Publication: Exploring the evolutionary history of a “supertramp” species
Scientists often assume species living on oceanic islands have strong dispersal ability, surmising that colonizing these isolated, far-flung land masses in the first place would have required the ability to travel vast distances. Oceanic islands are also often known for their endemic species — organisms that are found nowhere else. Taken together, these two statements constitute a famous paradox in the field of island biogeography, a scientific discipline focused on studying the distributions of island organisms. The paradox goes: If island species are able to cover the great distances required to colonize their homes, shouldn’t this ability also maintain sufficient gene flow (the process of migrants from one population interbreeding with another, which tends to make both more similar) to outweigh the processes of evolution that give rise to unique, endemic species?
In a paper my coauthors (Dr. Sarah Schaack and Dr. Jack Dumbacher) and I published this month in the journal Molecular Phylogenetics and Evolution, we investigated this paradox by examining patterns of genetic variation in a small songbird distributed on offshore islands in Papua New Guinea, the Louisiade White-eye (Zosterops griseotinctus). The Louisiade White-eye is a member of a family of birds (the White-eyes; Zosteropidae) known for their rapid speciation, with a large number of islands across the Pacific and Indian Oceans featuring an endemic species. But unusual for the White-eyes as a whole, the Louisiade White-eye is what Jared Diamond termed a “supertramp species:” an organism highly specialized for overwater dispersal.
Diamond’s concept of a supertramp draws mainly on patterns of distribution in South Pacific birds he observed during his extensive fieldwork in the region. Noting that some species were only found on low-lying, resource poor islands, and never on adjacent larger, higher-elevation islands, he hypothesized supertramps were skilled colonists and ecological generalists that competed poorly against more specialized species in richer habitats. He holds that the dispersal ability of supertramps is also an asset in providing the ability to move on to new habitat when resources were overexploited or otherwise became insufficient, and in escaping disturbances from cyclones, sea level rise, and volcanic eruptions. Intriguingly, in the few rare exceptions where supertramps were found on higher-elevation islands, there was evidence of shifts in their ecological niche towards more specialization.
If this change is accompanied by a reduction in dispersal, it might help explain the famous paradox mentioned above. Imagine a scenario in which a supertramp species arrives at a decently-sized, higher-elevation island lacking the kind of competitor species that have previously kept it to lower, smaller islands. On this new island, size and height mean disturbance is less prevalent and resource levels are less prone to catastrophic crashes. Dispersal ability therefore is less advantageous, and more individuals stay put and breed exclusively on their new home. As they more sedentary, gene flow is reduced to the point that populations are sufficiently isolated for long enough that processes such as natural selection and genetic drift become new species.
In our study, we examined the plausibility of this scenario using on a large number of samples of Louisiade White-eye tissue my coauthor Jack Dumbacher and his team collected via sailboat in 2009 and 2011. Jack focused on sampling small, coral atolls that had previously been overlooked by scientists. To represent the rare larger, taller islands where no collections had been made for nearly a century, we sampled tissues from toe-pads on specimens housed the American Museum of Natural History, originating from the seminal Whitney South Seas Expedition. Using a special technique known as ‘ancient’ DNA extraction, we were able to obtain DNA sequence from birds shot in the 1920s, perhaps the greatest “oh-shit-science-is-cool” moment of the research project. Coupled with sequenced DNA from the modern samples, we ran analyses to reconstruct the evolutionary relationships of the different island populations of the Louisiade White-eye (building a phylogeny), and assessed relative levels of divergence.
We were particularly interested in learning 1) whether there was genetic evidence for the supertramp idea, e.g., a signature of significant interbreeding among different island populations; and 2) whether there was genetic evidence for the loss-of-dispersal-ability explanation for the ‘famous paradox’, which would show significant divergence on the few high-elevation islands in our sampling. While our data lacked the resolution to come to unequivocal conclusions, our results provide preliminary support for both these hypotheses, which, of course, only lead to more questions. How frequently are migrants exchanged between populations, and where do they go? How do shifts in dispersal ability occur?