Jordan Martin

1 Introduction A population will evolve by natural selection whenever heritable variation occurs in fitness-relevant phenotypes (Darwin 1859). Measuring the fitness consequences of individual differences in highly labile behavioral, physiological, and morphological traits is, therefore, fundamental for explaining their adaptive evolution. Across a variety of phenotypes and taxa, repeatable individual differences have been observed in organisms' average trait values (Bell et al.

Abstract Explaining the rapid evolution of human cooperation and its role in our species’ biodemographic success remains a major evolutionary puzzle. To address this challenge, we tested a social drive hypothesis, which predicts that social plasticity and social selection in human groups cause indirect genetic effects that accelerate the adaptation of fitness, promoting population growth via feedback between the environmental causes and evolutionary consequences of cooperation.