You’ve committed a crime. You and your co-conspirator have been nabbed, but fortunately, there’s not enough evidence to put you both away for the time you deserve. You’ll be sentenced to just one year in prison. Unless, that is, you turn on your criminal associate, who will take the fall for a three-year sentence, while you walk free. The one catch: the prosecution have offered your partner the same Faustian bargain. If you both keep your mouths closed, you’ll each serve a year. But if you both spill the beans trying to profit at the expense of your partner, you both get two-year sentences. No conferring — you’re both in separate cells. What do you decide?
This is the Prisoner’s Dilemma, first framed in 1950 by mathematicians Merrill Flood and Melvin Dresher, and later given its evocative name by Albert Tucker. In its original form, the best option for both parties would be to each adopt a ‘no-squeal’ policy — in other words, cooperate with each other for mutual benefit. But if you think your partner is a bit of a soft touch who’s likely to cooperate, then as an individual, the option of betraying them (or defecting) becomes almost too tempting to pass up. Except, of course, that your partner could be thinking along exactly the same lines, which would lead to both of you getting a longer sentence than if you just cooperated. But do you cooperate, knowing that your compatriot might end up taking you for the sucker?
Since its invention, the Prisoner’s Dilemma has been applied to a whole gamut of problems that try to weigh the benefits of cooperation against the pull of self-interest. In global climate politics we can see the prisoner’s dilemma in full flight. Individual nations shirk their responsibility to commit to coordinated action to curb global warming, opting instead to let others bear the economic cost of reducing emissions, even though all nations will pay in the long term if no action is taken.
Cooperators will always come off second best when pitted against defectors, as the Prisoner’s Dilemma illustrates. So why, then, has cooperation evolved to be so ubiquitous in nature?
In a study published last week in Current Biology, J. David van Dyken and colleagues from Harvard University and the University of Tennessee tried to answer this conundrum using cooperator and defector strains of the bread-rising, beer-brewing, genetics lab superstar: the budding yeast, Saccharomyces cerevisiae.
S. cerevisiae feeds off sucrose (table sugar) by excreting an enzyme called invertase that splits the sugar into its two constituent monosaccharide units, glucose and fructose. These smaller sugars are readily imported and used as food by the yeast cells. In the cooperator/defector set-up that the researchers used, only the cooperator strain produces invertase. Since digestion takes place outside of the cells, defector strains that lack invertase are able to benefit from free-loading on the fruits of the cooperators’ labour — the invertase production.
In real-world evolutionary scenarios, where individuals within a population are competing for resources and mates, defectors aren’t usually completely dependent on cooperators for their survival. So the researchers conducted their experiments in growth medium containing 2% sucrose — which the cooperators can digest, benefiting both strains — as well as a small amount of protein that defectors can use as an inferior food source. The defector strains were also made to be resistant to the chemical cyclohexamide, which was added to the medium to represent a hypothetical ‘cost of cooperation’ for the cooperators. In the presence of cyclohexamide, the playing field was made more level: both cooperators and defectors were handicapped in some way.
When cooperator and defector strains were mixed in a flask of liquid growth medium, cooperators lost out to the free-loading defectors, whatever the cost of cooperation (concentration of cyclohexamide) imposed. The Prisoner’s Dilemma, where cooperators come off second best, was being played out.
In the experimental world of the Erlenmeyer flask, the multiplying yeast cells weren’t faced with any space restrictions. In nature, limited territory is a common problem forcing individuals to venture further afield to colonise new frontiers. One complication that these intrepid colonists face is a diminishing gene pool — there aren’t many individuals out on the frontiers to mate with, and sooner or later your first cousin starts looking rather appealing. Being closely related to those around you isn’t all bad, though — cooperation is favoured in populations with a high degree of relatedness. It makes sense to cooperate with your sister, for example, because helping her to survive and breed will help to propagate your own genes, half of which are shared by your sister.
So what happens with our cooperator and defector yeast strains when they are forced to expand into new territory?
This is what van Dyken and colleagues asked next. Instead of growing the yeast strains in liquid medium, they placed a droplet containing a mixture of both strains onto solid growth medium — again containing both sucrose and cyclohexamide. As the yeast cells multiplied and started to get crowded, cells at the edge of the mixed colony were forced outward onto fresh growth medium.
By tracking the defectors and cooperators using fluorescent labelling of defector cells, the researchers saw that cooperators were thriving the further away from the ‘homeland’ they got. Even when the cost of cooperation (cyclohexamide concentration) was high, cooperators won out. Expansion into new territory favoured cooperation.
One of the reasons that the cooperators were so successful was because in the process of expansion, whole sectors of the expanding territory pie became cooperator-only. Without the free-loading defectors encroaching on their hard-earned food, their growth increased, further enabling expansion. As the researchers explained, it was a case of ‘survival of the fastest’.
The dynamics witnessed on the petri dish could help to explain how cooperation evolved in other organisms. Perhaps our own migration out of Africa some 70,000 years ago helped to ensure that we would evolve into the cooperative creatures we are today.
Now, if only someone could remind our politicians of this.
Reference: Van Dyken, Müller, Mack & Desai. 2013. Spatial population expansion promotes the evolution of cooperation in an experimental prisoner’s dilemma. Current Biology. Published online before print May 9, 2013, doi:10.1016/j.cub.2013.04.026