Homoplasy and adaptation in the atelid postcranium

Homoplasy is a ubiquitous phenomenon in phylogenetic investigations, but it is rarely investigated on its own. As a case study in the pattern and basis of homoplasy in primates, the atelid postcranium is discussed here. Characters available from Ford's ([1986] in Erwin J, Swindler DR, eds: Comparative Primate Biology I: Systematics, Evolution, and Anatomy (New York: Alan R. Liss), p 73-135; [1994] in Fleagle JG, Kay RF, eds: Anthropoid Origins (New York: Plenum Press), p 595-674) analyses of New World monkeys are mapped onto alternative phylogenetic trees for the family Atelidae to contrast patterns of character evolution and to develop explanatory hypotheses for differences in the trees. In an unrooted phylogenetic network, pitheciines do not group together because those pitheciines that routinely adopt hind limb suspensory postures (Chiropotes, Cacajao) share traits with atelines. Ford's (1986) work on phylogeny has shown that these traits are homoplastic and also identified potential synapomorphies of a clade comprised of modern pitheciins and atelines. However, following that work, congruence between studies of craniodental and molecular data suggested a still broader definition of atelids (including Callicebus and Cebupithecia), and in this case only one trait may define atelids, and several traits arise in parallel. The homoplastic characters in this phylogeny suggest that the phylogenetic signal in this set of postcranial data is overwhelmed by parallel adaptations to the use of climbing behaviors in all of Ford's atelids and suspensory postures in a more restricted set of taxa. These parallelisms probably indicate a bias of selective pressures in the South American environment, especially given the frequent, independent evolution of suspensory mammals there. This highlights the fact that homoplasy can be a dominant source of similarity in data partitions strongly influenced by a particular behavioral regime, in this case positional behavior.