Biology Reference
In-Depth Information
length. That ratio does not fully describe the shape differences among species, but it
suffices to identify unknown specimens.
FINDING CHARACTERS
The use of morphometric data in phylogenetic studies has long been controversial.
Most often, debates among phylogenetic systematists have focused on two issues: (1)
methods for coding variables that overlap, sometimes considerably; and (2) the reliability
of the information obtained from the data for inferring phylogenies. Morphometric data
have been viewed with suspicion partly because it is difficult to determine where to draw
the line when there are no distinct gaps between the observed values. A wide variety of
techniques has been proposed and debated heatedly (see, for example, Colless, 1980;
Simon, 1983; Archie, 1985; Goldman, 1988; Chappill, 1989; Thiele, 1993; Swiderski et al.,
1998). Only very recently has the discussion begun to focus on a more fundamental prob-
lem: what to code? What is being extracted from the data and treated as a character?
Clearly, this issue must be addressed before the first one is even relevant; coding becomes
a moot issue if there are no characters to code, and if there are no characters, there is noth-
ing to test for homoplasy.
It is clear that partial warps should not be used as characters (for the reasons discussed
below), but it is not clear what ought to be used instead. It is not even clear that the prob-
lem has a solution. The major objective of the first part of this section is to define the prob-
lem we had hoped to solve using partial warps, then to explain why our approach was
flawed. In the next section, we discuss two alternatives; both rely on conventional multi-
variate methods, but neither is precisely tailored to the problem.
Defining the Problem
The general problem is to find features that differ among taxa and are shared by a sub-
set of them. The differences indicate evolutionary novelties and the similarities indicate
common ancestry, although it is not possible to determine which are novelties until the
phylogenetic analysis has been completed. You would not expect that an entire shape is a
character because species rarely have exactly the same shape (whether comparing the
whole organism or a single part); indeed, shape analysis is used precisely because shape
and its variation are complex. This suggests that if the problem is cast in terms of whole
landmark configurations, it will not be possible to make any progress. Yet, that is precisely
what the theory of shape demands of us; if we do not think of the problem in terms of
whole landmark configurations, we will be led to theoretically invalid solutions. This,
then, is the heart of the problem: to analyze entire configurations of landmarks and find
features that differ among taxa and are similar among a subset of taxa. Furthermore, to
say that a feature is a character, it is necessary to satisfy the criteria for recognizing phylo-
genetic or evolutionary homology, that is, to say
where
the feature is, and over how large a
spatial expanse it extends. A primary objection to traditional morphometric variables is
that
they are lines, having no spatial extent as individual variables. Attempting to
