Which subspecies deserve a species status?
The taxonomy of raptors can be complicated as shown by several species complexes, such as the Northern Goshawk (Accipiter gentilis) and Palearctic buzzards (genus Buteo). Another example is the Red-shouldered Hawk (Buteo lineatus), a common sit-and-wait predator from North America. Currently, ornithologists recognize five subspecies. One subspecies (elegans) occurs in the west of the US, while the remaining four can be found in the east (alleni, extimus, lineatus, and texanus). Do some of these subspecies actually represent distinct species? A recent study in the journal Ecology and Evolution tried to figure it out.
East vs. West
George Barrowclough and his colleagues collected samples across the entire range of the Red-shouldered Hawk. They sequenced the mitochondrial ND2 gene and two nuclear introns. These genes revealed a clear difference between the eastern and western populations. The western birds in California – originally described as a distinct species Buteo elegans by Cassin (1855) – are clearly a separate species. This conclusion is corroborated by morphological data: California birds have a much richer rufous coloration and can be diagnosed using the number and size of tail bands.
What about the four subspecies the eastern part of the range? Three subspecies – alleni, lineatus and texanus – were genetically similar. The fourth subspecies (extimus), however, was genetically distinct from the rest (see haplotype network below). This subspecies, which occurs in Florida, is substantially smaller and paler than the other three subspecies.
Does the Florida population represent a different species? The genetic analyses uncovered extensive gene flow between this population and the northern birds. Following the Biological Species Concept (which emphasizes reproductive isolation), the Florida birds would be a well-differentiated subspecies. However, the authors of the present study argue that the population in Florida should be considered a distinct species:
However, our opinion is that the Florida peninsula population has had a separate evolutionary history from that of the other eastern birds and consequently represents an appropriate unit for studies of diversification and historical biogeography; therefore, it represents a phylogenetic species. This would not be apparent were the taxon to be simply regarded as a subspecies in the B. lineatus complex.
In this regard, they follow the Evolutionary Species Concept, which states that “A species is an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies.” Hence, we can recognize three species in the Red-shouldered Hawk:
- Red-shouldered Hawk (B. lineatus)
- California Red-shouldered Hawk (B. elegans)
- Florida Red-shouldered Hawk (B. extimus)
You can check this recent blog post for a longer discussion on avian species concepts.
Barrowclough, G. F., Groth, J. G., Mauck, W. M., & Blair, M. E. (2019). Phylogeography and species limits in the red‐shouldered hawk (Buteo lineatus): Characterization of the Northern Florida Suture Zone in birds. Ecology and Evolution, 9(11):6245-6258.
This paper has been added to the Accipitriformes page