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

Re: Cladistic Massive Retaliation

At 09:17 PM 11/17/96 -0700, Jeff Martz wrote:
>     If cladistic groups are real, then how is it that all cladists do
>not produce identical cladograms?

        A clade is a real thing.  The group formed by a common ancestor and
its decendants is a real thing.  The *composition* of that group changes
with our understanding of the phylogeny.  We once thought that the
Dinosauria, which I will, for the sack of discussion, define as the common
ancestor of _Megalosaurus_ and _Iguanodon_ and all of its decendants, did
not include the birds, now we believe it does.  This does not mean that, in
"real life", the group of the common ancestor of _Megalosaurus_ and
_Iguanodon_ and all of its decendants has actually *changed*.  There is only
one of this group, and it only changes insomuch as birds are born every day.
Our understanding of the composition of this group *has* changed.
        Remember:  the composition and autopomorphies of a clade *do not*
define the clade.  Common ancestry defines the clade.  The composition and
supporting character states can change with new analysis, but the clade is a
real group.
        Cladists do not produce identical cladograms because they do not
have all the data, or, if you believe George, because their methodology is

>     And what about intermediates?  For a given trait, how fully
>developed does that feature have to be to say that all memebers of that
>clade possess it?

        Again, the character state distribution is how one finds clades, not
how one defines clades.  There are different methodologies practiced by
different workers, but for taxanomic purposes, we do not say that all
members of a clade posses a certain character, we say that the charater
supports a clade, because it changes state at the node of that clade in the
most parsimonious tree.

>at what
>point do you say: "THIS is a feather, this species had FEATHERS, and so
>this clade is defined by the possesison of feathers."

        Again, clades are not defined by characters.

>Without arbitrary
>decisions, this should be impossible, because there are ALWAYS
>intermediates**(see below).

        No one pretends that there isn't human judgement in science.  there
is simply *less* subjective judgement in a phylogenetic taxonomy.

>However, cladists make the decision and define a clade by that trait.

        Again, this is an incorrect assumption.

>this may seem like a nitpicky point but cladistics makes arbitrary judgements
>in another way: it assigns NAME, which are for our convenience only.

        I fail to see why this is any more arbitrary than any other system,
or even why this is flawed in any way.  Please elaborate.

>      **The creation of a paraphyletic clade that is defined by the
>loss of a trait is no more arbitrary than the creation of a
>monophyletic clade that is defined by the development of a

        True.  Paraphyletic taxa seem to be more often defined by the
*absence* of synapomorphy.  Loss of a trait is a potential synapomorphy
(although there seems to be some scholarly debate on their value).  Taxa
defined by the absence of a trait (interpreted as not being a synapomorphic
reversal) will very likely be paraphyletic, and potentially polyphyletic.
See Gauthier and deQuiroz 1990 for a discussion of the philosophical and
practical limitations of defining taxa based on character states.
        A taxon based on common decent is much less arbitrary than either
case.  One can, make a paraphyletic taxon based on common decent.  Example:
the most recent common ancestor of A and B and all of it's decendants,
excepting the mrca of A and C and all of it's decendants.  However, this
taxon *could* end up being meaningless, ie exclusive of all taxa, if B is
closer to C than to A.

>     Taxonomy should be based on descent, but it should ALSO help us to
>understand interrelationships and similarites of organisms which were NOT

        Why?  What added value is there in burdening your taxonomy with
trivia?  Is today's zoologist so overworked that he cannot simply look and
see which animals look superficially like which other animals?  How many
people are so confused that they can't recognize the very derived nature of
the avian wing when compared to the bird's other dinosaurian cousins.  Do we
need to confuse and mislead everyone, from scientist (see the recent
comments that have been made regarding taxonomy-based studies of diversity)
to layman about the obscurities of the evolutionary relationships of these
animals just to point out  what should be painfully obvious to any

>This was the idea behind "traditional" taxonomies, and **IF**
>the morphological similarities and differences reflect on phylogney, why is
>this bad?  Why can't an undertanding of evolved morphological
>differences of ALL kinds be laid over a simple cladistics framework of
>common descent?  Why can't a taxonomy cover as many phylogenetic bases as

        Morphological distance is *irrelevant* to phylogeny.  Phylogeny is
"who is more closely related to whom" in an absolute sense, and has nothing
to do with morphological similarity, even though morphological similarities,
in the person of synapomorphies, are used to deduce phylogeny.  Now, if
there were a way to objectively measure genetic distance for organisms, I
can see where it could be incorporated into a phylogenetic framework.
Presence or absense of characters is not phylogeny, it's card-cataloging,
and has no place in *Phylogenetic* taxon definitions.
        This confusion is a common one, as can be seen by Charig and
Milner's "classic" paper in _Dinosaur Systematics_.

        Tha Cladistic Assassin
| 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             |