Analyses of mitochondrial genomes might provide the answer.
Evolutionary biologists love to quote Charles Darwin in their papers. I could also not resist the temptation in a review paper on hybridization in geese. But there are so many other brilliant naturalists with insightful and beautiful quotes. Here is Alfred Russel Wallace in a paper from 1855.
“The facts proved by geology are briefly these: that during an immense, but unknown period, the surface of the earth has undergone successive changes; land has sunk beneath the ocean, while fresh land has risen up from it; mountain chains have been elevated; islands have been formed into continents, and continents submerged till they have become islands; and these changes have taken place, not once merely, but perhaps hundreds, perhaps thousands of times.”
I choose this sentence for a reason, because today’s blog post revolves around biogeography and – more importantly – Wallace’s Line. If you would travel from Borneo to Sulawesi, you will notice a significant change in animal diversity. In Borneo, the animals are related to Asian species, while on Sulawesi you will also encounter Australian animals. The line separating the biogeographical biotas of Asia and Australia was drawn by Alfred Russell Wallace. Most bird species did venture across this line, but there are some exceptions. A recent study in the journal Biology Letters figured out when frogmouths (family Podargidae) crossed the line.
Frogmouths are a species-poor group of birds, represented by 13 extant species. Interestingly, these species can be found on both sides of Wallace’s Line. On the west of the line, you can spot several Batrachostomus species, but you will have to visit the east side of the line to see species of the genus Podargus. The peculiar geographic distribution raises an obvious question: when did the frogmouths cross Wallace’s Line?
To solve this mystery, Paul Oliver and his colleagues sequenced the complete mitochondrial genomes of the Sunda frogmouth (Batrachostomus cornutus), the Solomons frogmouth (Rigidipenna inexpectata) and all three Podargus species. Analyses of these sequences indicated that the initial divergence of frogmouths across Wallace’s Line probably occurred between 44 and 27 million years ago. But how did they get there?
Geological studies of this time period (the mid-Oligocene) suggest that there were no islands directly between Asia and Australia that could have functioned as stepping stones. So, how did the frogmouths manage to cross Wallace’s line and reach Australia? The researchers suspect that the birds used a southwestern detour via the Pacific island arcs to exit the Asian region. There, they persisted for some time before colonizing Australia. This scenario is supported by the distribution of the Solomons frogmouth, which resides on the northern Solomon Islands. Perhaps other animals have followed the same route?
Oliver, P. M., Heiniger, H., Hugall, A. F., Joseph, L., & Mitchell, K. J. (2020). Oligocene divergence of frogmouth birds (Podargidae) across Wallace’s Line. Biology Letters, 16(5), 20200040.
Featured image on top: A tawny frogmouth (Podargus strigoides) in Australia © Benjamint444 | Wikimedia Commons