Researchers provide multiple lines of evidence to assess the species status of these plovers.
The recent surge in genomic resources allows ornithologists to uncover extremely fine-grained population structure. In a book chapter that I wrote with several colleagues, you can read that “In the past, statistical power from only a small number of genetic markers from distant regions of the genome has often been insufficient to unveil weak population structure, and increasing the number of markers has clearly shown that more markers give better signals.” This rise in resolution raises an important issue: Will a more fine-grained picture of population structure lead to the splitting of more and more species? Several recent papers on plovers (genus Charadrius) highlight this conundrum.
A Kentish Plover in India © David Raju | Wikimedia Commons
Genetic Differentiation
Are the Kentish Plover (C. alexandrinus) and the White-faced Plover (C. dealbatus) different species? Opinions about the species status of these two plovers vary and several researchers have attempted to answer this question using genetic data. In 2011, Frank Rheindt and colleagues used three mitochondrial and seven nuclear DNA markers to describe the genetic situation among these plovers. Despite clear morphological differences, they failed to find genetic differentiation between Kentish and White-faced Plover. They speculated that “diagnostic phenotypic characters may be encoded by few genes that are difficult to detect” or that “gene expression differences may be crucial in producing different phenotypes”.
Fast forward to 2019, when Xuejing Wang, Pinjia Que and co-workers revisited these plovers using three mitochondrial and sixteen nuclear loci. Their analyses revealed a clear genetic difference in the mitochondrial DNA: the species formed distinct groups in the haplotype networks. The nuclear networks, however, could not confidently discriminate between the putative species. This result is in line with the speculation from the 2011 study, namely that diagnostic phenotypic characters may be encoded by few genes.
Haplotype networks of (a) mitochondrial and (b) nuclear markers for Kentish (blue) and White-faced (yellow) plover. From: Wang et al. (2019) BMC Evolutionary Biology
Enter Genomics
To really know whether the morphological differences between these birds are encoded by a few genes, there is only one solution: sequence the whole genome! And this is exactly what Xuejing Wang, Kathryn Maher, Nan Zhang and colleagues did. As expected, comparing the genomes of Kentish and White-faced Plover showed low levels of genome-wide differentiation. However, the researchers did find a few differentiated genomic regions (so-called “islands of differentiation”) which might hold the key to the morphological differences. Which genes are in these regions and how they might contribute to species-specific differences remains to be investigated.
An example of a genomic island of differentiation (in blue, based on relative genetic differentiation Fst). Notice that this regions corresponds to a decrease in Dxy (absolute divergence) and π (genetic diversity), suggesting strong linked selection. From: Wang et al. (2019) Frontiers in Genetics
Multiple Lines of Evidence
In summary, the genetic differences between Kentish and White-faced Plover are concentrated in a few differentiated islands while the rest of the genome is largely undifferentiated (genome-wide Fst = 0.046). To come back to the question posed in the beginning: Are these genetic differences sufficient to consider these plovers distinct species? As I have argued in other blog posts (see for example here, here and here), the answer is clearly no. Taxonomy has become pluralistic, requiring multiple lines of evidence to justify lumping or splitting species. Luckily, the authors of the papers mentioned above provided some insights into the morphology, ecology and demography of Kentish and White-faced Plover.
There are clear morphological differences between these birds: White-faced Plovers lack the dark eye ring found in Kentish Plovers. Plumage-wise, they show lighter patterns and a brighter cinnamon cap. Moreover, Whiter-face Plovers have, on average, longer beaks, longer wings and a larger body mass. The differences in bill size suggest that Kentish and White-faced Plover use different food sources. This suggestion is supported by a stable isotope analysis which provides insights into the diet of these birds. It turned out that White-faced Plovers probably feed on a higher energy diet compared to Kentish Plovers. Finally, the genomic analyses mentioned above revealed distinct demographic histories. The population size of Kentish Plover increased, while the number of White-faced Plovers decreased during the Last Glacial Period.
Synthesizing all the results (based on genetics, morphology, ecology and demography), we can conclude that Kentish and White-faced Plover can be considered “independently evolving metapopulation lineages” and are thus – according to the General Lineage Concept – distinct species!
A Kentish Plover (left) and White-faced Plover (right) at Tanjung Tokong in Malaysia. © D.N. Bakewell | PLoS One
References
De Queiroz, K. (1999). The general lineage concept of species and the defining properties of the species. Species: new interdisciplinary essays. MIT Press, Cambridge, MA.
Ottenburghs, J., Lavretsky, P., Peters, J.L., Kawakami, T. & Kraus, R.H.S. (2019) Population genomics and phylogeography. Avian Genomics in Ecology and Evolution – From the lab into the wild (Edited by Kraus, R.H.S.), pp. 237-265, Springer Nature.
Rheindt, F. E., Szekely, T., Edwards, S. V., Lee, P. L., Burke, T., Kennerley, P. R., et al. (2011). Conflict between genetic and phenotypic differentiation: the evolutionary history of a ‘lost and rediscovered’ shorebird. PLoS One, 6(11).
Wang, X., Que, P., Heckel, G., Hu, J., Zhang, X., Chiang, C. Y., et al. (2019). Genetic, phenotypic and ecological differentiation suggests incipient speciation in two Charadrius plovers along the Chinese coast. BMC Evolutionary Biology, 19(1), 135.
Wang, X., Maher, K. H., Zhang, N., Que, P., Zheng, C., Liu, S., et al. (2019). Demographic histories and genome-wide patterns of divergence in incipient species of shorebirds. Frontiers in Genetics, 10, 919.
These papers have been added to the Charadriiformes page.
Avian Hybrids 