Evolution of Sex, Part 1: The Tease

Time for a science story that’s bound to be a winner. Over the next few posts, I’ll relate a story that spotlights important research from New Zealand, that makes us question the fundamental aspects of our own biology, that winds its way from snails to game theory to children’s novels and back again.

But the real selling point, the thing that makes this story extra juicy— it’s all about sex.

You’re hooked already, right? Of course you are, and with good reason. Humans seem to carry a hard-wired obsession with sex. Many argue that this obsession is all because of biology, because of an innate and inescapable desire to pass on genes. In its most radical incarnation, the argument proposes that the human body is just an elaborate capsule for the genes it carries, that every gene has an uncontrollable drive to replicate, and that every facet of human behavior or society can be traced back to this drive. These arguments can make for a fun and at times ridiculous mental exercise (why am I craving ice cream? Ah, right! It’s all about passing on my genes!), but there are questions that go even more to the root of the issue. Namely: where did sexual reproduction come from? How did it evolve? And what led it to such complete success?

The research story I’m going to share is concerned with uncovering this, the very origins of sex. But to understand the research, it’ll be good to start with a bit of background on reproduction.

In order for life forms to persist in the world, they have to pass on their genetic material. Reproduction allows organisms to create offspring that will carry their genes into the future, allowing their lineage to continue on. For humans, the avenue for passing on genetic information is sexual reproduction.You know this story: two individuals each provide a gamete (egg or sperm) with one set of their DNA, the gametes combine, and ta-da! The outcome is a lovely baby, with a fresh and unique double set of DNA. It is built out of Mom and Dad’s contributions, but ultimately is its own genetically distinct self.

This is the core theme of sexual reproduction, and it holds constant: two individuals each provide genetic information, and that genetic information is combined into genetically distinct offspring. Beyond that, sexual reproduction appears in a huge variety of guises– everything from the internal fertilization favored by us and our mammal brethren, to broadcast spawning strategies that seem fit for a science fiction novel. The variation in reproductive strategies is testament to how long sexual reproduction has been around. There’s been time for the evolution of many variations on the essential theme of sperm-meets-egg, which is strong evidence that it’s been a favorite way to pass on DNA for a long time.

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What do these creatures have in common? Sexual reproduction, of course!

But it hasn’t always been that way. There was a time when sexual reproduction didn’t exist. Passing on genes used to be a matter of asexual reproduction. Instead of requiring two individuals, asexual reproduction is a solo affair. An individual makes a complete copy of their genome and and blesses their baby with this genetic information. The offspring, the clones, are genetically identical to the parent.

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Asexual reproduction dominated the biosphere for a long time before the advent of sexual reproduction. In the right situations, it continues to dominate today. And there are some really good reasons for that.

To understand those reasons, it’s best to strip reproduction down to the cold, harsh numbers: how many offspring can each individual create? Judging by the rules of evolution theory, the strategy that results in more offspring and more gene copies will win.

First, let’s cover a few assumptions. What follows was proposed in 1977 by John Maynard Smith, an eminent evolutionary biologist and game theorist. He provided major insights into the conflict between sexual and asexual reproduction by turning it into a numbers game. And his numbers make a lot of sense, but rely on a few key assumptions. The first assumption is that in a sexual population, only females produce progeny. Males contribute genes, but don’t produce offspring of their own. Next, we’ll assume that sexual reproducers create offspring in a 1:1 sex ratio— meaning one daughter, one son. And finally, in an asexual population, there’s really only one sex (because where do “the sexes” fit into a world that’s asexual?). We’ll call them females, just to make things easier.

So, consider an asexual female. Every generation, she splits (I always imagine a sound effect here… bloop!) to create two clonal daughters. Each of those daughters will split— bloop, bloop— to produce a total of four clonal daughters. Compare that to a sexual female. The sexual female finds a male, and also produces two offspring. But since she’s sexual (and remember the earlier assumptions), she creates one male and one female. In the next generation, her female offspring finds a male and, again, creates one male and one female offspring. But those sons won’t produce offspring, right? They’ll contribute genes, but ultimately we’ll only count the offspring coming from the daughters. So for every two offspring the sexual female creates, she only has one daughter to carry on the line. And that daughter will have only one daughter. And so on.
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When you compare a few generations of each population, you can see that the sexual population is quickly swamped by the asexual. While the asexual population is able to double every generation, the sexual population is held back by needing to create and mate with a second sex. Evolutionary biologists have dubbed this “the two-fold cost of males” (snicker, snicker). Asexual reproduction doesn’t involve males, and so it bypasses that two-fold cost. Based on the numbers alone, that’s a massive evolutionary advantage compared to the sexual reproducers.

That advantage comes into play when the two reproductive strategies coincide. Were you to introduce just one asexual female into a sexual population, the asexual population would be quick to blossom. The increasing numbers of asexual individuals would soon gobble up all of the resources and drive the sexual population to extinction.

So we can all agree that sexual reproduction seems like a losing proposition, right? It results in fewer gene copies for any given individual, it’s quickly replaced by asexual reproduction…evolutionarily speaking, it ought to be a dud. But the thing is: sexual reproduction isn’t extinct. In fact, we seem to see just the opposite— for a majority of multicellular organisms, sexual reproduction is fixed in the population, it’s the best (well, or the only) way to reproduce. If asexual reproduction has such a massive numerical advantage, how did sexual reproduction evolve and become a preferable strategy? What’s the advantage to sexual reproduction that we aren’t seeing in the numbers game?

Basically— why sex?

There are plenty of theories to answer this question, and they work out well on paper. But a theory can only take you so far— at some point, you need evidence, data to back up your claims. To make any theory of the evolution of sex really powerful, you’d need to find a system where you could make predictions based on the theories, then actually test those predictions against the reality of the system.

But what sort of system would you need to test these ideas? You’d have to find an organism that can reproduce sexually and asexually, so you could compare the benefits of each approach.  And it would have to be a creature with a short enough lifespan that a researcher could examine the success of each generation in real-time. And, ideally, it would be naturally occurring– a situation where the rules of evolution have been allowed to unfurl genuinely, rather than a situation created artificially to test these ideas.

Which leads us to the question: where on earth could you find a system like this?

The answer, of course, lies in New Zealand.

Stay tuned,
Ellie

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