No evidence for hybrid speciation in the Ashy-throated Parrotbill

Admixed individuals do not necessarily point to a hybrid origin.

Hybrid speciation – the origin of new species through hybridization – seems to be a rare phenomenon in birds. In my 2018 review paper on this topic, I could only find convincing evidence for four putative hybrid bird species. In recent years, several additional species have been proposed to be of hybrid origin, such as Salvin’s Prion (Pachyptila salvini), Steller’s Eider (Polysticta stelleri), and Altai Snowcock (Tetraogallus altaicus). The evidence supporting these hybrid species is mixed (as you can read in the linked blog posts). Recently, another putative hybrid species was added to the list: the Ashy-throated Parrotbill (Paradoxornis alphonsiana).

In their Molecular Phylogenetics and Evolution study, Chuanyin Dai and Ping Feng presented genetic analyses of the Paradoxornis webbianus species complex across East Asia. Based on a dataset of one mitochondrial and five nuclear markers, they concluded that “P. alphonsianus was likely the result of hybridization between P. webbianus and P. a. ganluoensis.” Martin Päckert reevaluated the genetic evidence underlying this bold statement. He published his findings in the Journal of Avian Biology.

Admixed Individuals

As you might have guessed from the title of this blog post, Päckert does not agree with the hybrid speciation scenario. His main point of criticism relates to the simplistic reasoning of Dai and Feng. They found several admixed individuals in a genetic clustering analysis (using the software STRUCTURE) and immediately jumped to the hybrid species conclusion. As Päckert nicely put it: “the mere presence of admixed individuals in a population is insufficient evidence for hybrid speciation.”

Indeed, more analyses are needed to confidently discriminate between hybrid speciation and other processes, such as recurrent introgressive hybridization or incomplete lineage sorting. In another blog post, I proposed several approaches to test for a hybrid speciation scenario, such as coalescent modelling.

The genetic clustering analyses of Dai and Feng (2023) uncovered several admixed individuals (with blue and yellow ancestry). However, these patterns alone are insufficient to support a hybrid speciation scenario. From: Päckert (2023).

Genomic Power

Interestingly, Dai and Feng did not consider the possibility of incomplete lineage sorting (ILS). The genetic patterns in their STRUCTURE plot could also be the outcome of this process (see this blog post for more details). In fact, a previous genetic study on parrotbills reported that two nuclear markers (IRF and TGFB) were impacted by ILS. This information suggests that the set of molecular markers was not powerful enough to confidently discriminate between hybridization and ILS. Genomic analyses are needed to reassess the potential patterns of gene flow within the Paradoxornis webbianus species complex.

In his critique, Päckert provided a nice example to illustrate the difference in explanatory power between a handful of molecular markers and genome-wide data.

That Dai and Feng’s (2023) set of five intron markers was a suboptimal choice for their parrotbill study, is reflected by the strongly contrasting divergence and admixture patterns inferred from different sets of markers across a Chinese contact zone of bush tits (Aegithalos fuliginosus and Ae. bonvaloti) in Sichuan. A first study by Wang et al. (2014) relied on six nuclear loci only and could not detect any signal of gene flow between the two species. In contrast, a successive study on approximately 70 000 genome-wide SNPs could detect strong divergence in allopatric populations but also a signal of admixture in the contact zone of these two bush tit species (Zhang et al. 2017). 

In the end, the genetic evidence presented by Dai and Feng is insufficient to support a hybrid speciation scenario for the Ashy-throated Parrotbill. A good occasion to use on of my favorite Carl Sagan quotes: “extraordinary claims require extraordinary evidence”.

References

Dai, C., & Feng, P. (2023). Multiple concordant cytonuclear divergences and potential hybrid speciation within a species complex in Asia. Molecular Phylogenetics and Evolution, 180, 107709.

Päckert, M. (2023). No evidence of a hybrid origin of the ashy‐throated parrotbill Sinosuthora alphonsiana. Journal of Avian Biology, 2023(11-12), e03146.

Qu, Y., Zhang, R., Quan, Q., Song, G., Li, S. H., & Lei, F. (2012). Incomplete lineage sorting or secondary admixture: disentangling historical divergence from recent gene flow in the Vinous‐throated parrotbill (Paradoxornis webbianus). Molecular Ecology, 21(24), 6117-6133.

Featured image: Vinous-throated Parrotbill (Paradoxornis webbianus) © Alnus | Wikimedia Commons

Note on taxonomy: The genus name for these parrotbills differed between publications. Dai and Feng (2023) referred to Paradoxornis whereas Päckert (2023) used Sinosuthora. To avoid confusion, I followed Dai and Feng (2023) because I quoted from their paper at the beginning of the blog post.