Game theory study: Cooperative behavior meshes with evolutionary theory

Apr 06, 2009 By Anne Trafton
Physics postdoctoral fellow Jeff Gore, left, and physics professor Alexander van Oudenaarden are harvesting yeast cells for experiments exploring the evolution of cooperation. Photo / Donna Coveney

( -- One of the perplexing questions raised by evolutionary theory is how cooperative behavior, which benefits other members of a species at a cost to the individual, came to exist.

Cooperative behavior has puzzled biologists because if only the fittest survive, genes for a behavior that benefits everybody in a population should not last and cooperative behavior should die out, says Jeff Gore, a Pappalardo postdoctoral fellow in MIT's Department of Physics.

Gore is part of a team of MIT researchers that has used to understand one solution use to get around this problem. The team's findings, published in the April 6 online edition of Nature, indicate that if an individual can benefit even slightly by cooperating, it can survive even when surrounded by individuals that don't cooperate.

In short, the study offers a concrete example of how cooperative behaviors can be compatible with evolutionary theory.

Yeast may seem unlikely subjects for a study of cooperative behavior, but in fact they are perfectly suited to such studies, says Gore. Unlike humans, yeast have no emotions or thoughts that interfere with rational decision-making; their actions are solely driven by their genetic response to the environment.

"You can apply game theory to biological interactions and in some ways it's more broadly applicable than it is in humans," says Gore, the paper's lead author.

Game theory, traditionally employed by economists and military strategists, uses mathematics to predict individuals' behavior in certain situations.

Cooperators and cheaters

Working with MIT physics professor Alexander van Oudenaarden, also an author of the paper, Gore developed an experimental setup involving yeast sucrose metabolism. Sucrose is not yeast's preferred food source, but they will metabolize it if no glucose is available. To do so, they must secrete an enzyme called invertase, which breaks sucrose into smaller sugars that the yeast can absorb.

Much of that sugar diffuses away and is freely available to other yeast cells in the environment. In this scenario, yeast that secrete invertase are known as cooperators, while those that don't secrete invertase and instead consume the simple sugars produced by others are called cheaters.

If all of these simple sugars diffused away, with no preferential access to the yeast that produced it, then it would always be better to cheat, and the cooperators would die out.

The researchers observed that cooperating yeast have preferential access to approximately 1 percent of the sucrose they produce. That benefit outweighs the cost of helping others, allowing them to successfully compete against cheaters.

In addition, no matter the initial starting numbers of yeast in a given population, the microbes always come into an equilibrium state, with both cooperators and cheaters present. "It doesn't matter where you start. You always end up with equilibrium," says Gore.

This suggests that the yeast are playing what game theorists call a snowdrift game. The name of the game comes from a situation in which two drivers are trapped in cars behind a snowdrift. Each one can choose to get out and clear a path or stay put. If one driver does not shovel, the other must.

The best option is to "cheat" by staying in the car while the other driver shovels. However, the worst-case scenario occurs if both drivers cheat and no one gets home. Therefore, the best strategy is always the opposite of your opponent's strategy.

The same rules apply to the cheating and cooperating yeast: Like the driver who grudgingly gets out and shovels so that both she and her fellow motorist — snug inside his car — may continue on their journeys, the yeast who cooperate do so because there is a slight benefit for themselves. However, when most of the yeast are cooperating, it becomes advantageous for some individuals to cheat, and vice versa, which allows co-existence between cheaters and cooperators to arise.

Studies have shown that in the wild, yeast carry different numbers of copies of the invertase gene. This genetic diversity in the wild may be similar to the long-term coexistence of cooperators and cheaters observed in the laboratory, says Gore.

Provided by Massachusetts Institute of Technology (news : web)

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3.5 / 5 (2) Apr 06, 2009
At the risk of sounding naive, it seems to me that cooperative behaviour is a fundamental trait of life. Even deeper than that, it's a trait of all matter. Cooperation is what allows matter to group together to form life in the first place. It's what allows all the components of cells to interact and reproduce DNA and build up new cells. So, I don't really get the idea that it's somehow against survival of the fittest to cooperate, since cooperation is required to survive in the first place.
5 / 5 (2) Apr 06, 2009
That is true. But under survival of the fittest, the problem becomes do you do something that benefits only you, or do you act upon things and benefit the rest of the group as well. Although the second is the best strategy, the problem is that it fails survival of the fittest, because not one but many people will benefit. That doesn't benefit the individual directly, but it does benefit the group. But in general, the group requires balance with the individual because both rely on each other to survive which this study supports.
1 / 5 (2) Apr 06, 2009
So the yeast can THINK! Without inteligence everything STOPS! Without intelligence we can believe it all just HAPPENED! I am glad we were there to think this thing through!
4 / 5 (3) Apr 06, 2009
The term "Survival of the Fittest" is very simplistic and actual evolutionary theory is much more complicated than that. Darwin never actually used this phrase, instead it was made up by Herbert Spencer in his Principles of Biology of 1864.
5 / 5 (2) Apr 06, 2009
So the yeast can THINK! Without inteligence everything STOPS! Without intelligence we can believe it all just HAPPENED! I am glad we were there to think this thing through!

I just read THINK STOPS HAPPENED. Is this a message subconsciously being expressed by the creationist mind?
3 / 5 (3) Apr 07, 2009
Actually Richard Dawkins explained how selfish genes can lead to altruism in organisms in his 1976 book "Selfish Gene." Dawkins even uses game theory to explain stable strategies for a population of organisms with cooperators and cheaters. This article doesn't really explain anything new.
2.3 / 5 (4) Apr 11, 2009
I don't see how the yeast are cooperating. The important point in the article is that there IS an advantage to the yeast that secretes the enzyme, even if it is small. The yeast is not thinking, just doing what needs to be done to survive. The others eat some of the sugar that they did not "produce" just because it is there. Cooperating implies thinking. There is no thinking, therefore there is no cooperation.

Humans are different. We have our basic instincts, but then these are modified by our cognitive process. We, in effect, do what satisfies us or makes us happy. If cooperating makes us feel good, or virtuous, then we do it. Even if we consider it a "duty", we do it to feel good, or to avoid feeling bad. Those who don't see it as a duty, don't cooperate because it does not make them feel good, they see no virtue in the action.
2.3 / 5 (3) Apr 11, 2009
OH WOW! Correct me if I am wrong, but they just provided some evidence that Objectivism is a natural phenomona!

Forget the term Cooperators, and instead use the term that reflects reality: Producers.

The yeast that releases the enzyme is at the top of the production chain: It reaches the sucrose and releases the enzymes first because it has the best genetic design with which to produce. It is only acting selfishly, i.e. for its own benefit.

The nearby yeast benefit by consuming some of the sugars released by the producer. Left alone, they would extract their own food, but at a less efficient rate. They are not lazy, just not as productive as the top producer(s).

Thus, whenever there is food available that must first be proccessed, the best producers take the lead and create usefull food, possibly more than they could consume in one sitting, with the population at large benefitting from the efforts of the producers.

Each one acts in its own best interest. The producer is not a slave, because it produces by its own initiative and superior abilities. That innate ability acts as a "gift" to other, less able yeast. The other yeast are not slaves or pets. They, too, are acting out of self interest and simply consuming what is available.

Now, if all of them had the same ability, there would be competition just to get at the sucrose, possibly making the process less efficient. But they don't all have the same ability, and that is simply because of genetic variation, which is a consequence of mutation, but is also a requirement for successfull evolution. An optimum is probably often reached between producers (the best at producing in that given situation) and the rest of the population. If a new situation arises where a different genetic variation turns out to be the best, then the yeast with that variation will become the top producers.

This kind of reminds me of America, especially in the 1900's, the melting pot. Lots of genetic variation, coupled with Capitalism, allowing for a high level of production. Not everyone is the best, but we all benefit, not out of altruism, but out of our own self interest in producing. Thank you Ayn Rand! LOL

>>>Please note: This is not communism because it happens naturally. In communism or socialism, when you EXPECT the best producers to produce for everyone else, they then become slaves to the community, and production will die. The lesson here is that, when left alone, nature finds the best way, as does natural, free market capitalsim. Okay, now let the attacks begin. Give me your best shot! :P