The taxonomic story of the Stipplethroats

A recent phylogenetic study proposes nine distinct species.

Worldwide, there might be about 50 billion individual wild birds (according to this recent PNAS paper). Taxonomists classified all this diversity in about 10,000 species. Each species has been given a binomial name, consisting of genus and a species name. For example, the house sparrow is also known as Passer domesticus, ever since Linneaus named in 1758. The taxonomic position of the house sparrow has been stable for centuries, but other bird groups have been prone to more changes. Some have been promoted from subspecies to species rank (or the other way around), while others have received different genus names.

A nice example of this taxonomic instability concerns the stipplethroats of South America (currently in the genus Epinecrophylla). These small passerines were considered close relatives of the Myrmotherula antwrens and were classified in the same genus. Genetic studies revealed that the stipplethroats were actually more closely related to the bushbirds of the genera Neoctances and Clytotanctes. This finding resulted in the naming of a new genus for the group: Epinecrophylla. Since taxonomists have pinned down the genus name for the stipplethroats (at least for now), they turned to the species level. A recent study in the journal Molecular Phylogenetics and Evolution proposed to recognize nine distinct species. Let’s meet the stipplethroats!

Genetic Splits

The genus Epinecrophylla contains 21 recognized taxa, but their classification into species and subspecies is still a matter of debate. Using thousands of ultraconserved elements (UCEs), Oscar Johnson and his colleagues reconstructed the phylogenetic relationships between these taxa. At the base of the phylogeny, we find the checker-throated stipplethroat (E. fulviventris), followed by the ornate stipplethroat (E. ornata). The latter one showed deep genetic splits and clear population structure between three subspecies (meridionalis, hoffmannsi and atrogularis), suggesting that there might be multiple species hiding in this section of the phylogeny. However, the situation could be complicated by a potential hybrid zone between atrogularis and meridionalis in southern Peru. More research on the ornate stipplethroat is definitely warranted.

Next, the researchers reported a clear split between the rufous-tailed stipplethroat (E. erythrura) and the white-eyed stipplethroat (Epinecrophylla leucophthalma). These taxa are clearly distinct species, but the classification of subspecies within the white-eyed stipplethroat needs more work (currently containing leucophthalma, phaeonota, sordida and dissita).

Dated phylogenies for the genus Epinecrophylla based on (A) ultraconserved elements and (B) mitogenomes. From: Johnson et al. (2021) Molecular Phylogenetics and Evolution.

Short Branches

The classification of the first four species was rather straightforward, but now we arrive at the “Epinecrophylla haematonota group” which holds eight taxa that have undergone many taxonomic rearrangements. Apart from the position of the brown-bellied stipplethroat (E. gutturalis), the researchers found considerable disagreement between the phylogenetic methods regarding the relationships among the three other main clades in this group (see figure below). The rapid evolution of these birds probably resulted in very short branches between the clades, making it extremely difficult to uncover the exact branching order. More detailed analyses – perhaps using genomic data – might be necessary to solve this phylogenetic knot.

Despite this methodological issue, the researchers could identify four species that diverged at roughly the same time (about 2 to 3 million years ago): the rufous-backed stipplethroat (E. haematonota), the Rio Madeira stipplethroat (E. amazonica), the foothill stipplethroat (E. spodionota) and the Negro stipplethroat (E. pyrrhonota). The classification into genera and species seems to be quite stable, so now taxonomists can dive into the subspecies level.

Different phylogenetic methods lead to different outcomes. From: Johnson et al. (2021) Molecular Phylogenetics and Evolution.

References

Johnson, O., Howard, J. T., & Brumfield, R. T. (2021). Systematics of a Neotropical clade of dead-leaf-foraging antwrens (Aves: Thamnophilidae; Epinecrophylla). Molecular Phylogenetics and Evolution154, 106962.

Featured image: brown-bellied stipplethroat (E. gutturalis) © Hector Bottai | Wikimedia Commons

This paper has been added to the Thraupidae page.

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