Morphological Techniques

Morphological taxonomy relies on the selection of certain character states, the presence or absence of which groups species together. As species with a common ancestry evolve and diverge through time, anatomical features that their common ancestor shared will either be conserved or lost. Genetic mutations occur over time within a species' genome and alter or modify that species' phenotype. Therefore, if two species share many highly specialised anatomical features (termed homology), that are unlikely to have arisen by chance, then it is likely that they once shared a common ancestor which also possessed those traits, thereby meaning those species are relatively closely related.

The major anatomical domains utilised by systematists to define orders and families of placental mammals are the teeth, auditory region and proximal tarsus (ankle). Of the three major anatomical domains utilised in mammalian morphology, arguably the most revealing is the teeth. Indeed, Colbert (1991) stated that if all the eutherian mammals were extinct, and represented only by fossil teeth, "their basic classification would be essentially the same as the classification now drawn up on the knowledge of the complete anatomy of the animals". Others are less complimentary of the benefits of classification using dentition. Novacek (1992) for example, accuses "the abiding fascination with tooth structure" of being responsible for many confusing and highly incongruous higher mammalian relationships. Dental traits, it would appear, seem prone to rapid and substantial parallel changes.

The major problem of comparative morphology as means of determining phylogenetic relationships is homoplasy. Homoplasy (or analogy) is the expression of a very similar trait in two or more species that has arisen by some reason other than inheritance from a common ancestor. Such reasons include convergent and parallel evolution.

By the 1950's phylogenetics using comparative morphology was no longer the groundbreaking science it had been, and research was concentrating only on the smaller branches of the tree. Phylogeneticists it seemed had exhausted the available morphological data, and the techniques being used were not stringent enough in their distinction between homology and analogy. In reference to the decline of morphological phylogenetics, Paterson declared "the clues are poor, the trail is cold". Morphological studies may not therefore yield any further dramatic insights into the large-scale interordinal relationships of placental mammals, but comprehensive research is still carried out to determine intraordinal relationships. The future of using comparative morphology for phylogenetics may lie in utilising both morphological and molecular data. In this way, the phylogenticist receives a more rounded perspective of possible systematic relationships, and hopefully further helps to reduce the problems of homoplasy.