Recent study maps the geographic phenotypic variation in a highly variable woodpecker.
Biologists try to classify the living world into neat little boxes (which is not always possible). Some get carried away and grasp any opportunity to draw lines between supposed species or subspecies. A few feathers with a slightly different color can be used to define a new subspecies. Devising a realistic and biologically relevant classification for variable species can be tricky. The Velazquez’s Woodpecker (Centurus santacruzi), for example, exhibits striking variation in plumage and size across its range, which runs from northeastern Mexico to Nicaragua. Subdividing this diversity has resulted in at least 11 recognized subspecies. But how meaningful is this taxonomic arrangement? A recent study in the Journal of Ornithology took a closer look.
Pilar Benites and her colleagues collected specimens from all 11 subspecies (for interested readers: grateloupensis, veraecrucis, dubius, polygrammus, santacruzi, huglandi, pauper, leei, insulanus, canescens, and turneffensis). Based on extensive analyses of morphometrics and plumage patterns, the researchers uncovered three basic morphs:
- red nape/red belly with higher barring frequency and lower barring ratio
- red nape/yellow belly with intermediate barring frequency and intermediate barring ratio
- yellow nape/yellow belly with lower barring frequency and higher barring ratio
A closer look at the distribution of the 11 subspecies shows that they do not reflect the variation in morphology and plumage patterns. A taxonomic revision might thus be warranted here.
The patterns uncovered by this study raise an additional question: What environmental factors underlie this morphological variation? The researchers reported a significant correlation between plumage patterns and precipitation seasonality. However, because precipitation co-varies with numerous other environmental variables (e.g., habitat type) it is difficult to pinpoint the exact driver of this phenotypic diversity. Despite this uncertainty, it seems plausible that the plumage patterns are caused by local adaptation.
Genetic work on the Velazquez’s Woodpecker reported weak population structure, indicating that “divergence in the phenotypic traits probably evolved faster than neutral genetic markers.” Most likely, the plumage patterns in these birds are encoded by a few genes, similar to other woodpecker species. For instance, the genetic differences between three Sphyrapicus woodpeckers are due to 19 small genomic regions, one of which contains a candidate gene for plumage variation. Will a genomic analysis of the Velazquez’s Woodpecker reveal comparable patterns?
Benites, P., Eaton, M. D., García-Trejo, E. A., & Navarro-Sigüenza, A. G. (2020). Environment influences the geographic phenotypic variation in Velazquez’s Woodpecker (Centurus santacruzi). Journal of Ornithology, 1-14.