Mapping patterns of gene flow across numerous islands.
The study of island biogeography is full of fascinating concepts and questions. For example, I have covered the “paradox of the great speciator” in previous blog posts, mainly using white-eyes (genus Zosterops) as an example. This paradox relates to the observation that some lineages have the ability to disperse over many islands, but still manage to speciate despite their impressive colonization capacity. Why does regular gene flow not counteract the formation of new species? You can read some possible solutions to this paradox here. But in this blog post, I want to focus on another, related concept: “supertramps”. This term was coined by Jared Diamond and refers to vagile species that repeatedly colonize remote islands, but do not speciate due to high levels of gene flow.
When members of supertramps colonize an island, they will not evolve specialized traits because continuous gene flow will impede local adaptation. This might result in a seemingly random pattern of these traits across islands. Ernst Mayr noticed such a pattern in the body size of the Polynesian Wattled Honeyeater (Foulehaio carunculatus) and proposed two possible explanations. Perhaps the body size distribution follows Bergmann’s Rule with species of smaller size residing in warmer regions. Or the small island populations are subject to genetic drift, leading to random changes in body size. A recent study in the journal Molecular Phylogenetics and Evolution revisited the ideas of Mayr and used a genomic approach to reconstruct the evolution of the Wattled Honeyeater species complex.
Phylogenetic analyses of more than 4000 ultraconserved elements revealed nine distinct lineages within the Polynesian Wattled Honeyeater. Estimates of gene flow pointed to some genetic connectivity between adjacent islands, but not more distant ones. These genetic patterns are not as pronounced as you would expect for a supertramp species. It seems that these birds do not disperse over water that frequently. The current levels of genetic differentiation across the archipelago reflect a balance between dispersal capacity (helping the birds reach remote islands) and sedentary behavior (allowing for some genetic differences to evolve). This particular balance is just one possible combination on a continuum from extreme supertramps to sedentary species.
But what about the body size conundrum that occupied Ernst Mayr? The researchers detected no phylogenetic signal in the distribution of body sizes, which supports the explanation that the random variation in this trait is mainly due to genetic drift. However, the analyses also revealed that body size tends to increase with decreasing island area. This pattern is consistent with the island rule, which predicts that smaller species might increase in size on small islands due to lack of competition with other species. A nice observation that opens many avenues for further research.
Mapel, X. M., Gyllenhaal, E. F., Modak, T. H., DeCicco, L. H., Naikatini, A., Utzurrum, R. B., Seamon, J. O., Cibois, A., Thibault, J.-C., Sorenson, M. D., Moyle, R. G., Barrow, L. N. & Andersen, M. J. (2021). Inter-and intra-archipelago dynamics of population structure and gene flow in a Polynesian bird. Molecular phylogenetics and evolution, 156, 107034.
Featured image: Wattled Honeyeater (Foulehaio carunculata) © Duncan | Wikimedia Commons