A PHYLOGENETIC STUDY OF THE BLACKBIRDS BASED ON VARIATION IN MITOCHONDRIAL-DNA RESTRICTION SITES

We used a generalized parsimony model to estimate the phylogeny of 47 species in the avian subfamily Icterinae using restriction enzyme cleavage sites in mitochondrial DNA (mtDNA). Deep branches in the tree are poorly resolved, either because blackbirds radiated rapidly after the origin of the clade or because substitutions have saturated sites in the mtDNA. Our tree is similar to one published by Lanyon (1994, Evolution 48:679-693) based on mtDNA sequences, which suggests that lack of basal resolution might be attributable to closely spaced speciation events. Both data sets had similar consistency and retention indices, suggesting comparable information content in these two types of data. Neither the restriction-site tree nor the sequence tree support the monophyly of the genus Agelaius. The restriction-site tree suggests that several traditional taxonomic rearrangements are likely based on phenotypic resemblance rather than phylogeny and that the feature viewed as a key innovation of blackbirds (gaping) might not be the basal condition; gaping might be a composite of characters rather than a single homologous condition. The restriction-site tree suggests that the brood-parasitic cowbird species are not monophyletic, contra the sequence data. A combined analysis of sequences and restriction sites for cowbirds supports the sequence tree of Lanyon (1994); however, we suggest an alternative view of the evolution of brood parasitism that is also consistent with the combined data tree. Polygyny appears to be independently derived within blackbirds. Icterus galbula and I. bullockii, which hybridize and are therefore currently considered a single biological species, are not sister taxa; these are likely phylogenetic species.