Reed College, Portland, OR, August 2000
Coordinators: Jeff Fletcher and Athena Aktipis
The purpose of this workshop is to explore theoretical issues in multilevel selection and the role of evolutionary simulations in deepening our understanding of multilevel selection processes. The presentations and papers included here represent a variety of explorations into multilevel selection theory. Michael Wade’s theoretical paper explores the different kinds of constraints that result from the complicated interactions in evolving systems. These interactions include those between genes within genomes (individuals), between genes and environmental factors, and between genes among genomes (individuals). Wade emphasizes how these genetic and ecological interactions are essential components of micro-evolutionary processes and how they lead to adaptive local evolution and speciation in the absence of gene flow.
The paper by Richard Michod and Denis Roze emphasizes the conditions under which higher levels of organization can emerge and specifically the crucial role of cooperation in forming higher level units of selection. This cooperation between lower level units is facilitated by adaptations they call “conflict modifiers.” Michod and Roze’s model focuses on the origins of multicellular life, but can be applied up and down the hierarchical levels of the natural world. Jeff Fletcher and Martin Zwick emphasize the non-zero sum nature of aggregation in their model in which groups are based on the n-player prisoner’s dilemma. In this way they are able to represent the relative magnitudes of within-group and among-group selective forces precisely as two parameters of the n-player PD. In her paper, Athena Aktipis discusses how individually selected adaptations for social behaviors that lead to aggregation can play a role in forming groups upon which multilevel selection can act. She explores the role of externalities in modeling multilevel selection and social evolution as well as the importance of information processing in creating situations that change selection dynamics in favor of group selection. Leticia Avilés provides a general model of multilevel selection closely based on the real-world example of social spiders. These spiders exhibit female-biased sex ratios in their offspring, which is individually less fit than even ratios. She demonstrates how selection at within and among-group levels balances to determine the overall equilibrium sex ratio, and explores factors that effect this balance.
Two of these papers involve concrete examples of how environmental structure can influence the aggregation of individuals and therefore lead to group diversity and selection. In both of these papers the notion of a group is not defined explicitly and instead groups emerge as a consequence of individuals interacting with the environment. Joshua Mitteldorf and David Sloan Wilson’s paper explores the role of population viscosity with variable density in promoting altruistic behavior. They find that altruists’ capacity to expand more quickly than non-altruists can maintain altruism in the population when environmental conditions routinely provide lower density areas in which to expand. Mitteldorf and Wilson suggest that stochastic effects that forestall individual selection can increase the importance of group selection. In John Pepper’s paper groups form as a consequence of foraging for food which is unevenly distributed. Pepper explores how different patterns of food affect the maintenance of cooperative behavior. He finds that restraining food intake (cooperation) can be selected for when food is sparsely distributed in small patches and food replenishment is based on logistic growth. He also shows that part of this effect is not due to association among kin.