Groups of organisms, from microbes to humans, harbor cheats: individuals who don’t contribute to the common good but still benefit from the work of others. This classic social evolution dilemma was at the heart of a five-day working group that met for the second time in early June. The group built on the models developed in their first meeting last summer, exploring ways to model interactions between organisms and a public good, where both can diffuse in space.

The models developed in the working group identify four ways organisms can interact via a single compound: ways that help others through producing a common good or removing waste, and ways that harm others through producing toxins or consuming public goods. Whether or not a cheat disrupts the whole system depends on a variety of factors.

“One way to model this social dilemma is to abstract away the environment,” says Eric Libby, an Omidyar Fellow and co-organizer of the working group. Such models would only consider the dynamics between producers and cheats. “Our working group was primarily interested in what happens when you explicitly measure that public good along with the organisms, and allow chemicals and organisms to diffuse or move.”

Modeling the environment — the public good — and the dynamics of movement through space are actually quite important, says Libby. For instance, using traveling wave models, the group documented how movement can allow less productive but faster-moving organisms to surpass and eventually outnumber more productive but slower members of the population.

Including environment and movement in social evolutionary models may be important for better understanding diseases like cholera, says Libby. “When they first invade, Vibrio cholerae secrete molecules that make you sick. But when their population increases, they can detect this by quorum-sensing molecules they generate, and leave your system,” he says. “These models are a way of understanding what we believe to be more like real microbial ecologies where societies are deciding between these different types of strategies.”