Nicole,
There is no need to perform multiple residual-based analyses. If you wish to
obtain shape residuals where both the phylogeny and allometry (size) have been
taken into account, these are found as residuals from the PGLS analysis:
shape~size|phylogeny.
However, a question then is what will these size/phylogeny shape residuals be
used to investigate? If the intention is to then evaluate these relative to
some other factor (say groups), then the correct approach is just to perform a
factorial PGLS analysis, where: shape~size+factor | phylogeny (shape is a
function of size and some other factor, given the phylogeny). Additionally, if
that factor describes groups, you may wish to include the size:group
interaction term.
In fact, if that is indeed the case, it is advisable NOT to perform the
analysis in piecemeal fashion, where residuals from one regression are then
used in a subsequent linear model to test other effects. The reason is that if
there is some interaction between model effects (say, between size and groups),
then the residuals from the first regression are not correctly capturing the
observed patterns of variation. This is the multivariate equivalent of the
ANCOVA problem, and why an ANOVA on residuals from a regression is not always
the same as performing the ANCOVA analysis. The best solution is to simply
perform the factorial model, and account for size while examining other
effects. For this correct approach, one simply requires software that allows
one to perform factorial PGLS. Geomorph (and as I recall, NTSYS) will allow
uesrs to perform factorial PGLS.
Hope this is helpful.
Dean
Dr. Dean C. Adams
Professor
Department of Ecology, Evolution, and Organismal Biology
Department of Statistics
Iowa State University
www.public.iastate.edu/~dcadams/<http://www.public.iastate.edu/~dcadams/>
phone: 515-294-3834
From: Nicole Dzenowski [mailto:[email protected]]
Sent: Monday, February 1, 2016 9:18 PM
To: MORPHMET <[email protected]>
Subject: [MORPHMET] Questions regarding correction for allometry & evolutionary
allometry
Hi everyone,
I had some questions regarding allometry correction. I'll be working with
closely related specimens on a project where I think a moderate chunk of the
shape variation is due to allometric size differences. Should (or can) I
correct for both, as in, regress shape on size and take the residuals and then
use a phylogenetic comparative method on those residuals and the size data and
then do another multivariate regression and then use the residuals from that
final regression as my new shape variables?
Any help or direction is greatly appreciated.
Thanks!
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