How the Pleistocene glacial cycles drove the evolution of Arctic shorebirds

An extensive study of 69 species highlights the role of glacial and interglacial periods.

During my PhD on the evolution of geese (see this blog post for a summary), I came across the work of Pieter Ploeger. In 1968, he published an extensive overview on the distribution of arctic ducks and geese during the last ice age. Using a diverse set of data, he tried to pinpoint the areas where these birds resided during the last glacial maximum. The results of this exercise made intuitive sense, but were difficult to test at the time. The development of Species Distribution Models (SDMs) has allowed researchers to reconstruct the past distribution of species and test biogeographical hypotheses, such as the ones put forward by Ploeger. A recent study in the Journal of Biogeography used this approach to investigate how the glacial cycles during the Pleistocene affected the distribution and consequent evolution of arctic shorebirds.

Four Scenarios

Angel Arcones and his colleagues focused on 69 shorebird species and tested four scenarios that could explain the current morphological and genetic patterns. The first scenario (scenario A) assumes that the observed variation might predate the Pleistocene and the glacial cycles had thus no effect on the distribution of these species. The other three scenarios do entail an effect of the Pleistocene glacial cycles, but differ in timing. Populations could become isolated during the warmer interglacial periods (scenario B), the colder glacial periods (scenario C), or both (scenario D). To discriminate between these possibilities, the researchers used Species Distribution Models to determine the distribution of these shorebirds during the last glacial maximum (about 20,000 years ago).

The researchers made a distinction between species that show little morphological or genetic variation (i.e. monotypic species) and species that do. The results revealed that most of the monotypic species (over 65%) did not experience range fragmentation during the last glacial maximum. The more variable species, on the other hand, did show signatures of range fragmentation (62%). The most likely scenarios underlying these fragmentated distributions were roughly equally represented. These patterns confirm the idea that the Pleistocene glacial patterns have shaped the current morphological and genetic patterns in several arctic-nesting species.

The percentage of distributions of the monotypic shorebird species (orange) and species with subspecies (blue) that are explained by the four scenarios. From: Arcones et al. (2021) Journal of Biogeography.

Palearctic and Nearctic

A more detailed look at the results showed that the patterns differ between regions. In Beringia and the eastern Palearctic, climatic conditions were more stable and this area remained largely ice-free. Species residing in this region were thus less likely to be fragmented during the ice ages. The situation is drastically different in the western Palearctic and the Nearctic. Here, huge ice sheets extended to lower latitudes, pushing bird populations into several refugia in the south. The climatic differences between these regions need to be taken into account when reconstructing the evolutionary history of the local bird species.

All in all, this study highlights the importance of considering both the effects of glacial and interglacial periods in the evolution of shorebirds. And it emphasizes the significant climatic differences between biogeographical regions.

An overview of the different biogeographical regions. The Palearctic (red) and Nearctic (green) are most relevant here. © Carol | Wikimedia Commons.

References

Arcones, A., Ponti, R., Ferrer, X., & Vieites, D. R. (2021). Pleistocene glacial cycles as drivers of allopatric differentiation in Arctic shorebirds. Journal of Biogeography48(4), 747-759.

Featured image: Red Knot (Calidris canutus) © Hans Hillewaert | Wikimedia Commons

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