[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Cladospeak (Mammalia, Crurotarsi)
Ken Kinman writes:
not sure how "strong synapomorphies" might differ from "significant
syanpomorphies", but someone fluent in cladospeak might explain it."
A synapomorphy, by definition, is "a shared, derived, group-defining trait."
That said, aren't all synapomorphies "significant"? If they didn't define
a group, wouldn't they be "insignificant" and therefore not synapomorphies?
Ideally, we enter characters into a cladistic analysis we suspect may be
significant in defining groups and then determine which characters are
synapomorphies, apomorphies, or just convergence AFTER the analysis. The
difficulty in doing this unbiasedly is that we already know from previoius
cladistic analyses, and from previous phenetic or more traditional
evolutionary studies, what characters are synapomorphies already for many
groups. The "solution" to this bias is many-fold, but correlation and
consilience of results from many studies holds promise. If certain
characters more aptly define certain groups better than others in several
different phylogenetic analyses, we may have more confidence in these
characters as synapomorphies than others which cannot so easily be
replicated and correlated.
So, this debate about "strong" or "weak" synapomorphies might be better
reformulated as a character discussion. What are characters based on? We
use bony landmark or shape characters for much of vertebrate paleontology.
Are these characters reliable? Do similarities among a number of different
vertebrate fossil taxa indicate a synapomorphy or simply homoplasy? Many of
the characters that define the Dinosauria are locomotor characters -- these
characters must have been influenced by the function of the limbs and
muscles in the living animals. Do the similarities in dinosaur hindlimbs
reflect similar functional constraints and congruence, homology (i.e.,
synapomorphy), or both? None?
In my very humble opinion, it is the function-form complex and its effect on
character states that is really the issue with many of the dinosaur
phylogenetic debates. Function and phylogeny compliment each other, and
this observation is sometimes overlooked. Extremists on both sides either
just compile characters without examining how function effects form, and
strict functionalists too often want to impose convergence on everything.
Finally, phylogenetic analysis using parsimony (and hence the name of the
popular cladistic tool, PAUP) has proven to be a useful tool in
understanding systematics and evolution because it has made the phylogenetic
hypotheses being tested clear and has allowed other researchers the
opportunity to replicate the published results. To quote Randall T. Schuch
(2000: Biological Systematics):
"... it is now abundantly clear that systematic hypotheses are characters
and the hierarchic schemes of relationships they imply ... Failure of
agreement between theory and observation, once explained away with ad hoc
hypotheses, is now adjucated via the parsimony criterion in the search for
theories of relationships among taxa that show greatest agreement with the
Dept. Biological Sciences
Western Illinois University
Macomb, IL 61455
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp