Morphological disparity and landmark variation

[morphmet]

-------- Original Message --------
Subject: Morphological disparity and landmark variation
Date: Mon, 2 Nov 2009 09:58:55 -0800 (PST)
From: Murat Maga <m...@u.washington.edu>
To: morphmet@morphometrics.org

Hi all,

I have 3 sample groups, each of which have 5-12 individuals.  Two of
those are subspecies, and the third is a cross between them. I want to
estimate (1) what is the most morphologically different of these three
groups, and (2) for that group, which LMs are driving the difference
from the other groups. All of these groups are really similar, and I am
looking for small shape changes.

For (1) I looked into the Morphological Disparity (MD) equation provided
in Zelditch et al. (2004) and got confused a bit. Definition says the
measure of distance(or at least one of them) can be  the Procrustes
distance between the average shape of an individual species and the
grand mean of all groups.  I can, of course, calculate a mean shape for
each of my groups using 3 separate GLS, and then calculate the grand
mean from those three means. They also suggest to calculate a confidence
interval for MD using bootstrapping with resampling with replacement.
My understanding is, I bootstrap one group, take the new group mean
shape, calculate a new MD for that group, rank them, toss the upper and
lower 2.5%. to get a 95%. Provided that this correct, here are the parts
that I am confused about:

While bootstrapping, should I also calculate a new grand mean of groups?
Obviously, the group mean for the bootstrapped sample is not going to be
identical to my original mean for that group, and that should have some
effect on the grand mean of groups.  Should I worry about this, or stick
with the grand mean from the original analysis? Also when I do
resampling, should I resample the individuals or LMs?

For (2), I was thinking I can take the pooled sample,  calculate the
Euclidean Distance of each LM (for each individual) from the consensus
shape in tangent projected coordinates which will let me calculate LM
difference means for each group. But then I couldn't figure out what to
do with that. I guess I can calculate some sort of ratio, but would that
really give me what I want?

Am I totally off the track?

Best,

Murat

--
A. Murat Maga, PhD
Senior Fellow
University of Washington
Dept. Pediatrics, Division of Craniofacial Medicine
1959 NE Pacific St. HSB RR234
Seattle, WA 98195
(206) 616-9703



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