BEACON Researchers at Work: Cheaters never win

This week’s BEACON Researchers at Work blog post is by University of Washington graduate student Adam Waite.

Adam cooperating with some friends to get to the top of a boulder.

Adam cooperating with some friends to get to the top of a boulder.

Why do we cooperate? It’s easy enough to understand the benefits of cooperation. When we pay taxes, for example, we are contributing to the maintenance of the roads we bike on, the parks we enjoy, and universal access to education. However, no one enjoys paying taxes. Some people go through all kinds of complicated accounting schemes — which, somewhat ironically, cost them quite a bit of money — to minimize the amount they pay. Some people try to get away with not paying any at all, despite the potentially harsh punishments they will face if caught. Before they’re caught, these “cheaters” still drive to work on the roads and enjoy the parks maintained by the money of others.

For most of us, the threat of jail time is enough to deter the desire to “get something for nothing.” But what if there were no such thing as the IRS and no way to punish cheaters? No matter how unrealistically optimistic your opinion is on the general morality of the human race, I would feel comfortable betting a large sum of money that these roads and parks would not last very long.

So maybe we cooperate because of the fear of being punished somehow — in the form of fines or jail time for large offenses, but also because we want other people to like and respect us. We are incredibly social creatures, endowed with highly sophisticated cognitive systems that associate positive emotions with trust and friendship, and negative emotions with deception and treachery. And, for the most part, this “social sense” is all we need to remain cooperators and avoid becoming cheaters.

But I am an evolutionary biologist, and while this provides something of an answer as to why we cooperate right now, I am compelled to wonder, “But where did this sense come from?” Being biological creatures, our cognitive systems are the product of evolution by natural selection. And natural selection acts at the level of individuals, rewarding those who make the most use of resources available to them, even (and especially) if this comes at a cost to others. Imagine some pre-cooperative organisms, lacking any means of punishment or social sensibilities. Even if they did better when they worked together, the appearance of a “cheater” — through mutation of a previously-cooperating individual or invasion from outside — should quickly destroy them. Thus, the continued existence of cooperation seems to require the impossible: the maintenance of parks and roads by individuals acting purely in their own self-interest, with no possibility of being punished.

Although we are most familiar with cooperation as occurring among humans, it is actually found at all levels of biological complexity. Genes “cooperate” in the form of genomes; individual cells cooperate to form multicellular organisms; and cooperation within and between species is commonplace. As expected, all of these systems have their cheaters. For example, transposable elements cheat on genomes by selfishly replicating, often to the detriment of the host organism. Cells in multicellular organisms that ignore signals to cease dividing are responsible for cancer.

My interest lies in answering this seemingly impossible question: How does cooperation survive cheating? This is a big problem, and was recognized by Darwin as potentially fatal to his theory. Thus, decades of theoretical and experimental research have been focused on this problem. We now know that any mechanism allowing cooperators to preferentially associate with other cooperators can allow cooperation to survive cheating. This association can be achieved in simple ways, such as clustering with other cooperators, or it can be as complex as recognizing and remembering which organisms to cooperate with and which to avoid. And, everywhere we look, we find that these successful cooperative systems have one or more of these mechanisms at their disposal.

But what about cooperative systems — such as motile microorganisms — that exist in well-mixed environments and lack sophisticated methods of recognition? Presumably this type of cooperative system had to survive long enough to allow the appearance of more sophisticated mechanisms of cheater control. In order to study this question, I needed a cooperative system that had a clearly defined way of cooperating and was known to lack any mechanisms of cheater control. Since the existence of such a system is not known to exist, we made our own. My research group genetically engineered the commonly used species of baker’s yeast, Saccharomyces cerevisae, to cooperate and cheat.

Our system, which is called “CoSMO” (Cooperation that is Synthetic and Mutually Obligitory), cooperates through the exchange of essential metabolites: each cooperator produces a nutrient required by its counterpart. Cheaters require one of the nutrients but do not produce anything and, because they don’t pay the metabolic “tax” of production, they can divide a little bit faster than their cooperative counterpart.

Thus, my expectation was that, when mixed together, cheaters would deterministically displace their cooperative counterpart. Eventually, I reasoned, the amount of nutrient would not be sufficient to support the population, and the growth of the entire co-culture would slow down and eventually stop. To my astonishment, however, a fraction of the co-cultures continued to grow! Not only that, they were growing at a rate I would expect if cheaters were not present at all. Because we had fluorescently labelled each of the strains with a different color, I could quickly determine that, in fact, the growing co-cultures were almost completely devoid of cheaters. What was going on?

By isolating individual members from the cooperator-dominated co-cultures, I discovered that now they could always beat the ancestor cheater. The cooperators had evolved to become better cooperators! I could also isolate the few remaining cheaters from this population and they were always able to beat the ancestor cooperators… so they had improved, as well. However, when I mixed these improved cooperators with the improved cheaters, the cooperators always won. How strange! It was as if the cooperators and cheaters were in a race with one another to obtain the best mutations. The “winner” of this “adaptive race” would determine whether cooperators dominated the co-culture and continued to grow, or whether cheaters would dominate and destroy it.

But what was the selective pressure to which they were adapting? Luckily, yeast have small genomes and we know all their genes. So, to answer this question, I could sequence their entire genomes and look for mutations. I found mutations in both cooperators and cheaters that allowed them to grow at much lower concentrations of their required nutrient. In other words, the benefit obtained by adapting to the nutrient-limited cooperative environment was much greater than the disadvantage of being a cooperator, and allowed cooperators to outcompete cheaters.

So here was a way that cooperators could defeat cheaters without any mechanisms of cheater control, and it was through adaptation by natural selection, the very process that was supposed to destroy cooperation in the first place! And while in our system adaptation was to the nutrient-limited cooperative environment, any stressful environment — such as a change in temperature or salt concentration, or the presence of antibiotics — could potentially provide the fuel to drive the “adaptive race” and give cooperators a chance to drive cheaters extinct. Since changing — and therefore stressful — environments are an inescapable reality of biology, the “adaptive race” is a simple but effective mechanism to buy time for cooperation, allowing a fraction of cooperators to persist long enough to evolve more sophisticated mechanisms of cheater control.

For more information about Adam’s work, you can contact him at nodice at uw dot edu.

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