Large-scale analyses show how current selection on woodpecker drums might influence subsequent evolutionary trajectories.
A walk in the woods is often accompanied by the occasional drumming of a resident woodpecker. Experienced birdwatchers – or perhaps better: “bird-listeners” – can discriminate between the drums of different species. In fact, visualizing the different drum solo’s revealed that each species’ pattern conforms to a particular mathematical formula (see the figure below for some examples). The beauty of nature captured in a few symbols.
Similar to bird song, the drum rolls of woodpeckers serve a dual function. Males use these drums to attract females and to defend their territories against other males. These functions are the hallmarks of sexual selection. But which components of the woodpeckers’ drum – such as speed, length, cadence – are under sexual selection? A recent study in The American Naturalist compared the drums of about 200 woodpecker species to figure this out.
This woodpecker-wide comparison revealed that species that live in the same area (i.e. they are sympatric) tend to produce drums with very different rhythms. This suggests that sympatric woodpecker species use rhythm to find a partner of their own species and avoid – possibly detrimental – hybridization. In technical terms, rhythm is undergoing sexual character displacement. Interestingly, the same is not true for the speed and length of a drum. These components are under directional selection and might thus be more important in territorial competition among males.
Constraint and Potentiation
So, different rhythms allow the coexistence of certain woodpecker species. But these changes in rhythm have important implications for subsequent evolutionary trajectories. This is nicely illustrated by the Northern Flicker (Colaptes auratus) which contains two populations that might represent distinct species, namely the red-shafted (lathami) and yellow-shafted flicker (auratus). Individuals of the auratus population produce a linear cadence, while individuals of the lathami population have a constant rhythm. According to the estimates of this study, the lathami woodpeckers’ drum has the capacity to evolve more than ten times faster than the drum of auratus birds. In other words, the current drum rhythm “has introduced either a constraint on further signal evolution in auratus or a potentiation of further signal evolution in lathami.”
This phenomenon, where past selection influences the trajectory of future selection, is known as the recursive nature of evolution. This particular characteristic of the evolutionary process makes future evolutionary changes even harder to predict. Evolution is shaped by the interaction of deterministic and stochastic processes. Deterministic processes can be used to predict evolutionary outcomes, but the recursive feature of evolution can introduce stochastic elements in these deterministic changes, reshaping the evolutionary process in an unpredictable way.
You might be put off by this evolutionary quirk, but I think it only adds to the beauty of evolution. The authors capture this feeling nicely in their final sentence: “Altogether, out data illustrate how selection can be a source of contingency: it is a recursive process that modifies itself and, in doing so, leaves behind the exaptations and evolutionary ghosts that have fascinated biologists for decades.”
Miles, M. C., Schuppe, E. R., & Fuxjager, M. J. (2020). Selection for rhythm as a trigger for recursive evolution in the elaborate display system of woodpeckers. The American Naturalist, 195(5).