Fig. 2.

Evolutionary branching in ecological speciation models. Individuals compete for resources, which, in this example, are represented by seeds of different sizes. Depending on its phenotype (bill size), an individual can utilise only a limited fraction (indicated by the dashed resource utilisation curve) of the full distribution of resources (solid curve). a When the mean bill size in the population does not match the most abundant resource, directional selection (arrows) pushes the phenotype to an intermediate value. This eventually leads to strong competition for the most abundant seeds (b). Since resources in the tails of the distribution are left unexploited, selection favours individuals with either small or large beak sizes. c Under the influence of disruptive selection, the population splits into two ecological specialists that evolve an optimal compromise (arrows indicate that selection eventually becomes stabilising) between foraging on the most abundant seeds and avoiding competition