Abstract
Flight loss has evolved independently in numerous island bird lineages worldwide, and particularly in rails (Rallidae). The Aldabra white-throated rail (Dryolimnas [cuvieri] aldabranus) is the last surviving flightless bird in the western Indian Ocean, and the only living flightless subspecies within Dryolimnas cuvieri, which is otherwise volant across its extant range. Such a difference in flight capacity among populations of a single species is unusual, and could be due to rapid evolution of flight loss, or greater evolutionary divergence than can readily be detected by traditional taxonomic approaches. Here we used genetic and morphological analyses to investigate evolutionary trajectories of living and extinct Dryolimnas cuvieri subspecies. Our data places D. [c.] aldabranus among the most rapid documented avian flight loss cases (within an estimated maximum of 80,000–130,000 years). However, the unusual intraspecific variability in flight capacity within D. cuvieri is best explained by levels of genetic divergence, which exceed those documented between other volant taxa versus flightless close relatives, all of which have full species status. Our results also support consideration of Dryolimnas [cuvieri] aldabranus as sufficiently evolutionary distinct from D. c. cuvieri to warrant management as an evolutionary significant unit. Trait variability among closely related lineages should be considered when assessing conservation status, particularly for traits known to influence vulnerability to extinction (e.g. flightlessness).
Highlights
Organisms living in island environments frequently undergo remarkable evolutionary changes [1,2,3,4]
Based on Bayesian analyses of 10 million generations, divergence time estimates show consistent variation depending on the tree prior used (Yule estimates being older than Coalescent-Uniform estimates, and Coalescent-Uniform estimates being older than Coalescent-Inversegamma estimates)
We selected Yule and Coalescent-Inversegamma tree priors for our final two Bayesian analyses, thereby obtaining longer chains for the two tree priors that cover the full range of divergence estimates based on findings from shorter chains
Summary
Organisms living in island environments frequently undergo remarkable evolutionary changes [1,2,3,4] One such change is loss of flight, which has occurred worldwide in 26 bird families from 17 orders [5]. Despite the high incidence of avian flight loss on islands [8], the pace of evolutionary transitions underlying this trait is poorly known This is at least partly due to the fact that many insular flightless or poorly volant bird species are extinct, and the scarcity of cases [5,10,11,12] in which there exist gradations in flightlessness among or within extant lineages
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