How many rosy-finch species are there in North America?

Phylogenomic analyses try to solve this taxonomic puzzle.

Ornithologists have created several world bird lists to summarize avian taxonomy, such as the International Ornithological Community (IOC) World Bird List or the Howard and Moore Checklist. These checklists do not always agree on the classification of specific birds (see for example this study on raptors), leading to heated debates between taxonomists. Take, for instance, the rosy-finches of the genus Leucosticte in North America. The American Ornithologists’ Union recognizes three species: the gray-crowned rosy-finch (L. tephrocotis), the brown-capped rosy-finch (L. australis), and the black rosy-finch (L. atrata). In the Howard and Moore checklist, however, they are lumped into one species.

The lumped arrangement is based on a 2009 study in Molecular Phylogenetics and Evolution where Sergei Drovetski and his colleagues could not discriminate between the three American taxa using the mitochondrial gene ND2 and two autosomal genes. They attributed the lack of genetic differentiation to gene flow between neighboring populations. However, can we base a taxonomic decision on a handful of genes? These taxa clearly look distinct. Perhaps these morphological differences can be traced back to a few genomic regions, similar to golden-winged warbler (Vermivora chrysoptera) and blue-winged warbler (V. pinus) where a few “plumage genes” are responsible for their distinct plumage patterns. To test this idea, Erik Funk and his colleagues used genomic data to study the evolution of the rosy-finches in North America. Their findings recently appeared in the journal Systematic Biology.

Three Species?

Using whole genome sequencing data from 68 individuals, the researchers reconstructed the phylogenetic relationships between the different taxa. The analyses provided support for three American species: the black rosy-finch, the brown-capped rosy-finch, and the Alaska island rosy-finch. The first two are already considered distinct species by the AOU, while the populations on the Alaskan islands are currently classified as two subspecies within the grey-crowned rosy-finch (griseonucha and umbrina). The remaining subspecies of the grey-crowned rosy-finch are intermixed in the phylogeny, rendering this taxon paraphyletic (check this blog post for an explanation of paraphyly). A taxonomic revision might thus be necessary.

In line with the study by Sergei Drovetski et al. (2009), the researchers also found signatures of gene flow between different populations. This finding can explain why analyses based on a few genes were unable to discriminate between the North American species. The use of genomic data often results to the detection of minor differences between populations, which leads to an important warning. Just because you can discriminate between certain populations with genomic data does not mean they should automatically be considered separate species. Indeed, in a PNAS paper Jeet Sukumaran and Lacey Knowles nicely described this issue: “Until new methods are developed that can discriminate between structure due to population-level processes and that due to species boundaries, genomic-based results should only be considered a hypothesis that requires validation of delimited species with multiple data types, such as phenotypic and ecological information.” The taxonomic story of the American rosy-finches is just getting started.

The distribution (figure a) and phylogenetic relationships between the different rosy-finch taxa (figure d). From: Funk et al. (2020) Systematic Biology.

References

Funk, E. R., Spellman, G. M., Winker, K., Withrow, J. J., Ruegg, K. C., Zavaleta, E., & Taylor, S. A. (2021). Phylogenomic data reveal widespread introgression across the range of an alpine and arctic specialist. Systematic Biology70(3), 527-541.

Featured image: Gray-crowned Rosy-Finch (Leucosticte tephrocotis) © Alan D. Wilson | Wikimedia Commons

This paper has been added to the Fringillidae page.

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