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 Evolution, 159, 107106.

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

Unraveling the evolutionary history of the Galapagos Rail

When did this species reach the Galapagos Islands and where did it come from?

As more and more bird genomes are being sequenced (see this paper for the latest overview), it is surprising to come across bird species without genetic resources. But these species do exist, such as the Galapagos Rail (Laterallus spilonota). The lack of genetic studies on this rail might be an even bigger surprise when you took a closer look at its common name: this species occurs on the Galapagos, one of the most studied island archipelagos in the world. While some charismatic or historically relevant species attracted a lot of scientific attention – just think of the iconic Darwin’s Finches – the Galapagos Rail has not been studied with genetic tools yet. Hence, we know little about the evolutionary history of this rail and, more importantly, we have almost no knowledge about its conservation status from a genetic point of view. The Galapagos Rail might be heading for extinction and we would not have a clue. Luckily, a recent study in the journal Diversity provided the first genetic assessment of the Galapagos Rail.

Phylogenetics

Jaime Chaves and his colleagues sequenced the DNA of several recent and historical samples to reconstruct the evolutionary history of the Galapagos Rail. Phylogenetic analyses revealed that the ancestor of this species reached the Galapagos Islands about 1.2 million years ago. This timing is similar to other species, for example Darwin’s Finches arrived between 1 and 1.5 million years ago. The sister species of the Galapagos Rail turned out to be the Black Rail (Laterallus jamaicensis), which currently has a patchy distribution across North America and along the coast of Peru and Chile. The exact source population of Black Rails that gave rise to the Galapagos Rail remains to be determined with further sampling. It seems likely that the Galapagos Rail arose from birds dispersing out of the South American populations, but it is also possible that individuals migrating from North to Central America were blown off course and ended up on the Galapagos. Or perhaps an extinct “ghost” population was involved (such as in the Red-billed Chough). Plenty of hypotheses to explore.

The Galapagos Rail (#1) is most closely related to the Black Rail (#4) from which it split about 1.2 million years ago. From: Chaves et al. (2020) Diversity.

Genetic Diversity

More detailed analyses indicated little genetic differentiation between the island populations of the Galapagos Rail. This finding is rather surprising because this species is flightless and is thus not expected to disperse very far. The authors noted that these rails have been observed to forage near the coast and that they are capable of swimming significant distances. So, frequent movements between islands – which are on average 25 kilometers apart – are not impossible.

The researchers also reported low levels of genetic diversity in the Galapagos Rail, which can be explained by recent population bottlenecks. This species has suffered from human activities, such as habitat loss due to agricultural expansion and the introduction of non-native predators (mostly cats and rats). Currently, the Galapagos Rail can be found in higher numbers within restored habitats. They might thus recover from past population bottlenecks, although they still carry the genetic signatures of these events. Nonetheless, the information from this genetic study can help guide future conservation efforts to preserve the Galapagos Rail.

Only a few haplotypes (shared across different islands) were found for the different genetic markers. This low level of genetic diversity is probably due to past population bottlenecks. From: Chaves et al. (2020) Diversity.

References

Chaves, J. A., Martinez-Torres, P. J., Depino, E. A., Espinoza-Ulloa, S., García-Loor, J., Beichman, A. C., & Stervander, M. (2020). Evolutionary history of the Galápagos Rail revealed by ancient mitogenomes and modern samples. Diversity, 12(11), 425.

Featured image: Galapagos Rail (Laterallus spilonota) © John Gould | Wikimedia Commons

From the sky to the island: Flightless rail species converge on the same body plan

Several species independently evolved shorter sterna and stronger hindlegs.

Although birds are known for their aerial lifestyle, several species evolved towards a flightless existence. Common examples include the Ostrich (Struthio camelus) and the Southern Cassowary (Casuarius casuarius). Loss of flight occurred independently in numerous bird families and a recent study in the journal Science Advances showed that we are underestimating the frequency of this evolutionary transition if we only focus on the ca. 10,000 extant birds. Including data from 581 known human-related extinctions quadrupled the number of flightless species, suggesting that loss of flight has occurred independently at least 150 times. The pervasiveness of flightless birds across the avian Tree of Life is a nice example of convergent evolution: the independent evolution of similar features in distantly related species. But how convergent is this evolutionary change? Do birds just lose the ability to fly or do other traits change as well? The rails (Rallidae) provide the ideal bird family to explore these questions.

Taking into account recently extinct species reveals that loss of flight occurred numerous times during the evolution of birds. From: Sayol et al. (2020) Science Advances.

 

Running Rails

The rail family holds about 130 species of which more than 30 have lost the ability to fly. The majority of these flightless species are endemic to remote islands, suggesting that their ancestors could fly to the islands. Once on solid ground, the ancestral populations had little use for flying (perhaps because there were no predators to escape from) and gradually transitioned to a flightless life. Julien Gaspar and his colleagues assembled a dataset with 10 morphological traits for 90 rail species consisting of extant and recently extinct species.

The principal component analysis (PCA) depicted below nicely summarizes the findings of this study, which appeared in the journal Ecology and Evolution. A PCA captures the variation in a dataset – 10 morphological traits in this case – and condenses it into a few principal components. On the graph, you can see that the first component holds 41.8% of the variation while the second component explains 23.4%. These two axes clearly discriminate between flying (in red) and flightless (in black) rail species, although there is some overlap. The blue arrows show how different morphological traits correlate with the principal components. For example, the sternum depth and sternum length point upwards, indicating that species higher in the PCA have deeper and longer sternums. This makes sense because flying birds need a strong sternum to attach their flight muscles. Moreover, the arrows related to the sternum point away from the flightless species in the bottom of the graph. This means that flightless rails have smaller sterna.

A principal component analysis separates flying (red) from flightless (black) rails and shows which morphological traits (blue arrows) are associated with these lifestyles. From: Gaspar et al. (2020) Ecology and Evolution.

 

Genetic Path to a Flightless Life

Take your time to explore the PCA graph and you will see how different traits relate to flying and flightless birds. You should be able to deduce that flightless birds exhibit smaller sterna and wings than flighted taxa along with a wider pelvis and a more robust femur. These findings suggest that flightless rails became heavier and evolved stronger hindlegs to cope with a life on the ground. These traits evolved independently in several species, providing a nice example of convergent evolution on the morphological level.

The next step could be to investigate whether this convergence can be extended to the genetic level. Do mutations in the same genes underlie these morphological changes or are different genetic pathways under selection on different islands? A quick look at the reduction of wing size in other species provides some clues. In the flightless Galapagos Cormorant (Phalacrocorax harrisi), mutations in cilia-related genes contributed to the development of small wings. In the Emu (Dromaius novaehollandiae), however, reduced signaling in the early forelimb progenitor cells led to a delay in wing bud growth, culminating in shorter wings. There are thus several genetic paths to reduced wings. Does this also apply to other morphological traits?

 

References

Gaspar, J., Gibb, G. C., & Trewick, S. A. Convergent morphological responses to loss of flight in rails (Aves: Rallidae). Ecology and Evolution.

Sayol, F., Steinbauer, M., Blackburn, T., Antonelli, A., & Faurby, S. (2020). Anthropogenic extinctions conceal widespread evolution of flightlessness in birds. Science Advances.

Featured image: Guam Rail (Gallirallus owstoni) © Greg Hume | Wikimedia Commons