Molecular results are largely incongruent with morphological and behavioral data.
“I don’t trust the molecules,” said António Frias Martins during a field trip on the Azores (the Portuguese island group). We were looking for several snail species of the genus Plutonia that I would use for my Master thesis. António had been studying these slimy creatures for years and used morphological characters (mainly from reproductive organs) to reconstruct their evolutionary history. My goal was to use DNA to build a phylogenetic tree. I completely disagreed with him: genetic markers can be more reliable than morphology because phenotypes are vulnerable to convergent evolution (i.e. independent evolution of similar features). Convergent evolution can lead to homoplasy, a shared character between two or more species that did not arise from a common ancestor.
The difficulty of homoplasy is nicely illustrated by a recent paper in the journal Zoological Scripta that focused on the bird family Laniidae. This group of birds contains about 30 species of shrike, divided over four genera. Shrikes are also known as “butcherbirds” because they impale their prey on thorns and spikes. The evolutionary relationships within this family have been a matter of debate. Two studies used morphological and behavioral data to sort things out. Panov (2011) combined zoogeographical, morphological and behavioral characters to divide the shrikes into seven groups. Harris and Franklin (2000) used a different approach and recognized two main groups based on alarms calls. They identifed a “keer alarm group” and a “chatter alarm group”. Both classifications are depicted in the figure below.
As I mentioned in the beginning, morphological characters can be unreliable because of convergent evolution and consequent homoplasy. But this process can also occur in molecular data: the same nucleotide can evolve independently at the same position in the genome. However, because there are so many genetic characters in a sequence, it is unlikely that it happens very often. Moreover, there are several tools to detect homoplasy in DNA sequences and account for them (see for example this recent paper). In the end, homoplasy is less of a problem for molecular analyses. Hence, Jérôme Fuchs and his colleagues used three genes (one mitochondrial gene and two nuclear regions) to reconstruct the evolutionary history of the shrikes.
The results from the molecular analyses were drastically different from the previous studies (you can compare the three trees above). I will not discuss all the incongruences, but let’s have a look at one major difference. Based on their alarm calls, Souza’s Shrike (L. souzae) and Emin’s Shrike (L. gubernator) were placed in the “chatter alarm group”. However, the molecular tree groups them with several species from the “keer alarm group”, such as Newton’s Fiscal (L. newtoni) and Northern Fiscal (L. humeralis). The authors write that “Apparently, all of the traits analysed by earlier authors are highly homoplastic, with many cases of parallel evolution as well as cases of strongly divergent morphological differentiation.”
The genetic analyses also revealed some other interesting patterns. First, the genus Eurocephalus – which holds the Northern White-crowned Shrike (E. ruppelli) and the Southern White-crowned Shrike (E. anguitimens) – does not belong to the Laniidae family. Instead it is more closely related to a member of the Corvidae family: the Crested Jay (Platylophus galericulatus). The genus Eurocephalus should probably be placed in its own family. Second, the Yellow-billed shrike (Corvinella corvina) and the Magpie shrike, (Urolestes melanoleucus) are nested within the genus Lanius. It makes sense to include them in the Lanius-genus.
Trust the Molecules?
This study nicely shows that molecular data can be more reliable than morphological analyses. But this doesn’t mean that we should blindly go where the molecules take us. Genetic data comes with its own set of problems, such as discordance between gene trees, and we should be careful with drawing conclusions.
You might be wondering about my Master thesis. Well, I used two genes to construct a phylogenetic tree which agreed with the morphological analyses from António. But he still did not completely trust the molecules. So, we agreed to disagree…
Fuchs, J., Alström, P., Yosef, R., & Olsson, U. (2019). Miocene diversification of an open‐habitat predatorial passerine radiation, the shrikes (Aves: Passeriformes: Laniidae). Zoologica Scripta, 48(5), 571-588.
Harris, T., & Franklin, K. (2000). Shrikes and Bush‐shrikes: Including
wood‐shrikes, helmet‐shrikes, flycatcher‐shrikes, philentomas, batises
and wattle‐eyes. London, UK: Helm Editions.
Ottenburghs, J. (2011) Molecular relationships of endemic landsnails of the genus Plutonia (Pulmonata, Gastropoda) on the Azores (Macaronesia – Portugal). Master Thesis, University of Antwerp (Belgium).
Panov, E. N. (2011). The True Shrikes (Laniidae) of the world: Ecology,
behavior and evolution. Sofia and Moscow: Pensoft.