Sibley-Ahlquist taxonomy

The Sibley–Ahlquist taxonomy is a bird taxonomy proposed by Charles Sibley and Jon Edward Ahlquist. It is based on DNA–DNA hybridization studies conducted in the late 1970s and throughout the 1980s.

DNA–DNA hybridization is among a class of comparative techniques in molecular biology that produce distance data (versus character data) and that can be analyzed to produce phylogenetic reconstructions only using phenetic tree-building algorithms. In DNA–DNA hybridization, the percent similarity of DNA between two species is estimated by the reduction in hydrogen bonding between nucleotides of imperfectly complemented heteroduplex DNA (i.e., double stranded DNAs that are experimentally produced from single strands of two different species), compared with perfectly matched homoduplex DNA (both strands of DNA from the same species).

This revolutionary reordering was initially widely accepted by North American ornithologists, and the American Ornithologists' Union adopted some of its provisions. In other parts of the world its adoption has been more deliberative: it has been a major influence on existing classification schemes but hardly any authority adopted it in its entirety.

The classification appears to be an early example of cladistic classification because it codifies many intermediate levels of taxa: the "trunk" of the family tree is the class Aves, which branches into subclasses, which branch into infraclasses, and then "parvclasses", superorders, orders, suborders, infraorders, "parvorders", superfamilies, families, subfamilies, tribes, subtribes and finally genera and species. However the classification study did not employ modern cladistic methods, as it relies strictly on DNA-DNA hybridization as the sole measure of similarity.

The Sibley–Ahlquist arrangement differs greatly from the more traditional approach used in the Clements taxonomy. More recently published phylogenetic reconstructions based on cladistic and maximum likelihood analyses of DNA sequences lend credence to some of the DNA-DNA hybridization-based taxonomy, e.g. the recognition of palaeognathous birds as monophyletic and sister to all others. However, later studies failed to support many of the rearrangements in the Sibley–Ahlquist classification, such as the monophyly of the Corvida.

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