Are the Blue-faced and the Papuan Parrotfinch different species or not?

These birds are morphologically distinct, but have similar mitochondrial DNA.

Last year, I covered a study on the phylogeny of the estrildid finches (family Estrildidae). The original paper included samples of the Blue-faced Parrotfinch (Erythrura trichroa) and the Papuan Parrotfinch (Erythrura papuana). The authors – Urban Olsson and Per Alström – noticed that these two samples shared the same mitochondrial haplotype. They briefly commented that this result could be due to misidentified specimens and did not pay further attention to these samples. There might, however, be an interesting biological explanation for the identical haplotypes of these samples. Hence, a recent study in the Bulletin of the British Ornithologists’ Club took a closer look at these Parrotfinches.

Bill Morphology

Initially, the Papuan Parrotfinch was described as a subspecies of the Blue-faced Parrotfinch by Rotschild and Hartert. Later on, Hartert elevated the Papuan Parrotfinch to species level, based on differences in bill morphology. He drew parallels with the Darwin’s Finches by writing that “We have thus a similar case as in the genus Geospiza on the Galapagos Islands, a large and a small form occurring together.” Indeed, the bill of the Papuan Parrotfinch is significantly larger than that of its Blue-faced relative. However, a difference in morphology does not necessarily mean that we are dealing with distinct species. For example, in the African genus Pyrenestes, you can observe three distinct phenotypes in a genetically uniform population. The differences in morphology are due to adaptation to different resources. Could the same process be at work in the Parrotfinches?

The difference in bill morphology between the Papuan Parrotfinch (left) and the Blue-faced Parrotfinch (right). From: DeCicco et al. (2020) Bulletin of the British Ornithologists’ Club.

Three Scenarios

To solve this mystery, Lucas DeCicco and his colleagues sequenced the mitochondrial gene ND2 and compared the morphology of several specimens. They found that the Papuan Parrotfinch and the Blue-faced Parrotfinch were almost identical in the sequence of ND2, whereas they showed no overlap in several morphological measurements. This finding can be explained in several ways, nicely summarized in the discussion section:

(1) morphological differences arose in allopatry with either limited genetic divergence or gene flow upon secondary sympatry, (2) sympatric or ecological speciation is occurring with strong selection on different phenotypes, or (3) these two phenotypes represent a single panmictic population with a phenotypic polymorphism.

At the moment, we cannot draw a definitive conclusion yet. More genetic data – nuclear genes or genomic data – is needed to discriminate between these three scenarios. This study does provide the first step in unraveling the evolutionary history of these birds, showing that the result of Urban Olsson and Per Alström was not due to a misidentification. Instead, the sharing of mitochondrial haplotypes has a biological explanation. Which one remains to be determined. This how science works, slowly collecting pieces of the puzzle until we can see the bigger picture.

The two species clearly differed in several morphological measurements. From: DeCicco et al. (2020) Bulletin of the British Ornithologists’ Club.

References

DeCicco, L. H., Benz, B. W., DeRaad, D. A., Hime, P. M., & Moyle, R. G. (2020). New Guinea Erythrura parrotfinches: one species or two?. Bulletin of the British Ornithologists’ Club140(3), 351-358.

Featured image: Blue-faced Parrotfinch (Erythrura trichroa) © Nrg800 | Wikimedia Commons

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