## Information Geometry (Part 12)

#### John Baez

Last time we saw that if a population evolves toward an 'evolutionarily stable state', then the amount of information our population has 'left to learn' can never increase! It must always decrease or stay the same.

This result sounds wonderful: it's a lot like the second law of thermodynamics, which says entropy must always increase. Of course there are some conditions for this wonderful result to hold. The main condition is that the probability of a replicator being in one species or another evolves according to the replicator equation. But the other is the existence of an evolutionarily stable state. Last time I wrote down the rather odd-looking definition of 'evolutionary stable state' without justifying it. I need to do that soon. But if you've never thought about evolutionary game theory, I think giving you a little background will help. So today let me try that.

### Evolutionary game theory

We've been thinking of evolution as similar to inference or learning. In this analogy, organisms are like 'hypotheses', and the population 'does experiments' to see if these hypotheses make 'correct predictions' (i.e., can reproduce) or not. The successful ones are reinforced while the unsuccessful ones are weeded out. As a result, the population 'learns'. And under the conditions of the theorem we discussed last time, the relative information—the amount 'left to learn'—goes down!

While you might object to various points of this analogy, it's useful—and that's really all you can ask of an analogy. It's useful because it lets us steal chunks of math from the subjects of Bayesian inference and machine learning and apply them to the study of biodiversity and evolution! This is what Marc Harper has been doing:

But now let's bring in another analogy, also contained in Harper's work. We can also think of evolution as similar to a game. In this analogy, organisms are like 'strategies'—or if you prefer, they have strategies. The winners get to reproduce, while the losers don't. John Maynard Smith started developing this analogy in 1973, and eventually wrote a whole book on it:

• John Maynard Smith, Evolution and the Theory of Games, Cambridge University Press, 1982.

As far as I can tell, evolutionary game theory has brought almost as many chunks of math to game theory as it has taken from it. Maybe it's just my ignorance showing, but it seems that game theory becomes considerably deeper when we think about games that many players play again and again, with the winners getting to reproduce, while the losers are eliminated.

According to William Sandholm:

The birth of evolutionary game theory is marked by the publication of a series of papers by mathematical biologist John Maynard Smith. Maynard Smith adapted the methods of traditional game theory, which were created to model the behavior of rational economic agents, to the context of biological natural selection. He proposed his notion of an evolutionarily stable strategy (ESS) as a way of explaining the existence of ritualized animal conflict.

Maynard Smith's equilibrium concept was provided with an explicit dynamic foundation through a differential equation model introduced by Taylor and Jonker. Schuster and Sigmund, following Dawkins, dubbed this model the replicator dynamic, and recognized the close links between this game-theoretic dynamic and dynamics studied much earlier in population ecology and population genetics. By the 1980s, evolutionary game theory was a well-developed and firmly established modeling framework in biology.

Towards the end of this period, economists realized the value of the evolutionary approach to game theory in social science contexts, both as a method of providing foundations for the equilibrium concepts of traditional game theory, and as a tool for selecting among equilibria in games that admit more than one. Especially in its early stages, work by economists in evolutionary game theory hewed closely to the interpretation set out by biologists, with the notion of ESS and the replicator dynamic understood as modeling natural selection in populations of agents genetically programmed to behave in specific ways. But it soon became clear that models of essentially the same form could be used to study the behavior of populations of active decision makers. Indeed, the two approaches sometimes lead to identical models: the replicator dynamic itself can be understood not only as a model of natural selection, but also as one of imitation of successful opponents.

While the majority of work in evolutionary game theory has been undertaken by biologists and economists, closely related models have been applied to questions in a variety of fields, including transportation science, computer science, and sociology. Some paradigms from evolutionary game theory are close relatives of certain models from physics, and so have attracted the attention of workers in this field. All told, evolutionary game theory provides a common ground for workers from a wide range of disciplines.

### The Prisoner's Dilemma

In game theory, the most famous example is the Prisoner's Dilemma. In its original form, this 'game' is played just once:

Two men are arrested, but the police don't have enough information to convict them. So they separate the two men, and offer both the same deal: if one testifies against his partner (or defects), and the other remains silent (and thus cooperates with his partner), the defector goes free and the cooperator goes to jail for 12 months. If both remain silent, both are sentenced to only 1 month in jail for a minor charge. If they both defect, they both receive a 3-month sentence. Each prisoner must choose either to defect or cooperate with his partner in crime; neither gets to hear what the other decides. What will they do?

Traditional game theory emphasizes the so-called 'Nash equilibrium' for this game, in which both prisoners defect. Why don't they both cooperate? They'd both be better off if they both cooperated. However, for them to both cooperate is 'unstable': either one could shorten their sentence by defecting! By definition, a Nash equilibrium has the property that neither player can improve his situation by unilaterally changing his strategy.

In the Prisoner's Dilemma, the Nash equilibrium is not very nice: both parties would be happier if they'd only cooperate. That's why it's called a 'dilemma'. Perhaps the most tragic example today is global warming. Even if all players would be better off if all cooperate to reduce carbon emissions, any one will be better off if everybody except themselves cooperates while they emit more carbon.

For this and many other reasons, people have been interested in 'solving' the Prisoner's Dilemma: that is, finding reasons why cooperation might be favored over defection.

This book got people really excited in seeing what evolutionary game theory has to say about the Prisoner's Dilemma:

• Robert Axelrod, The Evolution of Cooperation, Basic Books, New York, 1984. (A related article with the same title is available online.)

The idea is that under certain circumstances, strategies that are 'nicer' than defection will gradually take over. The most famous of these strategies is 'tit for tat', meaning that you cooperate the first time and after that do whatever your opponent just did. I won't go into this further, because it's a big digression and I'm already digressing too far. I'll just mention that the Prisoner's Dilemma is still full of surprises. Just this week, some fascinating new work has been causing a stir:

I hope I've succeeded in giving you a vague superficial sense of the history of evolutionary game theory and why it's interesting. Next time I'll get serious about the task at hand, which is to understand 'evolutionarily stable strategies'. If you want to peek ahead, try this nice paper:

This is where I got the long quote by Sandholm on the history of evolutionary game theory. The original quote contained lots of references; if you're interested in those, go to page 3 of this paper.