Data Set Incongruence and Correlated Character Evolution: An Example of Functional Convergence in the Hind-Limbs of Stifftail Diving Ducks

Abstract

The unwitting inclusion of convergent characters in phylogenetic estimates poses a serious problem for efforts to recover phylogeny. Convergence is not inscrutable, however, particularly when one group of characters tracks phylogeny and another set tracks adaptive history. In such cases, convergent characters may be correlated with one or a few functional anatomical units and readily identifiable by using comparative methods. Stifftail ducks (Oxyurinae) offer one such opportunity to study correlated character evolution and function in the context of phylogenetic reconstruction. Morphological analyses place stifftail ducks as part of a large clade of diving ducks that includes the sea ducks (Mergini), Hymenolaimus, Merganetta, and Tachyeres, and possibly the pochards (Aythyini). Molecular analyses, on the other hand, place stifftails far from other diving ducks and suggest, moreover, that stifftails are polyphyletic. Mitochondrial cytochrome b gene sequences of eight stifftail species traditionally supposed to form a clade were compared with each other and with sequences from 50 other anseriform and galliform species. Stifftail ducks are not the sister group of sea ducks but lie outside the typical ducks (Anatinae). Of the four traditional stifftail genera, monophyly of Oxyura and its sister group relationship with Nomonyx are strongly supported. Heteronetta probably is the sister group of that clade, but support is weak. Biziura is not a true stifftail. Within Oxyura, Old World species (O. australis, O. leucocephala, O. maccoa) appear to form a clade, with New World species (O. jamaicensis, O. vittata) branching basally. Incongruence between molecules and morphology is interpreted to be the result of adaptive specialization and functional convergence in the hind limbs of Biziura and true stifftails. When morphological characters are divided into classes, only hind-limb characters are significantly in conflict with the molecular tree. Likewise, null models of synonymous and nonsynonymous substitution based on patterns of codon-degeneracy and chemical dissimilarity indicate that the nucleotide and amino acid changes postulated by the molecular tree are more plausible than those postulated by the morphological tree. These findings teach general lessons about the utility of highly adaptive characters (in particular those related to foraging ecology) and underscore the problems that convergence can pose for attempts to recover phylogeny. They also demonstrate how the concept of natural data partitions and simple models of evolution (e.g., parsimony, likelihood, neutrality) can be used to test the accuracy of independent phylogenetic estimates and provide arguments in favor of one tree topology over another.

Estado de conocimiento de las aves de aguas continentales de Chile

“…En este último trabajo se incluyen, además de Phoenicopterus chilensis, las especies Dendrocygna viduata, Oxyura jamaicensis y Nycticorax nycticorax. McCracken et al (1999) estudiaron los patrones de evolución de algunos caracteres morfológicos, en patos, apoyados en evidencia molecular, en cuyo análisis incluyen las dos especies de Oxyura presentes en Chile, y a Cygnus melanocoryphus como grupo externo. Existen más publicaciones sobre la filogenia de otros grupos de aves acuáticas, como por ejemplo para caradriformes (Joseph et al 1999;Crochet & Desmarais 2000), pero no es la finalidad de este trabajo entregar una descripción exhaustiva de esta literatura.…”

Heterochronic Detection Through a Function for the Ontogenetic Variation of Bone Shape

“…According to Boughton et al (1991), the ancestral condition for the character of interest should be estimated to determine the direction of the heterochronic change. The optimization of femur shape onto the phylogenetic tree of Anseriformes (modified from McCracken et al, 1999) shows that, as expected, the femur of the last common ancestor of Anas platyrhynchos and Bucephala clangula is also robuster than the femur of Anas platyrhynchos (Fig. 1).…”

“…Most phylogenetic relationships among and within different orders of Neoaves are uuresolved. Regarding Anseriformes and Ratites, we have used trees modified, respectively, from McCracken et al, 1999 andvan Tuinen &Hedges, 2001. Both molecular clock data (van Tuinen & Hedges, 2001) and the fossil record (Cracraft, 2001, and references herein) were used to estimate divergence times between avian clades.…”

“…1. Optimization of femur shape, the ratio bone length (the result of endochondral ossification)jbone diarneter (the result of periosteal ossification), onto a phylogenetic tree of Anseriformes (modified from McCracken et al, 1999). The estimation of ancestral character states was carried out through squared-change parsimony, a method that rninirnizes the sum of squared changes along the branches (Maddison, 19911.…”

Phylogenomic evidence for multiple losses of flight in ratite birds

“…There is no reason to expect convergence in the cranial characters of tinamous and any particular subset of ratites. However, convergence is common in the postcranial skeletons of other flightless birds (54), and morphological convergence certainly can mislead phylogenetic analysis (55). Flightlessness is expected to result in reduction or loss of the sternal keel; reduction in size, complexity, and number of wing bones; increase in size of leg bones; and nonaerodynamic changes in plumage structure.…”