years ago, our ancestors inhabited a world without sex. This was the era of the clones. In this strange world, all organisms reproduced by creating identical genetic copies of themselves, somewhat similar to how modern-day bacteria reproduce [1]. But this clonal strategy has a problem. Populations made up of identical twins are more vulnerable to infection. When a disease comes along, it doesn’t just wipe out a few individuals. It can take out the whole lot.

When sex arrived, it introduced a new pace to life. Organisms were mixing and matching genes in combinations never seen before. Imagine a world where you had to dress well to survive. In such a world, the invention of sex is like going from wearing uniforms to having your own wardrobe. You could pick a gene from here, another from there, and put together a novel offspring. And if a particular outfit were deemed ‘unfit’, it’s not a huge tragedy as there are plenty of alternatives.

In this way, sex helps us by innovating new evolutionary solutions and by protecting us from disease. But sex is not without its discontents. For one thing, sexual reproduction implies that you only pass down half your genes to your offspring. The other half come from the other parent, and they combine to make an offspring with a full set of genes. On the other hand, in asexual reproduction, the mother passes on a full set of genes to her offspring. So by adopting sex, your genes are travelling half as far. In evolutionary terms, this is a huge cost, and sex had better have a lot to offer for it.

Do the benefits outweigh the costs? We would certainly like to think so. But when evolutionary biologists did the math, they worked out that the answer is usually no. Your genes typically have more to gain if you reproduced asexually.

So what gives? Why, then, do so many species adopt a sexual lifestyle? Well, here’s a brilliant solution offered by Hamilton and others: if you are under constant attack by rapidly evolving parasites, then sex is a better strategy than cloning yourself. This idea came to be known as the Red Queen hypothesis and can be summarized in one line: it’s harder to hit a moving target.

According to this theory, the main purpose that sex serves is to rapidly change our genetic makeup in order to keep pace with the threat from parasites. The parasites themselves are also evolving in order to keep attacking us. It’s a race where everyone is running but no-one really gets ahead, quite like the race between the Red Queen and Alice in Through the Looking Glass. While this does have a nice literary ring to it, I prefer a more space-age analogy. I picture the eternal chase between the evil robots and the human race in Battlestar Galactica. Technology keeps evolving on both sides, and so the humans have to work just as hard as ever to stay one step ahead.

The Red Queen idea was a theoretical offshoot of evolutionary theory. And like any good theory, it made a clear, testable prediction. Species that are exposed to a greater load of parasites should be more likely to reproduce sexually. In the last few years this idea has found support in a beautiful series of experiments involving snails, led by Kayla King, a graduate student in the lab of Curtis Lively at Indiana University.

Snails have interesting sex lives. In many species, the snail has a choice – it can either mate with another snail, or it can directly clone itself. Hedging bets by ‘going both ways’ is a remarkably common strategy in the tree of life [2].

In a study published in 2009, the researchers focused on a type of snail called the New Zealand mud snail, that inhabited two different lakes in New Zealand. Crucially, both lakes also had a parasite that infected the snails. The parasite is called microphallus, a bit harsh for a worm that’s only a fifth of a millimeter long as an adult, in my opinion. And these parasites have a strange and alarming life cycle.

Picture this: the eggs of microphallus are eaten by the snails. They hatch into a larva, which begins to grow in the snail’s gut. The larva drills through the intestine, making its way to the reproductive organs. Here the parasite begins to multiply, and consumes much of the snail’s reproductive and digestive tissue, rendering it completely sterile. Eventually the body of the snail contains hundreds of tiny cysts. When a duck comes along and eats the snail, the next stage of the parasite’s life begins. These cysts hatch to form tiny worms, which spend their entire adult lives in the duck’s intestine. Here they meet other worms, mate and produce eggs – which completes the life cycle. If this picture sends a shiver down your spine, you’re not alone.

Now, ducks only live near the lake’s surface. And the parasite can’t survive without ducks, which means that it is basically confined to shallow water. If the Red Queen idea is correct, then a heavy parasitic load should lead to intense evolution of the host, through sexual reproduction. To test this idea, the researchers went to two different lakes and compared the snails that lived in shallow waters to those found in greater depths.

Here is what they found. If you collected both shallow and deep snails and exposed them to their local parasites, the shallow water snails had consistently lower rates of infection. However, if you tried to infect snails from one lake with parasites from the other lake, the shallow water snails would fare just the same as the deep water snails.

This meant that the shallow water snails were indeed co-evolving with their local parasites. The researchers also found that the frequency of snails adopting to reproduce sexually is significantly higher in the shallow water snails as compared to their deep water relatives. These results are just what you would expect if the Red Queen hypothesis were true.

In a follow up paper this January, the authors studied the relationship between parasitic infections and genetic diversity in more detail. They looked at snails collected from 17 independent streams in New Zealand, and screened them for their genetic diversity, whether they were clonal or sexually reproducing, and whether they were infected by parasites of any type.

None of the 17 populations had done away with sex entirely. They found that as the prevalence of infection increased in a population, so did the percentage of sexually reproducing snails. As in the previous experiment, this suggests that parasitic load is driving populations to adopt sex for reproduction. They also found that among the snails that were asexual, as the prevalence of infection increased, so did the diversity between clones. What this suggests is that parasitic load is doing more than just driving sexual reproduction. It is also alleviating one of the main problems of the clonal strategy – lack of genetic diversity.

The Red Queen hypothesis is an out-of-the-box solution to a scientific conundrum. It is creative, theoretically consistent, and makes clear cut predictions – the hallmark of good science. And personally, I find it cool because it teaches us that the little guys matter. The idea that tiny microscopic life forms are driving the evolution of macroscopic beings completely topples our notions of who’s in charge here.

Footnotes

[1] Many caveats here. For one, bacteria can engage in a kind of primitive sex.

[2] All species that have switched to a completely asexual lifestyle did so fairly recently, in evolutionary terms. This must mean that many who have tried to adopt such a strategy went extinct in the long term. There is one remarkable exception, and that is the Bdelloid rotifers, who have gone without sex for 80 million years!

If you’re interested in reading more about this subject, you may be interested in:

The Red Queen: Sex and the Evolution of Human Nature by Matt Riddley

And this Nova documentary called Why Sex?

References

King KC, Delph LF, Jokela J, & Lively CM (2009). The geographic mosaic of sex and the Red Queen. Current biology : CB, 19 (17), 1438-41 PMID: 19631541

King KC, Jokela J, & Lively CM (2011). Parasites, sex, and clonal diversity in natural snail populations. Evolution; international journal of organic evolution, 65 (5), 1474-81 PMID: 21521196

Image References

Mud Snail image and Life Cycle of Microphallus from Lively lab.

Alice and the Red Queen from Through the Looking Glass (Public Domain).

Snail ecosystem figure from King et al (2009).

This post has been submitted to the NESCent Evolution Blogging Contest

Out of the approximately 3 billion letters of DNA that make up your genome, there are about a 100 letters that neither of your parents possess. These are your own personal mutations. The machinery that copies DNA into new cells is very reliable, but it is not perfect. It makes errors at a rate equivalent to making a single typo for every 100 books filled with text. The sperm and egg cells that fused to form you carried a few such mutations, and therefore so do you.

Every child who grew up watching cartoons like X-men or the ninja turtles associates mutations with superpowers. But the sad reality is that, somewhat like a double edged sword, mutations are more likely to hurt you than do any good. Imagine if you were to change a few letters at random in a book. Chances are, you are not improving the story. A typo doesn’t usually do much. It’s easy to overlook and doesn’t change the essential meaning of a sentence. This is the idea of the neutral theory of evolution, that most mutations have little or no effect on the organism. While this may be the case, the rare events pack quite a punch. Beneficial mutations are rare, but they are the only road through which organisms become better adapted to their environments.*

Changes to DNA are more likely to be disruptive than beneficial, simply because it is easier for changes to mess things up than to improve them. This mutational burden is something that all life forms have to bear. In the long run, individuals that carry harmful mutations will, on average, produce fewer offspring than their peers. Over many generations, this means that the mutation will dwindle in frequency. This is how natural selection is constantly ‘weeding out’ disruptive mutations from our genomes.

There is a flip side to this argument, and it is the story of the blind cave fish. If a mutation disrupts a gene that is not being used, natural selection will have no restoring effect. This is why fish that adapt to a lifestyle of darkness in a cave tend to lose their eyes. There is no longer any advantage to having eyes, and so the deleterious mutations that creep in are no longer being weeded out. Think of it as the ‘use it or lose it’ school of evolution.**

A world without light is quite an alien place. There are many examples of fish that live in completely dark caves. Remarkably, if you compare these fish to their relatives that live in rivers or in the ocean, you find that the cavefish often undergo a similar set of changes. Their eyes do not fully develop, rendering them essentially blind. They lose pigmentation in their skin, and their jaws and teeth tend to develop in particular ways. This is an example of what is known as convergent evolution, where different organisms faced with similar ecological challenges also stumble upon similar evolutionary solutions.

The changes mentioned above are all about appearance, but what about changes in behavior? In particular, when animals sleep, they generally line up with the day and night cycle. In the absence of any daylight, how do their sleep patterns evolve?

A recent paper by Erik Duboué and colleagues addressed this question by comparing 4 groups of fish of the same species Astyanax mexicanus. Three of the populations (the Pachón, Tinaja, and Molino) were blind cavefish that inhabited different dark caves, whereas the fourth was a surface-dwelling fish.

The authors defined sleep for their fish to be a period of a minute or more when the fish were not moving. They checked that this definition met the usual criteria. Sleeping fish were harder to wake up (it took more pokes to elicit a response from them), and fish that were deprived of sleep compensated by sleeping more over the next 12 hours (these are both situations that any college student is familiar with).

The above figure from the paper summarizes their main result (click to enlarge). The black and white bars at the bottom are day and night, and the dots shows the fraction of time that the fish spend asleep. The black graph represents the surface fish. Like us, their sleep peaks at night and they are awake during the day (except for their mid-day nap). In comparison, all the cavefish populations (red, yellow and blue) get very little sleep. Their total amount of sleep has gone down dramatically, by a factor of 10 (see figure below). In fact, each cavefish population included many fish that did not sleep at all during the length of the study.

The researchers also tracked the speeds of all the fish, and found that, while they were awake, the cavefish moved faster or just as fast as the surface fish. This means that it’s not that the cavefish are constantly sleep deprived and in a lethargic, sleepy state. They are just as wakeful as the surface fish (if not more so), and genuinely need less sleep. These three cavefish populations all evolved independently, and yet they have converged on remarkably similar sleep patterns.

To study the genetics of this phenomenon, the researchers cross-bred the surface fish with the cavefish. The cave dwellers and surface fish all belong to the same species, which means that they can have viable offspring. They found that the mixed offspring (Pachón x surface and Tinaja x surface) had a reduced need for sleep that was indistinguishable from that of their cave-dwelling parent. Thus sleep reduction is clearly a genetic trait, and it is a dominant trait (Dominant traits are present in the offspring if they are inherited from just one parent. A recessive trait, on the other hand, will only be present if it is inherited from both parents.)

The scientists also checked that the reduction in sleep was independent of other characteristics of the cavefish, such as the size of their eyes or the presence of pigmentation. This means that sleep loss did not co-evolve with these other traits, but is an independent evolutionary event.

There are still many big questions to be answered. What is the driving force behind sleep reduction? It may be that sleeping less is beneficial to the cavefish, but there’s not yet any evidence to support this. The evolution of sleep and the role it plays in animals is still quite an open mystery. I imagine that the next step in this story would be to work out how the sleep loss evolved, by looking at the DNA sequences of these cavefish and tracing down the specific mutations and genes involved.

Unlocking the secrets of sleep is inherently cool science, and it also has the potential to help people suffering from sleep disorders. Who knows, it may even lead to the superpower of doing away with sleep altogether (though I’m quite sure that some of my fellow bloggers already have that covered).

Reference
Duboué ER, Keene AC, & Borowsky RL (2011). Evolutionary convergence on sleep loss in cavefish populations. Current biology : CB, 21 (8), 671-6 PMID: 21474315 Link

Footnotes
*I am talking about mutations loosely here, sweeping things like copy number variations, insertions, deletions and point mutations all into the same category.
**In fact, possessing eyes in a world without light may even be a hindrance as eyes are vulnerable to infection. It would then be an adaptation to lose them.

Image Credit
The topmost image of Astyanax mexicanus is taken by Joachim S. Müller and is licensed under Creative Commons.

If you enjoyed reading this post, you may like some of my other articles on evolution:
Why have sex? To fight parasites, of course!
Flies alter their ejaculate to get the best bang for the buck
Why moths lost their spots, and cats don’t like milk. Tales of evolution in our time
When nice guys finish first: a lesson from tiny robots