How much genetic material – if any – is moving between these species?
Distinct species can exchange genetic material through hybridization and backcrossing. This phenomenon – introgression – has been documented across the avian Tree of Life (check my Avian Research review for more details). However, hybridization does not automatically result in introgression. Sterile hybrids or low levels of hybridization can impede gene flow between species.
A Handful of Hybrids
In North America, Nuttall’s Woodpecker (Dryobates nuttallii) and Ladder-backed Woodpecker (D. scalaris) occasionally hybridize. In 1971, Lester Short published a long monograph on the “systematics and behavior of some North American woodpeckers.” He identified a handful of hybrids between Nuttall’s and Ladder-backed Woodpecker. Even today, birdwatchers sometimes report putative hybrids on eBird.
Low Levels of Gene Flow
But does hybridization between these woodpeckers result in introgression? To figure this out, Joseph Manthey and his colleagues compared the DNA of the two species, alongside a third related species, the Downy Woodpecker (D. pubescens). The genetic analyses indicated that these three species are distinct, although there were low levels of gene flow between Nuttall’s and Ladder-backed Woodpecker. The researchers write that “All of these patterns suggest that although D. nuttallii and D. scalaris can hybridize, little of the genomic content moves between species.”
In these woodpeckers, hybridization does not result in a lot of gene flow. Which processes underlie this pattern? The species might be reproductively isolated by behavioral differences. For example, Nuttall’s and Ladder-backed Woodpecker produce different calls and drumming sounds, which might allow them to avoid pairing up with a heterospecific bird. Another possibility could be reduced hybrid fitness. Perhaps hybrids are mostly sterile or unable to find partner (because they produce the wrong calls?). It remains to be determined what process limits gene flow in this system.
Manthey, J.D., Boissinot, S., & Moyle, R.G. (2019). Biodiversity genomics of North American Dryobates woodpeckers reveals little gene flow across the D. nuttallii x D. scalaris contact zone. The Auk, 136(2):ukz015.
Short, L.L. (1971) Systematics and behavior of some North American woodpeckers, genus Picoides (Aves). Bulletin of the AMNH, 145.
Winkler, H., & Short, L.L. (1978). A comparative analysis of acoustical signals in pied woodpeckers (Aves, Picoides). Bulletin of the AMNH, 160.
This paper has been added to the Piciformes page.