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Sibley-Ahlquist taxonomy
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Everything about The Sibley-ahlquist Taxonomy totally explainedThe Sibley-Ahlquist taxonomy is a radical 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 (Sibley & Ahlquist 1990). 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, (for example, 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 cladistic 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. In fact, the Sibley-Ahlquist taxonomy is merely a highly structured result of phenetic studies; no cladistic methods were used in producing it. Though the strong subdivision into taxonomic ranks suggests accuracy, the "parv"-ranks are simply considered unnecessary as of 15 years later for example.
Basal divergences of modern birds
based on Sibley-Ahlquist taxonomylist of birds. 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, for example the recognition of palaeognathous birds as monophyletic and sister to all others, and the recognition that fowl and waterfowl ( Galloanserae) are one anothers' closest relatives and sister to the remainder of all birds, Neoaves. However, new studies also categorically reject many of the rearrangements in the Sibley-Ahlquist classification, for example, the all-encompassing order Ciconiiformes, placement of buttonquails at the base of Neoaves, and the monophyly of various clades of songbirds, to name a few.
Even DNA-DNA hybridization studies produced more recently in other laboratories disagree with some of Sibley and Ahlquist's results, for example, inclusion of the limpkin in the sungrebe family, and the placement of flamingos near storks rather than next to grebes.
The major changes at order level are as follows:
- Enlarged Struthioniformes replaces the ratite orders Rheiformes (rheas), Casuariiformes (cassowaries), and Apterygiformes (kiwis) and Struthioniformes (ostriches).
Tinamiformes (tinamous) is unchanged.
A new, greatly enlarged Ciconiiformes includes the previous Sphenisciformes (penguins), Gaviiformes (divers), Podicipediformes (grebes), Procellariiformes (tubenoses), Pelecaniformes (pelicans and allies), Ciconiiformes (storks and allies), Falconiformes (birds of prey), Charadriiformes (waders, gulls, terns, and auks), and the family Pteroclidae (sandgrouse).
Anseriformes (ducks and allies) is unchanged.
New Craciformes chachalacas etc. Previously Galliformes
New Gruiformes Cranes
New Turniciformes button-quails etc. Previously Gruiformes
Columbiformes doves. Sandgrouse moved to Ciconiiformes.
Psittaciformes cockatoos and parrots unchanged
New Musophagiformes turacos. Previously Cuculiformes.
New Cuculiformes rest of cuckoos
New Strigiformes owls enlarged to include Caprimulgiformes nightjars
New Apodiformes swifts
New Trochiliformes hummingbirds. Previously Apodiformes.
Coliiformes mousebirds unchanged
Trogoniformes trogons unchanged
New Coraciiformes rollers
New Upupiformes Hoopoe, previously Coraciiformes
New Bucerotiformes hornbills, previously Coraciiformes
New Galbuliformes jacamars and puffbirds, previously Piciformes
Piciformes woodpeckers
Passeriformes perching birds unchanged.
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, and frigatebirds. According to their view, penguins are closer to those birds than herons are to storks.
This revolutionary reordering has been widely accepted by North American ornithologists, and the American Ornithologists' Union initially adopted some of its provisions. In other parts of the world its adoption has been more deliberative: it has been a respected major influence on existing classification schemes but hardly any authority adopted it in its entirety.
A more recent paper by van Tuinen, Sibley, and Hedges looked in more detail at the early ancestry of bird groups. The traditional view of avian evolution places ratites and tinamous at the base of the tree of modern birds (Neornithes), followed by old marine groups such as the penguins, grebes and divers.
The new research suggested that the ducks and gallinaceous birds are each other's closest relatives and together form the basal lineage of neognathous (non-ratite) birds, distinct from the others which are collectively called Neoaves. The ratites and tinamous are followed by the ducks and their allies and the pheasants and their allies. Penguins, grebes and divers are placed with other groups that were traditionally considered more modern.
The Galloanseres (waterfowl and landfowl) have found widespread acceptance and roundabout support. The ancientness was splendidly confirmed with the 2005 report on Vegavis iaai, an essential modern but most peculiar waterfowl that lived near Cape Horn some 66-68 million years ago, still in the age of the dinosaurs.
On the other hand, penguins, grebes, divers, and so on (colloquially sometimes called "higher waterbirds") are still considered very ancient neoavian orders - quite possibly together with the shorebirds (waders) which seem a bit older still, the most ancient ones. The supposed distinctness of the storks and herons as well as at least the supposed degree of closeness of penguins to frigatebirds have been refuted. They, as well as the "Ciconiiformes" assemblage, appear to be due to the shortcomings, both methodological and analytical, of DNA-DNA hybridization. Today, the AOU is still using the 1990 taxonomy as a baseline, but has adopted sweeping changes to the original layout.
Palaeognathae
Neognathae
Galloanserae
Turnicae
Picae
Coraciae
Coliae
Passerae
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