Kin competition and the evolution of cooperation

Abstract

Kin and multilevel selection theories predict that genetic structure is required for the evolution of cooperation. However, local competition among relatives can limit cooperative benefits, antagonizing the evolution of cooperation. We show that several ecological factors determine the extent to which kin competition constrains cooperative benefits. In addition, we argue that cooperative acts that expand local carrying capacity are less constrained by kin competition than other cooperative traits, and are therefore more likely to evolve. These arguments are particularly relevant to microbial cooperation, which often involves the production of public goods that promote population expansion. The challenge now is to understand how an organism's ecology influences how much cooperative groups contribute to future generations and thereby the evolution of cooperation.

Figure 1

Illustration of how local density dependence (a), global density dependence (b), and local carrying capacity elasticity (c) influence the evolution of a cooperative trait. In each, the black line demarks the population while the blue and red lines indicate the scale of cooperation and density dependence, respectively. Gray dots represent non-cooperative individuals while open dots represent cooperative individuals. In all cases, the green group has a higher initial (T = 0) frequency of cooperative individuals than does the yellow group. For each ecological scenario we then show the composition of both groups after cooperation and reproduction, but before the progeny disperse (T = 1). Figure 1a. Cooperative traits are unlikely to spread when local density dependence occurs at the same scale as cooperative benefits if the cooperative behavior does not increase the locally carrying capacity. In this example, the local carrying capacity is fixed at seven individuals for each group and for simplicity each group is assumed to stay at this carrying capacity. After cooperation and reproduction, the frequency of cooperative individuals decreases within both groups due to the cost associated with the cooperative behavior. Under these ecological conditions, local density dependence constrains group productivity such that cooperative groups do not contribute more to the next generation, preventing the spread of cooperative individuals in the global population. Figure 1b Global density dependence facilitates the spread of cooperative traits by allowing cooperative groups to contribute more to the next generation. In this example, the scale of density dependence is more global than the scale of cooperation. For simplicity, the global carrying capacity is fixed in this example. Under these conditions, cooperative individuals once again decline in frequency within both interaction groups; however their frequency increases globally because the group with more cooperative individuals contributes more to the next generation. The more cooperative group is shown larger at T = 1 than T = 0. It might not remain this way as the additional individuals can subsequently disperse away. Conditions that facilitate the potential of a cooperative interaction group exporting the benefits of cooperation (e.g. empty sites and kin-structured dispersal) help facilitate the evolution of cooperation. Figure 1c. Cooperative traits that increase local carrying capacity are able to spread under local density dependence. When a cooperative trait increases the local carrying capacity, cooperative groups contribute more to the next generation thereby facilitating the spread of cooperative traits. As before, the frequency of cooperators declines within both groups due to the costs of cooperation. Though differing in the scale of density dependence, this case is similar to that of Figure 1b in that both depend on elasticity of group productivity resulting from cooperation. Likewise, increased local population size in response to the cooperative behavior also facilitates the evolution of cooperation under global density dependence (Figure 1b and 1c).

Figure 1

Illustration of how local density dependence (a), global density dependence (b), and local carrying capacity elasticity (c) influence the evolution of a cooperative trait. In each, the black line demarks the population while the blue and red lines indicate the scale of cooperation and density dependence, respectively. Gray dots represent non-cooperative individuals while open dots represent cooperative individuals. In all cases, the green group has a higher initial (T = 0) frequency of cooperative individuals than does the yellow group. For each ecological scenario we then show the composition of both groups after cooperation and reproduction, but before the progeny disperse (T = 1). Figure 1a. Cooperative traits are unlikely to spread when local density dependence occurs at the same scale as cooperative benefits if the cooperative behavior does not increase the locally carrying capacity. In this example, the local carrying capacity is fixed at seven individuals for each group and for simplicity each group is assumed to stay at this carrying capacity. After cooperation and reproduction, the frequency of cooperative individuals decreases within both groups due to the cost associated with the cooperative behavior. Under these ecological conditions, local density dependence constrains group productivity such that cooperative groups do not contribute more to the next generation, preventing the spread of cooperative individuals in the global population. Figure 1b Global density dependence facilitates the spread of cooperative traits by allowing cooperative groups to contribute more to the next generation. In this example, the scale of density dependence is more global than the scale of cooperation. For simplicity, the global carrying capacity is fixed in this example. Under these conditions, cooperative individuals once again decline in frequency within both interaction groups; however their frequency increases globally because the group with more cooperative individuals contributes more to the next generation. The more cooperative group is shown larger at T = 1 than T = 0. It might not remain this way as the additional individuals can subsequently disperse away. Conditions that facilitate the potential of a cooperative interaction group exporting the benefits of cooperation (e.g. empty sites and kin-structured dispersal) help facilitate the evolution of cooperation. Figure 1c. Cooperative traits that increase local carrying capacity are able to spread under local density dependence. When a cooperative trait increases the local carrying capacity, cooperative groups contribute more to the next generation thereby facilitating the spread of cooperative traits. As before, the frequency of cooperators declines within both groups due to the costs of cooperation. Though differing in the scale of density dependence, this case is similar to that of Figure 1b in that both depend on elasticity of group productivity resulting from cooperation. Likewise, increased local population size in response to the cooperative behavior also facilitates the evolution of cooperation under global density dependence (Figure 1b and 1c).

Figure 1

Illustration of how local density dependence (a), global density dependence (b), and local carrying capacity elasticity (c) influence the evolution of a cooperative trait. In each, the black line demarks the population while the blue and red lines indicate the scale of cooperation and density dependence, respectively. Gray dots represent non-cooperative individuals while open dots represent cooperative individuals. In all cases, the green group has a higher initial (T = 0) frequency of cooperative individuals than does the yellow group. For each ecological scenario we then show the composition of both groups after cooperation and reproduction, but before the progeny disperse (T = 1). Figure 1a. Cooperative traits are unlikely to spread when local density dependence occurs at the same scale as cooperative benefits if the cooperative behavior does not increase the locally carrying capacity. In this example, the local carrying capacity is fixed at seven individuals for each group and for simplicity each group is assumed to stay at this carrying capacity. After cooperation and reproduction, the frequency of cooperative individuals decreases within both groups due to the cost associated with the cooperative behavior. Under these ecological conditions, local density dependence constrains group productivity such that cooperative groups do not contribute more to the next generation, preventing the spread of cooperative individuals in the global population. Figure 1b Global density dependence facilitates the spread of cooperative traits by allowing cooperative groups to contribute more to the next generation. In this example, the scale of density dependence is more global than the scale of cooperation. For simplicity, the global carrying capacity is fixed in this example. Under these conditions, cooperative individuals once again decline in frequency within both interaction groups; however their frequency increases globally because the group with more cooperative individuals contributes more to the next generation. The more cooperative group is shown larger at T = 1 than T = 0. It might not remain this way as the additional individuals can subsequently disperse away. Conditions that facilitate the potential of a cooperative interaction group exporting the benefits of cooperation (e.g. empty sites and kin-structured dispersal) help facilitate the evolution of cooperation. Figure 1c. Cooperative traits that increase local carrying capacity are able to spread under local density dependence. When a cooperative trait increases the local carrying capacity, cooperative groups contribute more to the next generation thereby facilitating the spread of cooperative traits. As before, the frequency of cooperators declines within both groups due to the costs of cooperation. Though differing in the scale of density dependence, this case is similar to that of Figure 1b in that both depend on elasticity of group productivity resulting from cooperation. Likewise, increased local population size in response to the cooperative behavior also facilitates the evolution of cooperation under global density dependence (Figure 1b and 1c).

Figure 2

Path diagram of factors influencing individual fitness for a cooperative trait that influences population growth or decline. This yields a modified version of Hamilton’s rule that includes the direct and indirect fitness consequences of the cooperative trait (c and b • r, respectively) as well as the direct and indirect effects of the trait’s impact on pre-dispersal population size (d • e_i and d • e_g • r, respectively).