All Politics is Local

James Lyons-Weiler weiler at ERS.UNR.EDU
Mon Nov 11 09:15:00 CST 1996

Michael Ivie <ueymi at GEMINI.OSCS.MONTANA.EDU> wrote"

>The current debate between morphological and molecular species
>descriptions is a false dicotomy...

I would hasten to add, however, that when phylogeny is recognized as a
process that _sometimes_ creates a pattern in the distribution of
characters among taxa that can be used to infer the history of that
process, and _sometimes_ it is a process that destroys the information
that might have been useful to that end, the onus is on the person
applying the data from whatever source to try as much as possible to
create an accurate estimate of phylogeny (i.e., trees).  More is not
always better; adding yet one more data set (say, molecular) where the
sites are saturated is tantamount to adding random data; adding one more
set of characters where the preponderance of assessment of homology is in
err is also like adding random data.  Generalizations about the
superiority of different sources of data are, by definition, going to be
wrong some of the time, and I think they are becoming rather passe.  It
doesn't even matter how often the generalizations are wrong; what matters
is whether or not they apply to data set A, data set B, and to data set
AB.  The influence of combining different data sets on phylogenetic
signal, for instance, is directly observable.  We know that taxon sampling
plays a major influence on phylogenetic accuracy, but we can't know if
adding just that one more taxon will screw up an otherwise good
phylogenetic estimate.  That is all changing; if the focus is moved away
from what an authority may have generalized about or personal biases about
the reliability of various type of characters to the specific, observable
consequences of how a data matrix is put together (taxon sampling,
character inclusion), then the phylogenetic estimates are better
justified.  For instance, unless one is a pattern cladist, I can't imagine
anyone knowingly publish a parsimony tree based on data that have been
shown to be devoid of phylogenetic signal, whether they be morphological
or molecular.  Naturally, I don't fault anyone for generalizing given they
were doing so in the absence of any other options.

James Lyons-Weiler


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