Multilevel selection on individual and group social behaviour in the wild

The degree to which phenotypes are shaped by multilevel selection – the theoretical framework proposing natural selection occurs at more than one level of biological organisation – is a classic debate in biology. Though social behaviours are a common theoretical example for multilevel selection, it is unknown if and how multilevel selection acts on sociality in the wild. We studied the relative strength of multilevel selection on both individual behaviour and group social structure, quantified with social networks and 19 years of data from a wild, free-living mammal, the yellow-bellied marmot ( Marmota flaviventer ). Contextual analysis revealed multilevel selection in specific fitness and life history contexts, with selection for group social structure being just as strong, if not stronger, than individual social behaviour. We also found antagonistic multilevel selection within and between levels, potentially explaining why increased sociality is not as beneficial or heritable in this system comparatively to other social taxa. Thus, the evolutionary dynamics of hierarchal or nested biological traits should be assessed at multiple levels simultaneously to tell a more accurate and comprehensive story. Overall, we provide empirical evidence suggesting that multilevel selection acts on social relationships and structures in the wild, and provide direct evidence for a classic, unanswered question in biology.