On the importance of evolving phenotype distributions on evolutionary diversification

Evolutionary branching occurs when a population with a unimodal phenotype distribution diversifies into a multimodally distributed population consisting of two or more strains. Branching results from frequency-dependent selection, which is caused by interactions between individuals. For example, a population performing a social task may diversify into a cooperator strain and a defector strain. Branching can also occur in multi-dimensional phenotype spaces, such as when two tasks are performed simultaneously. In such cases, the strains may diverge in different directions: possible outcomes include division of labor (with each population performing one of the tasks) or the diversification into a strain that performs both tasks and another that performs neither. Here we show that the shape of the population’s phenotypic distribution plays a role in determining the direction of branching. Furthermore, we show that the shape of the distribution is, in turn, contingent on the direction of approach to the evolutionary branching point. This results in a distribution–selection feedback that is not captured in analytical models of evolutionary branching, which assume monomorphic populations. Finally, we show that this feedback can influence long-term evolutionary dynamics and promote the evolution of division of labor.Author summary: Evolutionary branching is the phenomenon by which frequency-dependent selection causes a population with a unimodal phenotype distribution to diversify into multiple modes (strains). Simultaneous evolutionary changes in several traits results in divergence in different phenotypic directions. Here we show that branching preferentially occurs perpendicular to the direction of approach to the branching point. Employing different numerical techniques we demonstrate that this is due to changes in the shape of the population’s phenotypic distribution as it approaches the branching point: the population accumulates variance orthogonally to the selection gradient, which results in a predictable branching direction. The feedback between selection and the phenotypic distribution drives the preferred direction of branching, and ultimately affects the evolutionary equilibrium. For example, when populations are engaged in simultaneous snowdrift games, some branching directions promote division of labor. Thus, keeping track of the distribution is crucial to determine future states of the population.