Genetic analyses point to fifteen distinct lineages, but are they all separate species?
Morphology is not always helpful to resolve evolutionary relationships, especially when different lineages independently develop similar traits (i.e. convergent evolution). Take, for example, the Neotropical bird genus Dendrocolaptes: a morphological analysis of these brownish birds resulted in mixed clusters containing representatives from other genera, such as Xiphocolaptes and Hylexetastes. The most likely explanation is that species in these distinct genera convergently evolved similar plumage patterns (in this case, a “streaked” phenotype). One solution to clean up this morphological mess is to turn to genetic analyses. A recent study in the Journal of Zoological Systematics and Evolutionary Research did just that.
Using 43 specimens from all five recognized species in the genus, Antonita Santana and her colleagues reconstructed the phylogeny of these birds and performed a species delimitation analysis (using the software BP&P). These analyses revealed two main phylogenetic groups that correspond to the certhia and picumnus species complexes. Interestingly, these genetic groups mirror the morphological split into “barred” and “streaked” phenotypes. Past studies could not confidently place Hoffmanns woodcreeper (D. hoffmannsi) in one of these groups because of its intermediate plumage patterns. The genetic analyses show that it belongs to the “streaked” picumnus group.
The species delimitation exercise pointed to 15 lineages. However, this does not mean that there are 15 distinct species. These analyses are based on the multispecies coalescent model which captures genetic population structure. Whether these 15 lineages represent actual species remains to be determined with other data sources, such as morphology, song and ecology. As Jeet Sukumaran and Lacey Knowles warn in their PNAS paper: “Until new methods are developed that can discriminate between structure due to population-level processes and that due to species boundaries, genomic-based results should only be considered a hypothesis that requires validation of delimited species with multiple data types, such as phenotypic and ecological information.”
Based on their current knowledge, the authors suggest four species in the picnumnus group (D. hoffmannsi, D. picumnus, D. platyrostris, and D. transfasciatus) and seven species in the certhia group (D. certhia, D. concolor, D. juruanus, D. medius, D. radiolatus, D. retentus, and D. ridgwayi). The remaining lineages require further investigation.
This study nicely illustrates the importance of combining data from multiple sources to solve taxonomic issues. There is no “silver bullet” for decisions on species boundaries. Genetic analyses can reveal independently evolving lineages and indicate whether these lineages are still exchanging DNA. But other analyses are needed to quantify the level of reproductive isolation and determine if the proposed species can be diagnosed based on morphology or behavior. In some cases, however, taxonomists will uncover conflicts between different data sources, which can be explained by the gradual process of speciation (you cannot easily pigeonhole a continuum), convergent evolution or other processes. Here, a thorough understanding of the evolutionary history of the study species is crucial. This perspective was recently highlighted by Carlos Daniel Cadena and Felipe Zapata who argued that “studies using phenotypic data and methods properly grounded on evolutionary theory offer unique insight to delimit species because they shed light on the role of selection in generating and maintaining biodiversity.” I definitely agree.
Santana, A., Silva, S. M., Batista, R., Sampaio, I., & Aleixo, A. (2021). Molecular systematics, species limits, and diversification of the genus Dendrocolaptes (Aves: Furnariidae): Insights on biotic exchanges between dry and humid forest types in the Neotropics. Journal of Zoological Systematics and Evolutionary Research, 59(1), 277-293.
Featured image: Amazonian Barred Woodcreeper (Dendrocolaptes certhia) © Kent Nickell | Wikimedia Commons