Why Morphometrics is Special: The Problem With Using Partial Warps as Characters for Phylogenetic Inference

Abstract

There has been a dispute among morphometricians (Fink and Zelditch, 1995; Zelditch, Fink, and Swiderski, 1995; Adams and Rosenberg, 1998; Rohlf, 1998; Swiderski, Zelditch, and Fink, 1998; Zelditch, Fink, Swiderski, and Ludrigan, 1998) on the possibility of using morphometric data, ex? pressed as shape variables (particularly par? tial warps) extracted by geometric morpho? metrics methods, in phylogenetic analyses. Much has been said against and in favor of this particular combination of geometric morphometrics and cladistics, but the issue is not yet resolved. This is probably because the single point that makes the partial warps unsuitable as characters cladistic analysis, was not thoroughly considered: the lack of biological significance in partial warps when treated as single univariate characters. To project a multidimensional space into a set of single dimensions by different rotations parsimony analysis, the single dimensions must have biological significance by them? selves (what cannot be expected from partial warps, Rohlf, 1998). This problem is derived from the shape nonmonotonicity theorem, originally published by Bookstein (1980), that asserts: for any three triangles of land? marks, no two of which have exactly the same shape, and any of three triangles, there exist indefinitely many shape measures consistent with that ordering (Bookstein, 1994, p. 206). In other words, the ordination patterns in shape space are not consistent all possible shape variables, although the dis? tances among shapes are always preserved. Our perception of the world is deeply de? pendent on the dimensions we can observe. Natural phenomena are usually expressed in a multivariate fashion, even though most of us are limited to visualizing things in a max? imum of three dimensions. To understand