Genomic analyses point to speciation with gene flow in the Northern Saw-whet Owl

“Good advice is always certain to be ignored, but that’s no reason not to give it.”

– Agatha Christie

More than one year ago (in January 2018 to be precise), I wrote a blog post about the evolutionary history of the Northern Saw-whet Owl (Aegolius acadicus). A genetic study revealed that two subspecies – brooksi and acadicus – are genetically differentiated with extremely low levels of gene flow. In fact, there might not be any gene flow. I then provided the following advice: “a genomic approach is necessary to be sure.” The authors followed my suggestion (or they were already working on it) and revisited this system using genomic data. Has there been gene flow or not?

Three juvenile Northern Saw-whet Owls in Fossil, Oregon, USA. © Kathy & Sam | Wikimedia Commons

 

Heteropatric

The situation of the Saw-whet Owl is peculiar. The acadius subspecies, which breeds from Alaska to California, is mostly migratory. The brooksi subspecies, on the other hand, is sedentary and only occurs in the Haida Gwaii island. For most of the year, these subspecies are geographically isolated. But during migration and in winter, some acadius individuals reside on Haida Gwaii where they mingle with their brooksi relatives. This distribution has been described as heteropatric.

The distribution of the Northern Saw-whet Owl. The year-round
range of brooksi is shown in black (Haida Gwaii), the breeding
range of acadicus is shown in gray, and light gray indicates
areas where acadicus occurs only during migration. From: Winker et al. (2019) The Auk

 

Gene Flow?

The temporary co-occurrence of acadius and brooksi provides the opportunity for hybridization. Previous work, based on a subset of molecular markers, found some evidence for low levels of gene flow. But a genomic approach was needed to be completely sure. Kevin Winker and his colleagues used more than 2500 ultraconserved elements to provide a more detailed picture of Saw-whet Owl’s evolution.

Their analyses confirmed low levels of gene flow between the subspecies. Gene flow is stronger from brooksi into acadicus (about 4 individuals per generation) than the other way (about 1 individual per generation). The absence of hybrid specimens and the clear differences between the subspecies suggest that the detected gene flow is probably historical. The authors conclude that “this is a case of speciation with gene flow, and the Haida Gwaii owl (A. a. brooksi) might be considered a young biological species.”

Northern Saw-whet Owl at Reifel Migratory Bird Sanctuary, Delta, BC, Canada. © Brendan Lally | Wikimedia Commons

 

References

Winker, K. (2010). On the origin of species through heteropatric differentiation: a review and a model of speciation in migratory animals (No. 69). Ornitological Monographs, 69.

Winker, K., Glenn, T.C., Withrow, J., Sealy, S.G., & Faircloth, B.C. (2019). Speciation despite gene flow in two owls (Aegolius ssp.): Evidence from 2,517 ultraconserved element loci. The Auk, 136(2):ukz012.

Withrow, J.J., Sealy, S.G., & Winker, K. (2014). Genetics of divergence in the Northern Saw-whet Owl (Aegolius acadicus). The Auk, 131(1):73-85.

 

This paper has been added to the Strigiformes page.