TheSibley–Ahlquist taxonomy is abirdtaxonomy proposed byCharles Sibley andJon E. Ahlquist. It is based onDNA–DNA hybridization studies conducted in the late 1970s and throughout the 1980s.[1]
DNA–DNA hybridization is among a class of comparative techniques inmolecular biology that produce distance data (versus character data) and that can be analyzed to produce phylogenetic reconstructions only usingphenetic tree-building algorithms. In DNA–DNA hybridization, the percent similarity of DNA between two species is estimated by the reduction inhydrogen bonding betweennucleotides 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).
The classification appears to be an early example ofcladistic classification[clarification needed] because it codifies many intermediate levels of taxa: the "trunk" of the family tree is the classAves, 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 theClements taxonomy.
| Basal divergences of modern birds in the Sibley–Ahlquist taxonomy |
Showing major changes from Clements, the Sibley–Ahlquist orders are as follows:
Some of these changes are minor adjustments. For instance, instead of putting the swifts, treeswifts, and hummingbirds in the same order that includes nothing else, Sibley and Ahlquist put them in the same superorder that includes nothing else, consisting of one order for the hummingbirds and another for the swifts and treeswifts. In other words, they still regard the swifts as the hummingbirds' closest relatives.
Other changes are much more drastic. The penguins were traditionally regarded as distant from all other living birds. For instance, Wetmore put them in a superorder by themselves, with all other non-ratite birds in a different superorder. Sibley and Ahlquist, though, put penguins in the same superfamily as divers (loons), tubenoses, andfrigatebirds. According to their phylogenetic analysis, penguins are closer to those birds thanherons are tostorks.[3]
TheGalloanserae (waterfowl and landfowl) has found widespread acceptance. The DNA evidence of Sibley–Ahlquist for the monophyly of the group is supported by the discovery of the fossil birdVegavis iaai, an essentially modern but most peculiar waterfowl that lived nearCape Horn some 66–68million years ago, still in the age of thedinosaurs.[4]
