  During bouts of evolutionary diversification, such as adaptive radiations,
the emerging species cluster around different locations in phenotype space, How
such multimodal patterns in phenotype space can emerge from a single ancestral
species is a fundamental question in biology. Frequency-dependent competition
is one potential mechanism for such pattern formation, as has previously been
shown in models based on the theory of adaptive dynamics. Here we demonstrate
that also in models similar to those used in quantitative genetics, phenotype
distributions can split into multiple modes under the force of
frequency-dependent competition. In sexual populations, this requires
assortative mating, and we show that the multimodal splitting of initially
unimodal distributions occurs over a range of assortment parameters. In
addition, assortative mating can be favoured evolutionarily even if it incurs
costs, because it provides a means of alleviating the effects of frequency
dependence. Our results reveal that models at both ends of the spectrum between
essentially monomorphic (adaptive dynamics) and fully polymorphic (quantitative
genetics) yield similar results. This underscores that frequency-dependent
selection is a strong agent of pattern formation in phenotype distributions,
potentially resulting in adaptive speciation.
