What is the first feature that comes to mind when you think of a bird? Probably that it can fly! However, this distinguishing ability is not shared among all 11,000 bird species, as there are well-known examples of flightless bird, such as ostriches, kiwis, or penguins. But when the ancestral birds evolved from one of the dinosaur lineages, they could indeed fly, which means that ostriches, kiwis, and penguins have lost this ability over the course of evolution.
Less well-known than ostriches and kiwis is the family of birds that comprises most flightless species today, namely the rails (Rallidae). These birds are often secretive and many live their lives in so dense vegetation that they are seldomly spotted. Among the volant species, some species are sedentary while others migrate, but migration happens during the night and therefore generally escapes our attention. Nevertheless, rails are the most incredible dispersers! They have a strong tendency to fly out across the ocean, because most islands around the globe have been colonized by rails at one point or another. If the island is isolated, over time the island rail population will diverge from its mainland ancestor and become a separate species, edemic to the island or archipelago. If the islands are not home to predatory mammals, the island rails lose their flight ability over time, and hundreds or even over a thousand flightless rail species are estimated to have thrived on islands several hundred years ago. Then came waves of human settlement, and with them the introduction of mammals, which led to the rapid extinction of flightless island birds.
But how can flight loss evolve repeatedly in the same group of birds? We know that it is not because it first evolves on one island and is then spread to another, because once the rails lose flight, they also lose their dispersal capability. Instead, when arriving to a predator-free island, the rails face similar conditions. Flying requires large pectoral muscles and is metabolically very costly, so if you do not need to, instead allocating energy into sturdier legs is beneficial. Thus, the evolution towards flight loss if predictable and convergent. But what does convergent morphological evolution look like at a molecular level? Are the same changes happening independently in the genomes of different island lineages evolving flight loss? Or do a myriad of different genomic changes lead to the same result? Whether evolution is predictable at a molecular level is still a fundamental question in evolutionary biology. It also specifically offers understanding of the unique evolution of island biota, and may be relevant in consideration of conservation measures.
This action exploited the natural experiment that rails offer. Focusing on the Laterallus, Zapornia, and Hypotaenidia rails, replicates of insular flightless species and their volant sister species on the mainland were studied. The most recent common ancestors of some of those species pairs were also sisters species.
