Examining Basal avian divergences with mitochondrial sequences: model complexity, taxon sampling, and sequence length.

Traditional avian classiŽcations have generally indicated that the paleognathous birds (ratites and tinamous) represent the earliest divergence within the extant birds and the perching birds (Passeriformes) represent one of the most recently derived lineages (reviewed by Sibley and Ahlquist, 1990; Feduccia, 1995; see Fig. 1A). However, the conclusions of several recent studies using complete mitochondrial genome sequences from birds have challenged these classiŽcations (Mindell et al., 1997, 1999; Härlid and Arnason, 1999; Haring et al., 2001; but see Paton et al., 2002). In particular, these studies have suggested that the passeriforms are one of the most ancient groups of extant birds and that the paleognaths are derived. This conclusion has profound implications for avian evolution. An ancient origin of passerines could explain the remarkable diversity of this group, which represents over half of modern avian species, without invoking different rates of cladogenesis and/or extinction in the passerines and other groups of birds. Likewise, a derived position of paleognaths would require a neotenous origin of paleognath morphological characters (Härlid and Arnason, 1999) and a different model of avian sex chromosome evolution (Ellegren and Carmichael, 2001). Although complete avian mitochondrial sequences provide a large number of sites, it seems unlikely that mitochondrial data have provided an accurate picture of avian