Taxonomy
The rails are a bird family comprising nearly 150 species. Although the origins of the group are lost in antiquity, the largest number of species and the most primitive forms are found in the Old World, suggesting that this family originated there. However, the genus Rallus, the group of long-billed reed bed specialists to which the Water Rail belongs, arose in the New World. Its Old World members, the Water, African and Madagascar Rails, form a superspecies, and are thought to have evolved from a single invasion from across the Atlantic. Genetic evidence suggests that the Water Rail is the most closely related of its genus to the Pacific Gallirallus rails, and is basal to that group. The Water Rail was first described by Linnaeus in his Systema Naturae in 1758 under its current scientific name, Rallus aquaticus. The binomial name is the Latin equivalent of the English "Water Rail".
The subspecies R. a. indicus has, in addition to its distinctive plumage, very different vocalisations to the other subspecies, and it was considered a separate species in early works, including the first edition (1898) of Fauna of British India, but later demoted to a subspecies by E. C. Stuart Baker in the second edition (1929). It was restored as a full species, the Eastern Water Rail, R. indicus, by Pamela Rasmussen in her Birds of South Asia (2005). Rasmussen, an expert on Asian birds, also renamed the other forms as the Western Water Rail. Her treatment has not otherwise been widely adopted, but is followed in Birds of Malaysia and Singapore (2010). A 2010 study of molecular phylogeny further supported the possibility of specific status for R. a. indicus, which is estimated to have diverged from the western forms around 534,000 years ago. The paper also suggested that the differences between the three other races were clinal, and that they should all be merged into R. a. aquaticus.
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