The spatial spread of altruism versus the evolutionary response of egoists

Several recent models have shown that altruism can spread in viscous popula tions, i.e. in spatially structured populations within which individuals in teract only with their immediate neighbours and disperse only over short di stances. I first confirm this result with an individual-based model of a vi scous population, where an individual can vary its level of investment into a behaviour that is beneficial to its neighbours but costly to itself. Two distinct classes of individuals emerge: egoists with no or very little inv estment into altruism, and altruists with a high level of investment; inter mediate levels of altruism are not maintained. I then extend the model to i nvestigate the consequences of letting interaction and dispersal distances evolve along with altruism. Altruists maintain short distances, while the e goists respond to the spread of altruism by increasing their interaction an d dispersal distances. This allows the egoistic individuals to be maintaine d in the population at a high frequency. Furthermore, the coevolution of in vestment into altruism and interaction distance can lead to a stable spatia l pattern, where stripes of altruists (with local interactions) alternate w ith stripes of egoists (with far-reaching interactions). Perhaps most impor tantly, this approach shows that the ease with which altruism spreads in vi scous populations is maintained despite countermeasures evolved by egoists.