Miriam L. Zelditch, William L. Fink, and Donald L. Swiderski

Abstract.---It has been claimed that quantified features are inappropriate for phylogenetic analysis. We consider that claim to be true under most conditions for characters discovered by commonly used morphometric methods, including outline-based and conventional multivariate methods. The most important reason these characters are unsuitable is that one of the tests of homology, the test of similarity, may be difficult to apply to them. This test is not even possible if the methods for comparing forms, such as outline-based techniques, do not ensure that the characters are located in the same part of the anatomy. Conventional methods, including principal components analysis, have no explicit basis for localizing characters. In addition, unless the transformation between forms is homogeneous, conventional methods cannot dissect transformations region by region to discover characters. However, one morphometric method, the thin-plate spline decomposed by its partial warps (TPS), finds characters that can be subjected to the same tests of homology (conjunction, similarity, and congruence) that we would apply to all other characters. Among available methods, TPS is unique in being able to locate the center and spatial extent of regional differences in shape and ensures that the same regions are compared among forms. We provide an example using the teleost fishes piranhas, in which tests of homology are applied to a synapomorphy found by the method. [Morphometrics; homology; synapomorphy; thin-plate spline; character analysis; piranha; Pygocentrus; Pygopristis; Serrasalmus.]