Biology Reference
In-Depth Information
CHAPTER
13
Morph
ometrics and Syst
ematics
Systematists use morphometrics to answer three types of questions. The first, which we
label “taxonomic”, asks whether samples are drawn from multiple taxa and, if so, by what
variable(s) they are most effectively discriminated. The second, which we label “phyloge-
netic”, seeks to identify traits distributed among taxa in patterns that may be consistent
with their phylogeny and then infer that phylogeny from a consensus of those traits. The
third, which we label “evolutionary”, seeks to describe the historical evolutionary transfor-
mations of the features of interest. These are all interrelated issues, but there are important
distinctions that bear on choosing appropriate analytic methods, and also on the suitability
of different kinds of traits or trait descriptions.
One critical distinction is between discriminators and characters. It might seem obvious
that taxonomic discriminators are potential characters because both are features that differ
among taxa, however, taxonomic discriminators are not characters because discriminators
describe the net difference between taxa; they are vectors extending between (or among)
terminal taxa. The vector describes the direction in which the taxa can be distinguished
from each other, regardless of whether the features distinguishing them are unique to one
species, are shared by a group containing two species in the analysis, or are more broadly
shared (with taxa not included in the analysis). That vector need not be aligned with a
direction of evolutionary change; all that matters is that the discriminator exists (telling us
that the taxa are indeed different) and is successful (allowing us to identify unknowns cor-
rectly). In contrast to a discriminator, a character is a feature shared by members of a
monophyletic group. In principle, a character is a feature that is recognized as transform-
ing at the node of a cladogram, and thus represents a hypothesis of the direction of evolu-
tionary change. If shapes could be measured at successive nodes, characters could be
found by simple pairwise comparisons between them, but samples of taxa at successive
nodes usually are not available and, even if they were, the fact that those taxa represented
nodes cannot be determined before reconstructing the cladogram.
Another critical difference between types of systematic questions is between identifying
shape characters and reconstruction evolutionary shape changes. As noted above, changes
in shape characters,
traced on a cladogram, are intended to represent evolutionary
