One of the most hotly debated topics in evolutionary theory is the evolution of altruistic behavior. It would seem that selfish traits, such as consuming resources and reproducing as rapidly as possible, should quickly win out over altruistic traits, like reproducing less frequently when resources are scarce. The videos below demonstrate why it is important to consider the spatial distribution of populations and a provide an explantion for the evolution of altruism.

These animations were developed by Justin Werfel. For more on this subject, see The evolution of reproductive restraint through social communication (Justin Werfel and Yaneer Bar-Yam, PNAS 101, pp. 11019-11024 (2004).)


Animation 1: Time evolution of predator and prey populations

In each of these animations, green spots represent prey, red spots represent predators and prey together, and black spots represent empty areas. At each time step, prey may reproduce into neighboring empty spaces; predators may reproduce into neighboring prey spaces; and predators in a space may kill off all the prey in that space (which, since they've left nothing for themselves to live on, means their own death as well). Each of these events can occur with some probability at each time step. When predators reproduce, mutation means that the offspring may have a slightly higher or lower reproduction rate than do the parents.

In this animation, predators' reproduction rates are indicated by their color: dark red predators reproduce infrequently, while brighter red predators reproduce more frequently. Notice what happens to very bright predators (that is, more selfish ones, using up resources more quickly). Although they reproduce rapidly, giving them an immediate advantage over slower-reproducing competitors, they soon wipe out all of the prey available to them. When no prey remain, these strains die out.

Animation 2: Reproductive restraint through social communication

In this animation, a new type of predator (shown as blue) is introduced. When a predator of this type becomes surrounded by other predators, it sends out a short-range signal. Other blue predators may respond to this signal by temporarily changing their reproduction rate. Again, the extent of this response may be slightly higher or lower in offspring than in their parents, due to mutation. And the response can be to reproduce either more or less frequently when the signal is present.

A traditional view, based on individual fitness, would predict that the successful evolutionary strategy should be to reproduce as quickly as possible in all circumstances. In contrast, what the model shows to be successful in the long term is to reduce reproduction when in the presence of the signal. Moreover, predators with this capacity for communication (blue) have an advantage over non-communicating predators (red). Over the course of this simulation, you will see the blue predators thrive and dominate the environment, until in the end the red ones go extinct. By unselfishly restricting their reproduction in a coordinated way, the communicating predators avoid extinction due to depleted local resources. This evolution of altruism is a clear illustration of the important role of spatial distribution in population genetics.