A genetic test of the theory of island biogeography using the Fiji bush-warbler

Do patterns of gene flow follow the predictions of this classic theory?

Islands biology was one of my favorite courses during my Master at the University of Antwerp (Belgium). Not only did we travel to a collection of small islands of the coast of Croatia, we delved into the details of the evolution and ecology of island species. I still remember sitting outside while reading “The Theory of Island Biogeography“, a classic textbook by Robert H. MacArthur and E.O. Wilson (published in 1967). This books contains an iconic graph where they mathematically show how the number of species on an island is the equilibrium between colonization and extinction, taking into account the size and location of the island. Numerous ecologists have tested the predictions that came out of this simple mathematical model, resulting in several revisions of the theory of island biogeography (see for example this book by Jonathan Losos and Robert Ricklefs).

The rapid developments in genomic sequencing now allow scientist to test some classic predictions from a genetic point of view. For example, patterns of gene flow are expected to be influenced by the size and location of the island. A larger island can send out more migrants to smaller islands, so genes will probably flow from larger to smaller islands. In addition, remote islands are difficult to reach and will probably receive less migrants. Hence, gene flow will be negative correlated with distance between the islands. A recent study in the journal Molecular Ecology tested these straightforward predictions in Fiji, an isolated archipelago in the southwest Pacific.

A classic graph from “The Theory of Island Biogeography”, showing that the number of species on an island is the equilibrium between colonization and extinction. From: Geography Realm.

Demographic Analyses

Fiji is composed of four main islands (Taveuni, Vanua Levu, Viti Levu, and Kadavu), surrounded by hundreds of smaller islands. Ethan Gyllenhaal and his colleagues focused on a songbird that is endemic to the four large islands: the Fiji bush-warbler (Horornis ruficapilla). Using ultraconserved elements, the researchers mapped the genetic population structure of this island bird and estimated the levels of gene flow between the different islands. They nicely summarized their results in the abstract: “Our demographic analysis inferred low levels of gene flow from each large island to its small counterpart and little or none in the opposite direction. The difference in the distance between these two island pairs manifested itself in lower levels of gene flow between more distant islands.” These findings are in line with the predictions that we outlined above.

An unexpected finding was that the number of migrants from Viti to Vanua Levu was much smaller than that of Viti Levu to Kadavu even though Vanua Levu is closer (76 versus. 62 km) and acts as a larger target. This pattern does not follow the predictions of the theory of island biogegraphy and might require additional explanation. The researchers offer several possible mechanisms. First, the prevailing winds in Fiji are easterlies which would hamper the westward dispersal from Viti Levu to Vanua Levu. A similar pattern has been documented in penguins where gene flow follows the ocean currents between populations (see this blog post for the whole story). Second, there might be some degree of reproductive isolation between the island populations. Previous work reported that southern populations have more complex songs than their northern relatives, which could act as a reproductive barrier. Or there might even be some genetic differences that lead to lower fitness in inter-island hybrids. More research is needed to solve this mystery, perhaps leading to another refinement of the theory of island biogeography.

Patterns of gene flow between different island populations based on demographic analyses of ultraconserved elements. The arrows depict the presence and relative strength of gene flow. From: Gyllenhaal et al. (2020) Molecular Ecology.

References

Gyllenhaal, E. F., Mapel, X. M., Naikatini, A., Moyle, R. G., & Andersen, M. J. (2020). A test of island biogeographic theory applied to estimates of gene flow in a Fijian bird is largely consistent with neutral expectations. Molecular Ecology29(21), 4059-4073.

Featured image: Fiji bush-warbler (Horornis ruficapilla) © Lars Petersson | Oiseaux.net

This paper has been added to the Cettiidae page.

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