Selection for Rhythm as a Trigger for Recursive Evolution in the Elaborate Display System of Woodpeckers

Evolution is never truly predictable, in part because the process of selection is recursive: it operates on its own output to generate historical contingencies, so emergent traits can reshape how others evolve in the future. Studies rarely attempt to directly trace how recursion underlies present-day phenotypic pattern on a macroevolutionary basis. To address this gap, we examined how different selection regimes-each operating on a different timescale-guide the evolution of the woodpecker drum display. Approximately 200 species drum with distinctive speed and length, which are important for territorial competition. We discovered remarkable variation in drum rhythm, with some species drumming at constant rates and others changing speed along a range of mathematical functions. Rhythm undergoes divergent character displacement among sympatric sister species, a process that wanes as other reproductive boundaries emerge over time. Tracing the recursive effects of this process, we found that modifying rhythm may then potentiate or constrain speed/length elaboration. Additionally, increased sexual size dimorphism predicts the emergence of rhythms associated with constrained evolutionary rates of speed/length, implying that selection can also constrain itself. Altogether, our findings illustrate how recursion introduces contingencies that allow diverse phenotypes to arise from similar selection regimes.