Uncovering cryptic diversity in the lesser short-toed lark species complex

Genetic analyses reveal several distinct lineages within this morphologically uniform bird group.

“Few groups of birds show the same level of disagreement between taxonomy based on morphology and phylogenetic relationships as inferred from DNA sequences.” This statement refers to the bird family Alaudidae: the larks. These brownish birds mostly reside in open habitat where strong selection for camouflage resulted in little morphological differences. Such bird groups can be a taxonomists’ nightmare, especially if the researcher solely focuses on morphological characteristics to pigeon-hole the specimens. Luckily, molecular data can provide some clarity. A recent study in the journal Zoologica Scripta investigated one species complex within the lark family. Does molecular data work where morphology fails?

The short-toed lark, nicely camouflaged in its natural habitat. © Juan Emilio | Wikimedia Commons

 

No Consensus

An international team of researchers took a closer look at the lesser short-toed lark (Alaudala rufescens) species complex. Due to the morphological similarities, the taxonomy of these birds has been heavily debated and up to 16 different taxa have been recognized. Some authors treated all taxa as a single species, while others split it into two species: the lesser short-toed lark and the Asian short-toed lark (A. cheleensis). Because morphological analyses did not lead a consensus, the researchers turned to molecular data. They sequenced one mitochondrial and eleven nuclear genes.

The genetic analyses revealed four distinct lineages within the species complex that separated between 1.6 and 3.2 million years ago. The authors indicate that the complex could be separated into at least four species, namely the heinei clade, the raytal clade, the rufescens clade and the cheleensis + leucophaea clade. The latter two subspecies are grouped together because they could not be confidently separated by the nuclear genes. This cryptic diversity has been masked by the slight plumage differentiation within complex.

Genetic analyses revealed five separate groups that probably represent four distinct species. From: Ghorbani et al. (2020) Zoological Scripta

 

Evolutionary Questions

This study highlights the power of molecular data to uncover cryptic diversity and inform taxonomic decisions. Although it is important to create a solid taxonomic framework, the debates about which taxa should be classified as species or subspecies are secondary to more interesting evolutionary questions. As I have argued before, instead of focusing on minor morphological differences to draw arbitrary lines between “species”, we should try to understand how this diversity originated. In case of these larks, we can unravel the genetic basis of their impressive camouflage. Do the same genes underlie this diversity or are different genes under selection in different taxa. Such questions are more exciting than deciding to call something a species or not. The birds themselves couldn’t care less…

 

References

Ghorbani, F., Aliabadian, M., Zhang, R., Irestedt, M., Hao, Y., Sundev, G., Lei, F., Ma, M., Olsson, U. & Alström, P. (2020). Densely sampled phylogenetic analyses of the Lesser Short‐toed Lark (Alaudala rufescens) – Sand Lark (A. raytal) species complex (Aves, Passeriformes) reveal cryptic diversity. Zoologica Scripta.