[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Clade II



At 03:46 PM 1/25/97 -0700, Jeff Martz wrote:
>>         You miss the point.  If I can assume that there is a common ancestor
>> of any group, I can say that there exists a set of animals which decended
>> from that ancestor.  Read the ref.
>     They descended, but they weren't a "set" until someone decided to
>call them a "set".
        And yet this set represents a real phenomenon, common decent.  These
animals all decended from a common ancestor, exclusive of other animals.

>     If some of the fundamental units of the clade aren't monophyletic,
>how can the clade itself be monophyletic?  
        We are including a common ancestor and all of its decendants, but
you are theoretically free to subdevide this clade into any number of
paraphyletic groups you wish to.  This in no way implies that the whole
group is paraphyletic.
        Take a clade.  If I decide to make a paraphyletic group out of some
members of that clade, does this affect that monophyly of the whole group?
No, because the higher clade is defined to include the common ancestor and
all of its decendants, regardless of how we subdivide its constituants.  The
only way to make a taxon paraphyletic is to exclude some of the decendants
of its common ancestor, at which point it is no longer a clade.

>>[...]ask yourself where the sense is in
>> excluding some of the decendants of a once monophyletic group, thus making
>> it paraphyletic.  Why would you do this?
>      To emphasize other evolutionary phenomena besides common descent. 
        I've said it before, I'll say it again, why would you wish to
obscure the most confusing aspect of evolution behind those aspects which
are more readily discernable?  If any two-year old can tell the difference
between a bird and a non-avain dinosaur, why should we include this at the
risk of obscuring their theorised relationships?

>      Lets pretend someone found a series of historical documents written
>in Ancient Egypt.[...]
>     So: of the following means of classifying these documents, which
>makes the LEAST sense?:[...]
>     The best means of analysis is not neccessarily the best means of
>classification.
        I should point out here that, in a literary and historical study of
the Bible (at least the Old Testament), the most important classification is
not by book, by subject, or by concept, but is BY AUTHOR (in this case this
is roughly the same as your "by translation" catagory), as this contains the
information which is of LEAST OBVIOUS UTILITY to the reader.  This
classification gives one a better sense of the differing agendas and
differing perspectives of the various biblical authors, and incidentally a
better handle on the "spin" which later editors have put on these stories to
bring them into line with modern dogma.
        This corresponds roughly with the utility of phylogenetic taxonomy.
It makes the subtle obvious, and leaves the obvious for two-year-olds.

>I've never heared of evolution simply defined
>as "the descent of one organism from another."  Darwin's term "descent
>with modification" is better.  You are more then happy to use the
>modifications to guide your analysis, why not consider them equally
>important to guiding the taxonomy?
        The modifications are inexorably bound to the decent, yes.  However,
which of these concepts is the more specific?  Modifications can be in
parallel, or shared.  Descent only happens one way.

>[...]I simply want to take it a step
>further considering that branching and the creation of monophyletic
>groups by extinction are the DIRECT RESULT of macroevolutionary phenomena.   
        And this is best done in the context of common descent.  PT gives us
a vocabulary for this context, and is thus more useful and meaningful.

>1. All grouping of taxons are artifical, regardless of if they are defined
>by common descent, morphology, ecology, etc...
        Yes, but they can represent real groups.  I await your reading of
the article.

>2. Taxonomy is not by definition based solely on common descent; that is a
>cladistic interpretation, and in my opinion not as useful to the
>evolutionist as one that also takes into account other macroevoltionary
>trends besides common descent. 

        I will paraphrase and add to the statements made by another member
of the list who chose not to post his views.  "Evolutionary taxonomists"
admit that phylogeny should have a place in taxonomy.  And yet they do what
could be described as a "half-keystered" job of actually doing this, by
obscuring the phylogeny with character state data.  The addition of
character state data makes the resultant taxa less useful on a phylogenetic
basis, as one cannot discerne which are monophyletic and which are not, and
paraphyletic taxa are not useful from a phylogenetic viewpoint.  Those taxa
which are based solely on common decent fail the task of describing the
character states of their members.  Thus, the resultant taxonomy fails to
provide an adequate description of phylogeny or character states.
        Phylogenetic taxonomy clears this issue right up, by making the
criteria for inclusion in a taxon relatively straightforward, and by
including the assumption that the membership of any such grouping is
hypothetical and subject to change.  It provides a framework for the
discussion of phylogeny and of evolutionary change, by readily describing
the evolutionary paths of taxa and providing an accessable means of
determining whether traits are shared, convergent, or parallel.

        On a related topic, note that I do not recognize the objection to
the term "monophyly", and the insistance on the use of "holophyly", which is
raised by some scholars.  With perhaps the exception of pheneticists,
polyphyletic groups are not accepted by modern taxonomicists.  Any
non-polyphyletic group of animals may be considered "monophyletic" in the
strictest sense if the common ancestor is included in the group.  Since
inclusion of the ancestor is required to avoid polyphyly, all
non-polyphyletic groups are "monophyletic".  Thus, paraphyletic groups are
already "monophyletic" in this regard, and the distinction between
"monphyly" and "holophyly" is not relevant, except in an historical context.

        Wagner

+--------------------------------------------------------------------------+
| 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, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
+--------------------------------------------------------------------------+