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.
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.
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?
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