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the Metazoan-Choanozoan cut




Jaime,
Obviously I was not there to witness the evolution of a choanoflagellate into a primitive poriferan. And even if I was there, I would have difficulty drawing a precise line between populations----this population is choanoflagellate-becoming-poriferan, and this succeeding population is more poriferan-than-flagellate.
Evolution is a virtual continuum, far too continuous for us to justify drawing a line in any particular place. Fortunately extinction is far more prevalent than survival, especially at species level. And this coupled with an incredibly spotty fossil record leave gaps that make drawing such lines easier.
The preponderance of the evidence indicates that some form of choanoflagellate gave rise to Metazoa in the form of a primitive sponge, and extinction has created a gap that makes this a convenient place for human beings to draw a line between them, and one which is almost universally recognized by zoologists and microbiologists who specialize in protists. We could include choanoflagellates in Metazoa and still have a perfectly good holophyletic (cladistically monophyletic) group, but the gap between choanoflagellates and other protist groups seems much narrower and other cuts would cause unnecessary doubt and confusion.
In fact, since I published my book in 1994, a new group, Order Corallochytrida has been added to Phylum Choanozoa. Whether they should be separated as their own class (or not) is rather arbitrary and not particularly important. But the gap between choanoflagellates and chytrid fungi is narrowing, and this makes the gap between choanoflagellates and sponges (and thus Metazoa) comparatively wider and a more appropriate place to make an admittedly arbitrary cut between Kingdoms. Some cuts are more convenient and less arbitrary than others, but all cuts of a continuous tree will have some element of arbitrariness, inconvience, and there potential controversy.
My biggest conflict with strict cladists is convincing them that cladistic cuts are necessarily any less arbitrary, and that some paraphyletic groups are in fact inevitable. Mayr, Ashlock, and others have been trying to get that point across for decades. If you limit yourself to one group of organisms, you can easily work under the illusion that your cuts are totally unarbitrary and non-paraphyletic, but anyone who has to classify broader groups of organisms eventually comes to the realization that it is an illusion, and that sister groups are a Hennigian convention made possible by "gaps". Even Hennig understood this. They are a convenient way to classify and analyze diversity, but sister groups have no true existential reality (I will probably be jumped on for using that phrase, but I am too tired to come with anything better at bedtime).
So with that clarification, Jaime, I would now ask you if our positions are all that different. I look forward to your response, because I don't think we are really all that far apart. Classification does not have to be an either/or proposition, where strict cladists are right and strict eclecticists are wrong, or vice versa.
I am convinced that universally acceptable classifications are possible, and the inevitable disagreements can mostly be shifted into some kind of modifiable code, that leaves the major clades virtually universally recognized and stable, while the coding and ordering of these clades remains fluid enough to reflect the various controversies. That is why I recognize two Orders of dinosaurs (which are almost universally recognized), and code them as sister groups (a dinosauria clade for which the evidence is strong, but not as strong as the holophyly of the separate orders).
I make the cuts where I perceive the biggest phenetic gaps to be, taking into consideration other factors (cladist vs. eclecticist; scientist vs. lay public; and even tradition vs. a natural longing for novelty; and so on). That leaves me open to attack from both sides, but in the long run I believe it to be worth it. I guess I have gone on long enough, and will leave it at that. It's been a long day.
Sincerely, Ken Kinman
*********************************************************
From: "Jaime A. Headden" <qilongia@yahoo.com>
To: dinosaur@usc.edu
CC: kinman@hotmail.com, m_troutman@hotmail.com
Subject: The Difference Between Therapsida and Theropsida
Date: Fri, 30 Jun 2000 17:42:35 -0700 (PDT)

Ken Kinman wrote:

<In your example, Tyler would be a choanoflagellate
(almost an animal but not quite, as you put it). Marla
would be a primitive sponge.>

  As stated, _Tyler_ is an outgroup to Animalia, and
_Marla_ is itself a basal (-most?) animalian. Assuming
_what_ each is more explicitly is not done, because no
example was made, and this is entirely the point.

  Unless there was an observation of the event from
which one form offsprung another, you cannot assume
the descent of that form. Saying one's a sponge and
the other's a choanpflagellate is suggesting you know
precisely the evolutionary events that entailed the
appearance of these two forms. Simply, you do not.
_Tyler_, evolutionarily, may lie between
choanoflagellates and animals, rather than being one
or another, and the system you put forward would not
demonstrate this.

  Please, and tell me if I'm mischaracterizing your
position, Mr. Kinman, don't confuse cladistics with
those systems which assume reality through
hypothetical phylogenies applied by cladistics
(strictists) ... there is a difference.

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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