UCLA

The extent the evolution of sociality was shaped by multilevel selection – a theoretical framework for natural selection occurring at levels of biological organization other than the gene – is a classic debate in biology. Though common examples are focused on social behavior, we do not know if multilevel selection significantly acts on social behavioral phenotypes in the wild. For multilevel selection to contribute to evolution, social phenotypes must be variable, replicable, and have unique fitness consequences from two or more discrete levels (e.g., the individual and the group). While the individual fitness consequences of the individual social phenotype (e.g., how connected or social an individual is) have been demonstrated, the individual fitness consequences of the group’s social phenotype (e.g., the structure and pattern of all social interactions in the group) for each individual who lives in the group is largely unknown. Here I quantify individual fitness consequences of the group social phenotype, providing evidence that the individual and group social phenotypes are discrete and quantifiable levels of biological organization. Chapters 2 and 3 show how residing in more connected social groups is associated with decreased individual reproductive success but increased individual winter survival in a wild, free-living population yellow-bellied marmots (Marmota flaviventer), a harem polygynous, facultatively social, hibernating rodent with variable and genetically heritable social behaviors. Not only does the type of group an individual resides in have fitness consequences for those individuals, but the fitness consequences of the group social phenotype are different than those of the individual social phenotype (e.g., less social individuals in more connected groups experience higher winter survival). Thus, chapter 3 quantifies the independent contributions of the individual and group social phenotypes to individual fitness by exploring the two phenotypes together in a unified, multilevel selection framework. We show that multilevel selection is indeed occurring, with selection for the group social phenotype stronger than selection for the individual social phenotype. Thus, given the social phenotype is variable, heritable, and under selection in this system, this work has shown that evolution (directly or indirectly genetic) for both social position and social structure as discrete phenotypes is possible.