Two recent genetic studies attempt to distinguish between woodpeckers species.
The majority of bird species have been described based on morphological or vocal differences. The application of molecular data has put these taxonomic decisions to the test: in some cases new cryptic species popped up, while in other cases morphologically distinct birds turned out to be genetically indistinguishable (redpolls and wagtail subspecies are nice examples of the latter situation). Two recent studies on woodpeckers illustrate this ornithological struggle between morphology and genetics.
A Flicker of Hope
Let’s start in North America. Here, you can find one of the best studied avian hybrid zones, namely the one between red-shafted flicker (Colaptes auratus cafer) and yellow-shafted flicker (C. a. auratus). The species complex to which these woodpeckers belong also includes the gilded flicker (C. chrysoides). Previous studies, using traditional molecular markers – such as allozymes – could not discriminate between these three taxa. Even a recent genomic analysis was unable to tell them apart.
Nevertheless, Stepfanie Aguillon and her colleagues decided to give it another go. They sampled birds far from the hybrid zone and used a dataset of thousands of SNP loci to see if they could distinguish among the three taxa. And despite low levels of genetic divergence, they were able to “clearly and conclusively distinguish the three taxa genetically for the first time.”
Confusion in Celeus
Moving on to South America, where two woodpecker species have ornithologists scratching their heads. The taxonomy of scaly-breasted woodpecker (Celeus grammicus) and waved woodpecker (C. undatus) has been a topic of debate: are we dealing with one or two species here? Larissa Sampaio and her colleagues addressed this question by sequencing the DNA (3 mitochondrial and 3 nuclear markers) and comparing the plumage of these birds.
The genetic nor the morphological analyses could distinguish between the proposed species. These results suggest a very recent and possibly incomplete separation – estimated at about 50,000 years ago – of these lineages. The authors conclude that scaly-breasted woodpecker and waved woodpecker and best treated as a single species.
Patterns vs. Processes
But what if the researchers had used genomic data to study these South American woodpeckers, similar to the study on flickers? Would they have succeeded in discriminating between the species? Is more genomic data always better? In my opinion, these are not the right questions to ask. The goal is not to genetically discriminate between taxa that look alike, but rather to better understand the processes behind the genetic patterns. It is difficult to discriminate between these woodpeckers (in both the North and South American examples). Why is this? Are we dealing with a recent split? Or is gene flow preventing genetic divergence? Maybe there are a few genomic regions that determine the differences in plumage? These woodpeckers provide the ideal systems to study such questions. I am sure you will read about them soon at Avian Hybrids!
Aguillon, S.M., Campagna, L., Harrison, R.G. & Lovette, I.J. (2018) A flicker of hope: Genomic data distinguish Northern Flicker taxa despite low levels of divergence. The Auk 135, 748-766.
Sampaio, L., Aleixo, A., Schneider, H., Sampaio, I., Araripe, J. & Sena de Rêgo, P. (2018) Molecular and plumage analyses indicate incomplete separation of two woodpeckers (Aves, Picidae). Zoologica Scripta 47, 418-427.
These papers have been added to the Piciformes page.