Species diversity, origin and biogeography
Species diversity: The tradition of recognizing perching birds (Passeriformes) as the most diverse and rapidly radiating clade has been questioned because there are few obvious “key innovations” that should cause systematists to recognize Passeriformes over any other arbitrarily larger or smaller monophyletic group within birds (Raikow, 1986). One point that has been missed in debates on this issue is that the branch leading to the songbirds (oscines), a group comprising 80% of extant perching birds, is the longest internal branch on the DNA hybridization tree produced by Sibley and Ahlquist (1990). This branch has also been one of the few to be well resolved in applications of mtDNA sequences to higher level questions in birds, presumably because it is long. Given the large number of clades that will require names under phylogenetic taxonomy, perhaps the length of branches leading to particular clades should be one criterion whereby systematists decide which of the many clades to name.
Origin and biogeography of passerines: The temporal and geographic origin of passerine birds is obscure. Traditionally the group was thought to have originated in the Tertiary, at about the same time as extant orders of mammals. Some recent workers favor a later, Eocene origin (Feduccia, 1995; Wilson, 1989), but the DNA -DNA hybridization data again favors an earlier origin (Sibley and Ahlquist, 1990). Recently some of the oldest oscine fossils have been uncovered in Queensland, Australia (Boles, 1995); this and other paleobiogeographical data suggest that passerines may have in fact originated in the Southern hemisphere (Olson, 1989).
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