How convincing is the evidence to split the Gentoo Penguin into four species?

A critical look at the genetic and morphological data supporting this taxonomic proposal.

Taxonomy is often in flux. As new data are collected or novel methods are being developed, the classification of certain sections on the Tree of Life might change. For example, a recent study in the journal Ecology and Evolution presented morphometric and genetic evidence to split the Gentoo Penguin (Pygoscelis papua) into four distinct species. Last year, this study attracted some media attention (including the BBC and the Oceanographic Magazine), but I remained somewhat skeptical about this taxonomic revision. I added the paper to my writing-list where it gathered dust from several months (there were so many other interesting papers to cover on the blog). Now, it has finally resurfaced and we can assess the evidence. How strong is the case for four species of Gentoo Penguin?

Genetic Lineages

In recent years, taxonomy has become more pluralistic, combining several lines of evidence to support taxonomic decisions (see for example this blog post on larks). The researchers in the penguin study also took an integrative approach and collected genetic and morphological data. The genetic analyses – based on more than 10,000 markers – pointed to four clearly distinct lineages, corresponding to populations from several islands (i.e. Falklands, South Georgia Island, South Shetland Islands + Western Antarctic Peninsula, and Kerguelen). In addition, species delimitation analyses supported a model that considers these four lineages as distinct species. However, as I have discussed in other blog posts (see here and here), genetic population structure does not necessarily coincide with species boundaries. Other data types are needed to validate the delimited species.

Genetic analyses revealed four distinct lineages. But do they represent different species? From: Tyler et al. (2020) Ecology and Evolution.

Morphological Overlap

Next, the researchers performed several statistical analyses on a set of morphological traits. A MANOVA test indicated that “all genetically distinct populations are significantly distinct from each other overall.” However, a closer look at the results of this MANOVA test shows that the p-values are just below the significance threshold of 0.05. For example, the p-value separating the Falklands from Kerguelen is 0.0446. Statistically significant, yes. But is this difference biologically relevant?

My skepticism towards the morphological patterns was reinforced by the output of the linear discriminant analysis where the authors reported that “a small number of specimens occupying positions closer to other lineages.” Moreover, assigning individuals to the genetic lineages using the morphological data resulted in an error rate of 10% (i.e. one individual out of ten was assigned to the wrong lineage). Clearly, the morphological differences are not absolute.

Linear Discriminant Analysis of the morphological data. Circles represent individual specimens with triangles showing the lineage mean. Notice the overlap between SGI (pink) and FALK (blue). Tyler et al. (2020) Ecology and Evolution.

Verdict

So, how convincing is the evidence to recognize four species of Gentoo Penguin? There appears to be some conflict between the genetic and morphometric results. We can recognize four distinct genetic lineages, but they overlap morphologically. And that is a logical finding when you keep in mind that speciation is a gradual process in which different traits evolve at different rates (more details in this blog post). In this case, the Gentoo Penguin populations have become genetically distinct, but the morphological separation might still be under way (or it might stabilize in the current situation). In my opinion, these penguins are in a “taxonomic grey zone”. One could make a case to treat them as subspecies. Or one could argue that the morphological differences are large enough to classify them as distinct species.

An additional argument to recognize four distinct species concerns their conservation status. In the press release, the researchers say that “regarding the four populations as separate species, gives conservationists a better chance of protecting their diversity because if there’s a decline in one of them it will change the threat status as defined by the IUCN Red List”. This is not a biological reason for a taxonomic revision, but a political one. And that is no problem. In light of the current biodiversity crisis it is important to protect as many species as possible. We could have endless academic discussions whether to classify these penguins as species or subspecies, but that won’t safeguard their future. Let’s focus on what matters.

References

Tyler, J., Bonfitto, M. T., Clucas, G. V., Reddy, S., & Younger, J. L. (2020). Morphometric and genetic evidence for four species of gentoo penguin. Ecology and Evolution10(24), 13836-13846.

Featured image: Gentoo Penguin (Pygoscelis papua) © Ben Tubby | Wikimedia Commons

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