Disruptive selection as a driver of evolutionary branching and caste evolution in social insects

Theory suggests that evolutionary branching via disruptive selection may be a
relatively common and powerful force driving phenotypic divergence. Here,
we extend this theory to social insects, which have novel social axes of phenotypic
diversification. Our model, built around turtle ant (Cephalotes) biology, is
used to explore whether disruptive selection can drive the evolutionary
branching of divergent colony phenotypes that include a novel soldier caste.
Soldier evolution is a recurrent theme in social insect diversification that is
exemplified in the turtle ants. We show that phenotypic mutants can gain
competitive advantages that induce disruptive selection and subsequent
branching. A soldier caste does not generally appear before branching, but can
evolve from subsequent competition. The soldier caste then evolves in association
with specialized resource preferences that maximize defensive performance.
Overall, our model indicates that resource specialization may occur in
the absence of morphological specialization, but that when morphological specialization
evolves, it is always in association with resource specialization. This
evolutionary coupling of ecological and morphological specialization is consistent
with recent empirical evidence, but contrary to predictions of classical
caste theory. Our model provides a new theoretical understanding of the ecology
of caste evolution that explicitly considers the process of adaptive phenotypic
divergence and diversification.