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Re: cladobabble.../Some clarification... (long)



At 01:56 PM 11/18/97 -0500, Norm King wrote:

>I agree that clades do not exist in material form.  Previously some 
>discussants claimed that they are the REAL objects out there, but I 
>disagreed, saying animals themselves and fossils are the only real 
>objects.  I can hit you over the head with a fossil, but not with a 
>clade; clades are mental constructs (hope I used that term correctly).

Interesting definition of "real" (that is, "real" as "physical, tactile").
Would you agree that your (or mine, or anyone else's) personal geneology is
an historic fact?  That we have a particular, singular sequence of parents,
grandparents, great-grandparents, etc?

If you agree that a geneology is real, then we are working from the same
page.  If you believe that geneologies are just mental constructs, then we
are operating under different paradigms, and will (naturally) disagree about
concepts and metaconcepts deriving from these terms.

>Tom Holtz said (11/18/97; 8:35a):
>
>>Actually, a clade (from Greek clados, branch) is any monophyletic group:
>>an ancestor and all its descendants.  Accepting, based on other data, 
>the
>>existence of evolution as modification with descent, there exist real 
>>clades in nature external to human reality.  
>
>Now maybe I am being overly reductionist, but as mental constructs, 
>clades
>would not exist in nature external to human reality.  So, they are either 
>not what I say they are, or not what Tom says they are.

Or they aren't what either of us say they are.  (Just to be comprehensive).

>>We can propose names for real clades based on the relationship of any
>>two taxa (or one character, but (as discussed earlier) there is a chance
>>of homoplasy spoiling such definitions). 
>
>So, the definition of a clade depends on an earlier determination, which 
>may not be true.

If, by "earlier determination", you mean "previously named taxa", you have a
point.  That is why the trend is towards definitions based ultimately on
type specimens, which (except in chimerae) have the minimal problem of
identity: a type specimen is definitely that taxon, whether any other
specimens belong to it or not.
[Some snippage, to make Mickey happy...]

>T. H. Huxley hypothesized the theropod bird 
>connection in 1869, and nothing has been discovered in the meantime to 
>disprove it or to require a different phrasing other than development of 
>a new technique for assessing evolutionary relationships. 

I disagree with this to the strongest degree!  Huxley established a
dinosaur-bird link, and Ostrom a theropod-bird link, but Gauthier DID manage
to accomplish something by describing explicitly *which* theropods shared a
most recent common ancestor with birds.  This helps sort out all kinds of
problems, from the particulars of the anatomy of the direct ancestors to
birds to timing of origin to (possibly) place of origin, etc.

>Maybe start by adopting a defined procedure for characterizing clades, 
>like node-based.  Node-based would probably have the least disagreement 
>with earlier taxonomic concepts and hypotheses of descent, and that's 
>what I think we want.  (We shouldn't throw the baby out with the bath 
>water!).

Unfortunately, as already mentioned, this is not always the case.
Ichthyologists have long preferred stem-type concepts, even if they didn't
always use stem-based definitions.

This is why I feel that combinations of stem-based and node-based are
useful.  For an invertebrate example, we can use "Chelicerata" for a
well-defined, well-studied group (perhaps definable by all descendants of
the most recent common ancestor of _Limulus polyphemus_ and some favorite
spider).  A stem-based "Cheliceromorpha" can then be useful for chelicerates
and all forms closer to them than to (for example) Trilobita.  This captures
a bunch of Paleozoic forms which would otherwise be outside of Chelicerata,
but which are not "trilobitoids" or crustaceans or the like.  We can then
also ask questions/set up study programs to decide, for example, if the
agalaspids are cheliceramorphs, or trilobitomorphs, or something else (which
answer will modify our thinking of the anatomy and timing of origin of
particular groups).

(Hey, guess what the topic of today's Invert Paleo class was... :-)

>Don't name every clade--the proliferation of names may 
>ultimately produce a structure as as complex as the tree of life itself 
>(as G. O. has pointed out).

And, as noted numerous times, most people do NOT name every potential clade.
However, sometimes some people need names for parts of the tree that others
find less interesting (i.e., basal dinosaur relatives may not intrigue
people only interested in ankylosaurs, but they do interest me!).

>The whole superstructure will get bogged 
>down by its own nomenclatorial complexity,

As I have said before, no non-professional is obligated to use ALL the
different names if they aren't working on that subject!  There is no way
I'll ever know all the different bivalve taxa, but that doesn't mean that
malacologists have to stop defining and diagnosing clades because I don't
like it.

(It is this particular line of arguement that I find the most annoying.)

>to say nothing of the 
>background chaos caused by the inevitable disagreements over diagnostic 
>characters and clade "definition."

Actually, even Simpson noted the differences between diagnosis, definition,
and the like.

I'll deal with the rest later.  Till then...

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661