Science Question from a Toddler: How ants evolve

Consider, if you will, the issue of sex and the single ant. Male ants are born into what is, essentially, a giant sorority house, vastly outnumbered by female workers. But that doesn't mean male ants are living out some Hugh Hefner harem fantasy. Most of those many, many females in the ant colony are completely uninterested in sex.

Only queen ants breed. During the course of their lives, they will produce all the baby ants born in the colony. In fact, in some (but, contrary to popular belief, not all) species, the drones' options are narrowed down to exactly one queen—effectively turning that sorority house into a sausage fest. A virgin queen goes on her mating flight, and the drones will get one shot to pass on their genetic material. Afterwards, the males die, and the queen uses their seed to give birth to daughters upon daughters ... most of which will be sterile workers.

On the surface, that system doesn't make a lot of sense. Evolution works because of natural selection, right? And that's based on sex—who manages to survive to adulthood and who manages to find a mate or mates. It's all about passing on your genes to the next generation.

So why would any species evolve a whole class of individuals who never have sex, and never have offspring?

That was basically the question posed by anonymous reader who asked, "If only the queen ant breeds, how does natural selection work for the other ants?" And it's a damn good question, something that confuses people far older than toddler age. In fact, that very conundrum stumped Charles Darwin himself and, today, it sits at the heart of a knock-down fight within the fields of evolutionary biology and social insect studies.

First, a quick reminder about how evolution works.

It starts with natural selection. Different traits improve an individual's chances of surviving to adulthood, finding a mate (or mates), and producing offspring. Other traits diminish those chances. Selection favors those randomly generated traits that help themselves be passed on to the next generation. Over time, some of those traits might prove beneficial enough that they get incorporated into a species gene pool in a fundamental way.

Environment matters, too, because natural selection is all about what works in your physical and social environment. The traits that help a fish survive and reproduce are different from those that help humans. Those that help humans in a desert are different from those that help humans in the Arctic. Selection happens at the level of individuals, but it can alter populations.

If you keep things this simple, then ants really don't make a lot of sense. How can natural selection happen, if the designated breeder is chosen by birth, and not by fitness? And how does not breeding benefit all the individual female workers enough to make that trait stick, as a fundamental aspect of a species?

The answer, of course, is that evolution is a lot more complicated than the simplified explanation makes it sound.

Kith and Kin

Ever since Darwin's time, scientists have looked for ways to reconcile the theory of natural selection with the weird lifestyles of bees, ants, and other social insects. Today, the generally accepted explanation goes something like this: When your relatives benefit, you benefit, too.

That's because, to some degree, you and your relatives share genes. If you don't have any children, but you help ensure that your sister's children survive to adulthood and have kids of their own, then you've scored a point or two in the game of natural selection. Some of your genes got passed down.

Sure, your score would be higher if you'd had kids of your own who carried more of your genes. But natural selection isn't all-or-nothing. Your nieces, nephews, and cousins are better than no genetic descendants at all.

That's kin selection, and it's the mechanism behind something called inclusive fitness theory. The idea there: Species will evolve traits for cooperation—even traits that force them to sacrifice themselves for others—if the benefit outweighs what they loose. So, you might forgo having kids of your own if, by doing that, you enable enough nieces and nephews to survive and breed. You can see already how this relates to the ant problem.

"Any gene that rendered you sterile but enabled you to make other copies of that gene would survive," says Jerry Coyne, a professor in the department of ecology and evolution at the University of Chicago. "If you become sterile but you help your mother produce more workers and make bigger colonies, that would have an advantage. It seems maladaptive but it would be actually adaptive."

Worker ants, the maiden aunts of the ant kingdom, make sense because they enable their mother, aunts, or sisters (remember, colonies often have more than one queen) to produce hundreds of other workers and drones, and to produce the next generation of queens.

But not everybody buys that explanation. One of the most interesting things about this particular Science Question from a Toddler is the way it ties into current events. The tiny ant turns out to be the perfect tool for reminding us that science doesn't happen by reputation alone. Even the biggest names can, and will, be called out if they publish easily disproven research.

Move That Rubber Tree Plant

Among the minority of researchers who think kin selection and inclusive fitness aren't the right way to explain the existence of worker ants: Edward O. Wilson.

You probably recognize that name, and with good reason. Wilson is one of the most well-known names in science, certainly the most well-known researcher of social insects. The man has two Pulitzers. When science journalists talk about "what E.O. Wilson thinks" we usually assume that he's right.

But I can't do that this time.

In August of 2010, Wilson published a paper with two other colleagues that aggressively dismissed inclusive fitness theory entirely. The paper was, to say the least, not well received. The formal rebuttal, published in March, was co-signed by no fewer than 103 different scientists, including Jerry Coyne. I'm not sure whether that's the most people to ever co-author a rebuttal together in a scientific journal, but it's got to be close to the record. Clearly, E.O. Wilson has hit a nerve.

Why? Thats where things get complicated. Nearly everybody agrees that E.O. Wilson was wrong, but there's some disagreement on why he was wrong and just how wrong he was. Meanwhile, Wilson's co-author, Martin Nowak, thinks the people critiquing the paper aren't actually understanding his and Wilson's point very well. This is not a particularly easy debate to summarize.

"I think, honestly, no one understands their arguments, including themselves," says Terry McGlynn, associate professor of biology at California State University Dominguez Hills, and the current president of the North American section of the International Union for the Study of Social Insects. He didn't co-sign the rebuttal, but agrees that Wilson's paper was worth rebutting.

As McGlynn sees it, there are two key problems with the Wilson paper: First, it makes claims about the scientific research surrounding kin selection and inclusive fitness that are demonstrably false.

One of the key mistakes is that the paper claims there's no empirical evidence to support inclusive fitness theory at all. That's blatantly wrong, say McGlynn, Jerry Coyne, and all those co-authors on the rebuttal paper. The truth is that inclusive fitness has been an important part of understanding a range of real-world behaviors, from cooperation to cannibalism.

However, that doesn't mean it's perfect. The Wilson paper stresses that predictions made about ant societies based on inclusive fitness—i.e., how related a group of ants ought to be in order to make the workers' sacrifices make sense—aren't usually correct.

That's true, say McGlynn and Coyne, but it's a matter of degrees, not a matter of the ants turning out to be completely unrelated.

"Inclusive fitness works. Relatedness matters. What you see is that we theoretically expect the ants to be 66% related, but they're only something like 14% related," McGlynn says. "There are clearly benefits to relatedness or these colonies wouldn't be even that related.

This is what brings us to the second problem. The Wilson paper sets up inclusive fitness as a stone fortress to be assailed. It assumes that most other researchers think inclusive fitness is the only explanation you could possibly need for how social systems like those of ants evolved. Then, the paper says that because inclusive fitness doesn't explain everything about ant society, it must be fatally flawed.

And that's just a straw man argument, McGlynn says. Because nobody denies that there's more going on than just relatedness.

For instance, he says, the ants that share a colony have more in common than genetics. "They're living in defensible, constructed nests. The fact that they all live together and have shared defense and shared the benefits of foraging matter," McGlynn says.

Without those things, eusociality—the scientific term for cooperative social systems where labor is divided into strict castes and some of those castes don't breed—doesn't evolve. Kin selection is important, McGlynn says. But it's likely that these shared risks and benefits mattered a lot, too.

Of course, not everybody would agree with McGlynn, either. The last thing you really have to to understand about this debate is that it's really rooted in another—between researchers like McGlynn, who think that the evolution of eusociality has a lot to do with both kin selection and the shared risk/benefit scenarios of what's called "group selection," and researchers like Jerry Coyne, who think that group selection doesn't explain anything that isn't already explained by kin selection. Coyne, and a lot of other evolutionary biologists, pretty much wrote off group selection a long time ago. In fact, that position is well-established enough to often be the default in college classes. But, in recent years, there have been plenty of other researchers—particularly social insect researchers—who've challenged that status quo.

And, while it's very clear that E.O Wilson's paper is wrong—kin selection is important and has not been disproven—it doesn't look like we can say, with as much certainty, that the idea of group selection is wrong.

This should give you the basic overview of the debate. In future stories, I hope to delve more deeply into why group selection is a controversial idea, and why it refuses to die. For now, tell your toddler about kin selection. There's plenty of time to add complexity to the explanation when they're older.

Special thanks to the awesome Zach Shaffer at Arizona State University for turning me on to this debate.

Image: Adrian A. Smith, used with permission and deep gratitude.

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