Genetic and morphological data support the recognition of two distinct species.
“We echo calls for integrative taxonomy in which genomic and phenotypic data are considered on equal footing when delimiting species.” This concluding remark was stated by Carlos Daniel Cadena and Felipe Zapata in their recent review paper on species delimitation. I wholeheartedly agree with this statement. In fact, I have advocated this integrative approach to taxonomy in other blog posts (see for example here). The rationale behind this approach is quite straightforward: different taxonomic concepts and methods are combined in drawing species limits. Within this context, two general frameworks can be used: integration by congruence and integration by cumulation. The congruence approach entails that different data sets, such as molecular and morphological characters, support the decision to recognize certain taxa as valid and distinct species. In the cumulation approach, evidence from different data sets is gathered, concordances and conflicts are explained within the specific evolutionary context of the taxa under study, and based on the available evidence a decision is made.
The best way to highlight the importance and usefulness of integrative taxonomy is to see it in action. Luckily, a recent study in the Journal of Ornithology applied this strategy to the Long-tailed Rosefinch (Carpodacus sibiricus). The distribution of this species extends across a large part of eastern Asia. Based on its discontinuous distribution and some subtle differences in plumage, five subspecies have been proposed: the northern sibiricus, ussuriensis and sanguinolentus, and Chinese endemics henrici and lepidus. Simin Liu, Chentao Wei and their colleagues collected samples from all five subspecies and performed detailed genetic, morphological and acoustic analyses to determine how distinct these subspecies are. Perhaps some can be elevated to species rank?
Two Clades with Similar Songs
Genetic analyses of the mitochondrial gene COI uncovered two main clades that correspond to the northern and southern grouping of subspecies. The northern clade consists of sibiricus, ussuriensis and sanguinolentus, while the southern clade comprises henrici and lepidus. Interestingly, the subspecies ussuriensis and sanguinolentus form a mixed group, suggesting that they should be considered one subspecies instead of two. The other three subspecies are clearly distinct lineages.
In contrast to the genetic analyses, there were no clear differences in songs. The authors write that “Overall, there was much overlap among the taxa and no clear separation was found, and there was no clear division between the songs of the northern and southern groups.” The lack of song divergence might be due to the recent origin of these groups (ca. 1.36 million years ago) or their allopatric distribution. The birds might not need species-specific songs because they rarely encounter other subspecies. There might thus be no strong selection that could lead to different songs.
Particular Plumage Patterns
What about morphological differences? Previous studies indicated subtle variation in plumage patterns. Although the authors did not perform a quantitative morphological analysis, they do describe notable differences between the northern and southern groups.
While adult males from the northern group have the entire forehead and crown silvery-pink, in the southern group the feathers of the forehead are silvery-pink, whereas the crown is reddish. The mantle of northern birds is pink to deep red with moderately broad brown streaking, while the mantle of southern birds is more brownish and less reddish. The northern taxa have broader median and greater covert wing bars, while the wing bars of southern birds are relatively narrow. Finally, the three outermost tail feathers are extensively white in northern birds, while in southern birds only the outermost feathers are extensively white, and the white part only covers less than half the length of the second outermost tail feathers.
Based on the gathered evidence, the researchers propose to split the Long-tailed Rosefinch into two species, namely the Siberian Long-tailed Rosefinch (C. sibiricus comprising the subspecies sibiricus and sanguinolentus), and Chinese Long-tailed Rosefinch (C. lepidus comprising the subspecies henrici and epidus). Would you agree?
Regardless of whether the northern and southern group will be recognized as distinct species, it is worthwhile to have a closer look at their evolutionary history. This north-south division between a Siberian-Japanese clade and a Chinese clade has been documented in other bird species and can probably be attributed to the mountains of northern and central China. The combined effects of the geographical isolation of these mountains and the changing vegetation during the Pleistocene turned this area into a formidable barrier for passerines, resulting in genetic divergence between separated populations. Similarly, these two taxa in the southern group likely diverged in separate mountain ranges: henrici in the Hengduan mountains and lepidus in the Qinling Mountains and Yan Mountains.
Within the northern group, we see a east-west divide between sibiricus and ussuriensis/sanguinolentus. This pattern can be explained by historical glacial isolation in Siberia. It is possible that the eastern and western groups were connected by gene flow in the past. Indeed, one ussuriensis (in yellow) individual is nested within the sibiricus (in blue) group. Whether this concerns a misidentified individual or a signature of gene flow remains to be determined. I am secretly hoping for the latter explanation.
Liu, S., Wei, C., Leader, P. J., Carey, G. J., Jia, C., Fu, Y., Alström, P & Liu, Y. (2020). Taxonomic revision of the Long-tailed Rosefinch Carpodacus sibiricus complex. Journal of Ornithology, 161(4), 1061-1070.
Featured image: Long-tailed Rosefinch (Carpodacus sibiricus) © Попов Евгений | Wikimedia Commons