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| Meshing Cooperative Behaviors with Evolutionary Theory |
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| SciMed - Evolution | |||
| TS-Si News Service | |||
| Tuesday, 07 April 2009 15:00 | |||
 Cambridge, MA, USA. Evolutionary theory raises a number of perplexing questions. One of them has to do with cooperative behavior, which benefits other members of a species at a cost to the individual. Biologists are puzzled 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. So, how did such behavior come to exist? Jeff Gore is Pappalardo postdoctoral fellow in MIT's Department of Physics, part of an MIT team that has used game theory to understand one solution that yeast use to get around this problem.
Yeast may seem unlikely subjects, but Gore says they are perfectly suited to such studies. 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.
The team's findings, published in 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.
The Snowdrift GameConsider 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. In The Snowdrift Game the best strategy is always the opposite of your opponent's strategy.
This is an example of the interplay between cheating and cooperation. One driver grudgingly gets out and shovels so that both she and the other motorist — inside his warm car — may continue their journey. The driver who cooperates does so because there is a slight benefit to herself, which allows co-existence between a cheater and a cooperator.
In this game, individuals gain direct benefits from cooperative acts, which may indicate why cooperation is favored by natural selection.
For comparison, consider The Prisoner’s Dilemma. Two prisoners being questioned; each one has a choice: betray the other and assign guilt or defend the other’s innocence. As in the Snowdrift game, the best option is to betray your opponent while he defends you. There is a lesser reward for both of you to defend each other. As in the Snowdrift Game, if both betray the other, there is no payoff for either person.
The difference between the two games lies in the greater risk posed by the Prisoner’s Dilemma when you cooperate while your opponent defects. There is always a benefit to shoveling snow, even when the opponent sits and does nothing. Defending an opponent who betrays you results in the worst possible outcome for you: no payoff at all.
Game theorists often view these two games as variants of Chicken (also known as Hawk-Dove), an influential conflict model for two players.
The basic principle is that while each player prefers not to yield to the other, the outcome where neither player yields is the worst possible for both players. The game has also been used to describe the mutual assured destruction of nuclear warfare.  Working with MIT physics professor Alexander van Oudenaarden, also an author of the paper, Gore (pictured) 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. [sidebar] 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.
FundingThis research was funded by the National Institutes of Health (NIH) and the National Science Foundation (NSF).
ContributorsHyun Youk, an MIT graduate student in physics, is also an author of the paper.
CitationSnowdrift game dynamics and facultative cheating in yeast. Jeff Gore, Hyun Youk & Alexander van Oudenaarden. Nature 2009; doi: 10.1038/nature07921.
Abstract The origin of cooperation is a central challenge to our understanding of evolution. The fact that microbial interactions can be manipulated in ways that animal interactions cannot has led to a growing interest in microbial models of cooperation and competition. For the budding yeast Saccharomyces cerevisiae to grow on sucrose, the disaccharide must first be hydrolysed by the enzyme invertase. This hydrolysis reaction is performed outside the cytoplasm in the periplasmic space between the plasma membrane and the cell wall. Here we demonstrate that the vast majority (99 per cent) of the monosaccharides created by sucrose hydrolysis diffuse away before they can be imported into the cell, serving to make invertase production and secretion a cooperative behaviour. A mutant cheater strain that does not produce invertase is able to take advantage of and invade a population of wild-type cooperator cells. However, over a wide range of conditions, the wild-type cooperator can also invade a population of cheater cells. Therefore, we observe steady-state coexistence between the two strains in well-mixed culture resulting from the fact that rare strategies outperform common strategies—the defining features of what game theorists call the snowdrift game. A model of the cooperative interaction incorporating nonlinear benefits explains the origin of this coexistence. We are able to alter the outcome of the competition by varying either the cost of cooperation or the glucose concentration in the media. Finally, we note that glucose repression of invertase expression in wild-type cells produces a strategy that is optimal for the snowdrift game—wild-type cells cooperate only when competing against cheater cells. Of Related InterestGendered Division Of Labor: Humans Over Neanderthals?http://ts-si.org/evolution/2288-gendered-division-of-labor-humans-over-neanderthals.html. TS-Si News Service. TS-Si.org (05 Dec 2006).
Giving In To Get It Done: Co-evolving Choosiness And Cooperationhttp://ts-si.org/relationships/2868-giving-in-to-get-it-done-co-evolving-choosiness-and-cooperation.html. TS-Si News Service. TS-Si.org (13 Jan 2008). Human cooperation in social dilemmas: comparing the Snowdrift game with the Prisoner’s Dilemma. Kümmerli, Rolf, Colliard, Caroline, Fiechter, Nicolas, Petitpierre, Blaise, Russier, Flavien, and Keller, Laurent. Proc. R. Soc. Bdoi: 10.1098/rspb.2007.0793.
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