How did rails spread across the globe?

The importance of dispersal-related traits in historical biogeography.

“The distribution of species on islands and continents throughout the world is exactly what you’d expect if evolution was a fact.” This quote from evolutionary biologist Richard Dawkins nicely illustrates the importance of evolution to understand the present and past distribution of species on our planet. Historical biogeography is the discipline that deals with the interface of evolutionary history and the changing distributions of species. Using probabilistic models, scientists try to reconstruct the journey of a group of species across space and time. Most of these models do not take biology into account, but consider all species as interchangeable units. This is obviously not the case. Species differ in many traits, which could impact the way they spread across islands and continents. A recent study in the journal Molecular Phylogenetics and Evolution provides a nice example on how to include species-specific traits into biogeographical models.

Flightless Rails

Juan Garcia-R and Nicholas Matzke focused on the evolution of rails (family Rallidae). Numerous rail species have lost the ability to fly (see for example here and here). Being able to fly or not obviously affects a species’ capacity to disperse. Hence, the researchers included this trait in their biogeographical models. First, they build an evolutionary tree for the rails, using morphological and genetic data. This phylogenetic framework – containing 129 extant and 29 extinct taxa – provided the basis for a comparison of several biogeographical models. The model including trait-dependent dispersal outperformed all the other models. Clearly, the ability to fly matters.

Time-calibrated phylogeny and ancestral range estimation based on total-evidence data of the family Rallidae. From: Garcia-R & Matzke (2021).

Ancestral Area

The final model provided some interesting insights into the evolution of flightlessness in rails. The ability to fly was lost at least 22 times independently. And the earliest transition to a non-flying lifestyle occurred about 12 million years ago, which overlaps with the age of the flightless Litorallus and Priscaweka from New Zealand. However, including trait-dependent dispersal did not allow the researchers to pinpoint the exact origin of the rails. The most likely ancestral areas are the Afrotropical and Neotropical regions (see figure above), but the uncertainty surrounding this ancestral distribution is quite large. This biogeographical mystery can perhaps be solved by including even more species-specific traits.

References

Garcia-R, J. C., & Matzke, N. J. (2021). Trait-dependent dispersal in rails (Aves: Rallidae): Historical biogeography of a cosmopolitan bird clade. Molecular Phylogenetics and Evolution159, 107106.

Featured image: Buff-banded Rail (Gallirallus philippensis) © J.J. Harrison | Wikimedia Commons

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