Altruism in a volatile world

Abstract

The evolution of altruism – costly self-sacrifice in the service of others – has puzzled biologists since The Origin of Species. For half a century, attempts to understand altruism have been built on the insight that altruists may help relatives to have extra offspring in order to spread shared genes . This theory – known as inclusive fitness – is founded on a simple inequality termed ‘Hamilton’s rule’. However, explanations of altruism have typically ignored the stochasticity of natural environments, which will not necessarily favour genotypes that produce the greatest average reproductive success. Moreover, empirical data across many taxa reveal associations between altruism and environmental stochasticity, a pattern not predicted by standard interpretations of Hamilton’s rule. Here, we derive Hamilton’s rule with explicit stochasticity, leading to novel predictions about the evolution of altruism. We show that of offspring produced by relatives. Consequently, costly altruism can evolve even if it has a net negative effect on the average reproductive success of related recipients. The selective pressure on volatility suppressing altruism is proportional to the coefficient of variation in population fitness, and is therefore diminished by its own success. Our results formalise the hitherto elusive link between bet-hedging and altruism, and reveal missing fitness effects in the evolution of animal societies.