Abstract

Recent research has cast doubt on the reliability of bones and teeth for reconstructing phylogenetic relationships among higher primate species and genera. Herein, we investigate whether this problem is confined to hard tissues by examining the utility of higher primate soft-tissue characters for reconstructing phylogenetic relationships at low taxonomic levels. We use cladistic methods to analyze 197 soft-tissue characters for the extant hominoids and then compare the resulting phylogenetic hypotheses with the group's consensus molecular phylogeny, which is widely considered to be accurate. We show that the soft-tissue characters yield robust phylogenetic hypotheses that are compatible with the molecular phylogeny. Given the strength of the evidence for molecular phylogeny, these results indicate that, unlike craniodental hard-tissue characters, soft tissues are reliable for reconstructing phylogenetic relationships among higher primate species and genera. Thus, in higher primates at least, some types of morphological data are more useful than others for phylogeny reconstruction.
The bones, teeth, and soft tissues that make up the primate body have long been assumed to be useful for phylogenetic reconstruction at all relevant taxonomic levels. In recent years, however, researchers have begun to question the reliability of bones and teeth for reconstructing phylogenetic relationships among higher primate species and genera (1–14). This skepticism is based partly on the fact that phylogenetic analyses of fossil primates have thus far yielded conflicting and weakly supported hypotheses of relationships (4, 15, 16), partly on the fact that comparisons between craniodental phylogenies and reliable molecular phylogenies have found that the former disagree with the latter (1, 14), and partly on the fact that we are developing a better understanding of the processes involved in the generation of osteological and dental similarities and differences among primates (5–9, 12, 13, 17, 18). In this article, we describe a study that examined whether the reliability problem is confined to the bones and teeth of higher primates or whether their soft tissues are also unreliable for reconstructing species- and genus-level relationships.
To assess the phylogenetic utility of higher primate soft-tissue morphology, we carried out cladistic analyses of an extensive soft-tissue data set for the extant hominoids and then judged the resulting phylogenetic hypotheses against the group's consensus molecular phylogeny, which is widely considered to be accurate (19). This approach assumes that a match between morphological and molecular phylogenies is evidence that the morphological evidence is reliable, whereas incongruence indicates the converse (1, 14). There are several reasons for assuming that conflicts between the molecular and morphological phylogenetic hypotheses result from limitations of the morphological evidence. First, in phylogenetics, morphology can never be more than a proxy for molecular data, because phylogenetic relationships are genetic relationships. Second, the consensus molecular cladogram for the extant hominoids is supported by several sets of independent data (19). Agreement among multiple independent data sets is the strongest support possible for a phylogenetic hypothesis. Lastly, the methods of molecular phylogenetics have been successfully tested on taxa of known phylogeny, whereas comparable tests of morphological phylogenetic methods have proved unsuccessful (20–22).