Monophyly and Sister Group
Historically, it is generally agreed that the Passeriformes constitute a monophyletic group. Raikow (1982) established this monophyly in an explicitly phylogeneticcontext. He noted that Passeriformes possess a suite of distinguishing characteristics, including a unique sperm morphology, a distinctive morphology of the bony palate, a simple yet functionally diverse foot with three toes forward and one (the hallux) oriented backwards, and a distinctive fore- (wing) and hindlimb musculature. There are few if any species which pose problems for avian systematists as to whether they are or are not passerines. Most of the controversy lies in relationships within the clade.
The sister group of the Passeriformes is not so much hotly contested as it is poorly resolved by existing data sets. Traditionally, Passeriformes have been considered closely related to a large group known as the “higher non-Passerines”. These include a number of clades such as cuckoos (Cuculiformes), hornbills, kingfishers and related lineages (Coraciiformes), and woodpeckers and relatives (Piciformes). Many of these groups possess a zygodactyl foot, a condition in which two toes point forward and two point backward. The sister relationship of Passeriformes to woodpeckers, the hornbill group and allies is reflected in Joel Cracraft’s phylogenetic hypothesis for major groups of birds based on cladistic interpretation of morphological and molecular characters (Cracraft, 1988). However, in the other major classification bearing on the relationships of perching birds, that based on DNA-DNA hybridization, Passeriformes appear as the sister group to a large, diverse group containing pigeons and doves (Columbiformes), cranes and rails (Gruiformes) and storks (Ciconiiformes)! These latter three groups share few obvious morphological characteristics with Passeriformes. However, the DNA hybridization tree links Passeriformes with these groups at a very deep level in the tree, rendering this result tenuous. A recent study of nuclear DNA sequences by Hackett et al. (2008) finds Psittaciformes (parrots) to be the sister group of passerines, with Falconidae (falcons) also close. Clearly, more work on the sister-group relationship of Passeriformes is needed, since this relationship will be the basis of any study seeking to identify whether or not Passeriformes are a particularly diverse group (e.g., Nee et al. 1992).
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