Genetic study indicates that the Plain-brown Woodcreeper is paraphyletic.
Every taxonomist enjoys a nice monophyletic group. Paraphyletic or polyphyletic groupings are best avoided. But what do these terms actually mean? We owe this wonderful terminology to the German entomologist Willi Hennig who published the book Phylogenetic Systematics. This book introduced cladistics, a new method to classify organisms based on their morphological differences and similarities. Cladistics came with a whole suite of new tong-twisting term, such as symplesiomorphy and autapomorphy. But let’s focus on the phyletic terms for now.
Cladistics for Dummies
The evolutionary relationships between different organisms can be depicted in a cladogram. The endpoints of a cladogram – the twigs if you will – correspond to living species, whereas the nodes represent extinct ancestors. A group of organisms (several twigs) is called a clade. A monophyletic clade comprises an ancestor and all of its descendants. For instance, in figure a, species D, E, G and H can be traced back to a common ancestor B.
We could decided to cluster species E and G (see figure b), but then we would create a so-called polyphyletic group. These species originate from different ancestors (C and F) that are not included in the group. We could also cluster species J and K (see figure c), but now we ignore the many descendants of their common ancestor (A). The result is a paraphyletic group.
Fish do not exist
A famous example of a non-monophyletic group concerns fish. They share a common ancestor with all other vertebrates. By excluding these vertebrates from the fish-group, we are creating a group that does not include all descendants of this common ancestor. In other words, fish are paraphyletic. Technically, fish do not exist.
An example of a polyphyletic group can be found in the bible where birds and bats were grouped together: “These are the birds you are to regard as unclean and not eat because they are unclean: the eagle, the vulture, the black vulture, the red kite, any kind of black kite, any kind of raven, the horned owl, the screech owl, the gull, any kind of hawk, the little owl, the cormorant, the great owl, the white owl, the desert owl, the osprey, the stork, any kind of heron, the hoopoe and the bat (Leviticus 11:13-19).” A clear sign that the bible is not a science book…
But let’s focus on birds (this is after all a blog about our feathered friends). A recent study in the journal Molecular Phylogenetics and Evolution reconstructed the evolutionary history of the Plain-brown Woodcreeper (Dendrocincla fuliginosa), which has been divided into 12 subspecies. Eduardo Schultz and his colleagues assessed the relationships between these subspecies using genetic data from over 200 individuals.
The genetic analyses revealed that two other species are embedded with the Plain-brown Woodcreeper, namely the Tawny-winged Woodcreeper (D. anabatina) and the Plain-winged Woodcreeper (D. turdina). This phylogenetic result renders the Plain-brown Woodcreeper paraphyletic because it does not include all descendants of the common ancestor. A possible solution is to elevate some subspecies to the species level, creating several monophyletic groups.
The study also mentioned some possible hybridization events. The observation of a individual of the subspecies rufoolivacea within the range of the subspecies fuliginosa suggests ongoing gene flow. But this will need to be confirmed with denser sampling in that region.
Interestingly, rufoolivacea is known to interbreed with another subspecies (atrirostris). The paper describing this situation speculated that the hybrid zone originated because Amazonian rivers (Tapajos and Teles-Pires) split an ancestral population in two (a so-called vicariant event). However, the current study indicates that rufoolivacea and atrirostris are not each others closest relatives, suggesting that they established secondary contact due to changes in their distribution. No need for a vicariant event.
Schultz, E. D., Pérez-Emán, J., Aleixo, A., Miyaki, C. Y., Brumfield, R. T., Cracraft, J., & Ribas, C. C. (2019). Diversification history in the Dendrocincla fuliginosa complex (Aves: Dendrocolaptidae): Insights from broad geographic sampling. Molecular Phylogenetics and Evolution 140,106581.
Weir, J. T., Faccio, M. S., Pulido-Santacruz, P., Barrera-Guzman, A. O. & Aleixo, A. (2015). Hybridization in headwater regions, and the role of rivers as drivers of speciation in Amazonian birds. Evolution 69, 1823-1834.