Population Ecology, Nonlinear Dynamics, and Social Evolution. I. Associations among Nonrelatives

Abstract

Using an individual-based and genetically explicit simulation model, we explore the evolution of sociality within a population-ecology and nonlinear-dynamics framework. Assuming that individual fitness is a unimodal function of group size and that cooperation may carry a relative fitness cost, we consider the evolution of one-generation breeding associations among nonrelatives. We explore how parameters such as the intrinsic rate of growth and group and global carrying capacities may influence social evolution and how social evolution may, in turn, influence and be influenced by emerging group-level and population-wide dynamics. We find that group living and cooperation evolve under a wide range of parameter values, even when cooperation is costly and the interactions can be defined as altruistic. Greater levels of cooperation, however, did evolve when cooperation carried a low or no relative fitness cost. Larger group carrying capacities allowed the evolution of larger groups but also resulted in lower cooperative tendencies. When the intrinsic rate of growth was not too small and control of the global population size was density dependent, the evolution of large cooperative tendencies resulted in dynamically unstable groups and populations. These results are consistent with the existence and typical group sizes of organisms ranging from the pleometrotic ants to the colonial birds and the global population outbreaks and crashes characteristic of organisms such as the migratory locusts and the tree-killing bark beetles.

It takes grouping and cooperation to get sociality

“…Following Avilés (1999Avilés ( , 2002 and Avilés et al (2002Avilés et al ( , 2004, we assume that the number of o spring produced by an individual is a function of the size of the group it is in, of the cooperativeness of the other group members, and of the cooperativeness of the individual itself. We also assume that this function is hump-shaped with respect to the size of the group.…”

“…In Avilés et al (2002Avilés et al ( , 2004) the rst three factors are presented together, while the last one is presented separately. Here we present the compound function, because for the analytical derivations it is more convenient to have the whole formula at once.…”

“…The same holds for grouping; whether or not it pays to form larger groups may depend on the level of cooperation as well as on the size of the group. In a series of recent papers Avilés (2002) and Avilés et al (2002Avilés et al ( , 2004) use an individual-based and genetically explicit simulation model to explore the evolution of grouping and cooperation, treating these two traits as coevolving dynamic variables. These simulations are used to describe how group size and cooperation respond jointly to ecological and demographic parameters for this particular model.…”

“…We consider three alternative models. The rst is the model from Avilés (1999Avilés ( , 2002 and Avilés et al (2002Avilés et al ( , 2004. The second and third model are alternative possibilities for how cooperation and grouping can a ect tness.…”

“…The reason why these two might diverge, is that the analytical approach makes the simplifying assumption that populations are monomorphic in equilibrium. In the simulation model of Avilés (2002) and Avilés et al (2002), that is not the case; because grouping and cooperative tendencies are coded as polygenic traits, and because they evolve to intermediate levels, sexual reproduction maintains some variance in the population. In addition, group size is inherently stochastic in the simulation model, while in the analytical model we assume…”