MOLECULAR AND OSTEOLOGICAL HERON PHYLOGENIES - SOURCES OF INCONGRUENCE

Payne and Risley's (1976) comparison of 33 osteological characters of herons was the first cladistic estimate of heron phylogeny. Among thei r findings were two major clades: (1) Boat-billed Heron (Cochlearius c ochlearius), night-herons, and bitterns; and (2) tiger-herons and day- herons. In contrast, more recent DNA-DNA hybridization comparisons, cl adistic analyses of vocalizations, and mtDNA sequence data portray a m ore asymmetric phylogeny, with day-herons and night-herons forming a c lade with bitterns as their sister group, and tiger-herons and the boa t-billed Heron branching basally. To explore the source of the disagre ement between these phylogenetic estimates, we reanalyzed the osteolog ical data using modern cladistic methods and compared the results with the DNA-DNA hybridization tree using taxonomic congruence analysis. C haracter-by-character comparisons between trees and among lineages wit hin trees suggest that similar cranial morphology in the relatively un related tiger-herons and day-herons has resulted in the misleading att raction of these two lineages in osteological estimates of phylogeny. Apparent convergence in bill morphology and modifications of orbital s tructures for nocturnal feeding in night-herons and Boat-billed herons have led to further disagreement between data sets. In part, problems in the osteological data stem from the relatively small character mat rix of Payne and Risley (1976), but ultimately they may derive from us ing highly adaptive characters to reconstruct phylogeny. In this case, the cranial characters are functionally correlated as part of the pis civorous heron Bauplan. As such, they relate to the forces responsible for speciation and divergence in the early history of the group but m ay not be useful for phylogenetic inference. The discovery of bias in cranial characters underscores the value of taxonomic congruence analy sis and the need to explore cases of phylogenetic incongruence.