Dear Annelid enthusiasts,
Greg and Kristian's phylogeny paper seems to have a kicked off a lively
debate that is branching out in several directions, rather like a
cladogram :).
I don't profess in any way to understanding molecular genetics, except
that my daughter is a biochemist and active in the field and I have a
cousin who is heavily funded by NIH in the same area. Based upon
comments with my relatives, I take some exception to Kirk Fitzhugh's
suggestion that studies of molecular sequence data will be of no use in
understanding phylogentic relationships at the phylum level.
Given that the evolution all existing phyla was probably complete by
the beginning of the Cambrian (600 MY), and that many other phyla came
and went during that same Period, we have little in the way of direct
evidence as to what the ancestral soft-bodied organisms were that gave
rise to what we are looking at today (assuming a monophyletic origin).
It seems to me that a comparison of gene sequences among related groups
of organisms can only help in an understanding of their relationships.
The problem however, is which sequences to focus on. Given how much
time and energy has gone into the Human Genome Program, such an effort
for invertebrates will of necessity be limited to identifying those
gene sequences responsible for characters that currently are used to
define phyla. We are probably talking about some very fundamental
anatomical features, not chemosensory and adaptive characters such as
nuchal organs.
I agree with Kirk that outrageous claims are being made, but these are
probably because investigators are rushing their work into print,
making claims that are based upon sparse data, sometimes speculation,
and often reliance on what a cladistics software program reveals. I
have seen some recent examples of very bad cladistics using traditional
morphology, where the full extent of character state richness was not
developed not to mention the use of very inapproprite characters in the
matrix to begin with. When inappropriate character matrices are used,
there will obviously be problems in interpreting the results. I think
that molecular sequence data will be difficult to use initially, but it
should not be discounted because it may be the only real link we may
have to understanding the origins of phyla. If we do not consider all
available information in the development of a phylogeny, then our
hypotheses become exercises in fantasy.
Jim Blake
ENSR, 89 Water Street
Woods Hole, MA 02543
(jablake at ix.netcom.com)
