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Re: Cladistic Massive Retaliation
At 02:52 PM 11/18/96 -0700, Jeff Martz wrote:
>A clade may be a real
>thing, but cladists have different ideas about WHICH clades are the genuine
>ones (meaning, they make different judgemnts as to HOW common descent is
>traced for an organism, based on abitrary interpretations of the data).
This has nothing to do with the naming and defining of taxa, but has
to do with the methodology used to assign OTUs to higher taxa. A cladistic
taxon is never the result of arbitrary judgement. There are no ingenuine
(to use your term) cladistic taxa. Each cladisitic taxon, and ancestor and
its decendants, is monphyletic, real, genuine, whatever. Some may describe
similar groups, some may even be synonymous, but they are all proper.
As for tracing one taxons phylogeny, yes, there is some judgement
involved here, but this has nothing to do with allowing paraphyletic taxa,
as the phylogeny of one organism does not affect the definition of a
cladistic taxon, it may only affect the hypothesized composition or the
supporting characters of that taxon.
> You make it sound so simple. You might as well say "Is today's
>paleontologist so overworked that he cannot simply look at the fossils
>and see which animals are descended from other animals? Why do we
Taxonomy may have two purposes (I am simplifying): A card catalog,
an inventory, if you will, of animals, or a grouping of animals based on
their evolutionary relationships. One of these is a convenience for human
observers, one is a tool for the exploration of evolution.
Which concept is more readily accessable, an animal's appearance and
adaptations, or its phylogeny? It seems clear to me that the latter is more
obscure. If we are then to choose a classification which makes the obscure
clear, we must choose a taxonomy based on phylogeny. If we do this, we must
pick the method which makes evolutionary relationships most clear, the one
which does not demand a priori groupings, the one which changes with
changing ideas, but remains stable at the same time. Why should we
sacrifice clarity of that which is difficult to understand, yet natural, for
that which is easily seen and subject to human interpretation?
> In any case, it isn't SUPERFICIAL similarities I am interested in.
>You are forgetting that similarites can also result from analogy. [...]
>in and of itself in understanding evolution. Why shouldn't that
>information be included in the taxonomy?
This information obscures the evolutionary relationships in the
phylogeny by grouping animals based on homoplaisy. It serves to confuse the
casual reader (anyone reading Romer 1966, for example), and the scientist
(Romer in Romer 1966) alike as to the true path of evoltion. Any value this
information may have is outweighed by power it has to decieve.
Furthermore, with a truly phylogenetic taxonomy, these similarities
can be cast in an evolutionary light. They can be shown to be homoplastic,
and thus teach a much more useful lesson about evolution. To use an hoary
old example, should we group birds and mammals together because they are
both warmblooded, to illustrate the importance of this novelty, or should we
seperate them, to emphasize that it is convergent?
> What is wrong with arbitrary group divisions?
Natural ones are better, because they better represent the natural
world, which is ostensibly what a taxonomy should be doing.
> If they do not
>contradict the phylogeny (by creating POLYphyletic groups), and they
>help understand the differences and similarities between groups AS A
>RESULT OF EVOLUTIONARY CHANGES, why shouldn't these arbitrary grouping be
>included? How are such changes "not real" just because they are a little
>more difficult to identify?
The changes *are* real for the more inclusive group, but they are
not for the excluded one. For example, when birds evolved, the rest of the
dinosaurs didn't suddenly up and become something different. So why should
their designation change because of what their progeny became?
Making a paraphyletic group for the taxa left behind does emphasize
the difference of the newcomers, but is this necessary? Why can't you just
accept that the characteristics of the more derived group are emphasis
enough? It was all the emphasis nature needed.
> All I am saying is that it is helpful for taxonomy to have a further
>function in undrstanding evolutionary relationships and change than just
>"who is descended from who".
And it does, within an evolutionary context. Which are you more
concerned with elucidating, selective forces or phylogeny? If you
compromise your understanding of phylogeny for an understanding of selective
pressures or whatever it is you think morphology will give you, your system
no longer elucidates phylogeny. You get a Frankenstein's monster like
"evolutionary systematics", which is good at neither elucidating phylogeny
nor emphasizing the roles of selective pressures.
The understanding you seek *is* helpful, but phylogenetic
systematics will provide *better* tools for your search. Because of the
nature of phylogenetic systematics, you can speak of similarities between
groups as being homoplastic, without worrying about how some of the group
have these characters due to convergence and some through inheritance. Look
at the Dinosauria (sensu paraphyletic). Some of them have retroverted pubes
like birds due to homoplaisy, some due to common ancestry with birds. Does
this make the situation clear? With a phylogenetic taxonomy, one has but to
say that the condition in maniraptors results from common ancestry with
birds, while the condition in segnosaurs and ornithiscians is homoplastic.
A subtle, but tangible difference.
> Morpholgical distance can be INDICATIVE of phylogeny, is the
I am afraid that if you were not persuaded by my prior arguments, I
have no means of convincing you. I disagree with your point. Morphological
distance is something made up purely to emphasize difference, phylogeny is
about sameness. The two concepts are married only in the minds of those who
confuse cladograms and taxonomy.
> As an example: birds (read: avian dinosaurs) survived the K-T, while
>dinosaurs (read: non-avian dinosaurs) did not.
Since there is already a dandy grouping which describes the
survivors, I hardly see why the others need to be granted a taxon. All one
must say is "the birds alone amongst the dinosaurs survived the K-T
boundry". This emphasizes exactly what you want it to, that it was
something special about the birds which allowed them to survive while their
cousins did not.
> Doubtless, everyone will have different ideas about what
>arbitrary groups to use, and changes will occur with time, but the
>cladistics NONARBITRARY system is no less in flux. Considering that
>cladograms change as new information comes in, I do not see how ARBITRARY
>changes to this ARBITRARY system are any more problematic.
Cladograms do not equal phylogenetic taxonomy, and their fluctuation
has little effect on the stability of cladisitic taxa. Flux in the
phylogenetic hypotheses presented does not mean flux in the taxonomu,
because taxa are not defined based on cladograms, but on common decent. I
think what I'm saying is not getting through. Not trying to be a jerk, but
could you maybe reread my past two posts?
| Jonathan R. Wagner "You can clade if you want to, |
| Department of Geosciences You can leave your friends behind |
| Texas Tech University Because your friends don't clade |
| Lubbock, TX 79409 and if they don't clade, |
| *** firstname.lastname@example.org *** Then they're no friends of mine." |
| Web Page: http://faraday.clas.virginia.edu/~jrw6f |