ONE strain of E. coli bacteria produces a poison, to which it is immune, that kills competing strains. It takes resources to produce the poison, and the strain that produces it pays a cost in reproduction for the privilege of killing competitors. If the poisoner strain evolved from a more peaceful strain of E. coli, how did it get started? According to the large-population, random-encounter model discussed in Chapter 1, it can't. A few mutant poisoners would cause little change to the average fitness of a large peaceful group. They would nevertheless bear the cost of producing the poison, and so have lower average fitness than the natives. Theory is confirmed in the laboratory. If a few mutant poisoners are added to a well-stirred culture of peaceful E. coli, the mutants are gradually eliminated.

But when the same experiment is performed on agar plates rather than in a well-stirred solution, the poisoners can invade and eventually take over the population. Here theory followed experiment, and theoretical treatments of the local interaction involved also followed, providing analytic explanations of the experimental results. I won't tell the full story here, but I hope that I have told enough to illustrate the importance of spatial structure, location, and local interaction for evolutionary dynamics.

Another illustration, in a rosier hue, comes from the effect of spatial structure on the evolution of cooperation in prisoner's dilemma games. If prisoner's dilemmas are played in a well-mixed large population, the evolutionary dynamics drives cooperation to extinction.