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

Phylogenetic Taxonomy [was Re: Cladistic Massive Retaliation]

At 11:56 AM 11/21/96 +1030, Nick L. wrote:

>        And no, cladistic groups are NOT NECESSARILY REAL!

        Not if by "cladistic groups" you mean phylogenetic taxa, which is
what Jeff and I were discussing.  In this case, you are quite incorrect, see
Gauthier and deQuiroz 1990.  Cladistic taxa, defined by a common ancestor
and all of it's decendants are very real.  Their composition may not be what
we believe it to be, and their diagnostic characters may also not be what we
think they are, but the group is a real entity.  

>There are phylogenetic relationships which simply cannot be
>expressed in the caricature of cladistics!

        Such as?  What monolphyletic group of animals cannot be expressed as
a common ancestor and its decendants?  Oh, are you stuck on that old "what
if three species of birds are isolated at the same time" thing?  Answers to
this are available in the litterature.  Check Sereno 1990.

>Clades are human constructs which are meant to mimick phylogeny as
>closely as possible, but there are very real reasons why they cannot
>actually represent a phylogeny with complete accuracy, and it's
>important that we remember that.

        A clade is a monophyletic grouping of organisms.  In so far as that
goes, clades are not human constructs, they are real enitites.  We may not
be able to know for sure that a clade contains any more OTUs than the one or
two used to define it and the common ancestor (for node-based and
apomorphy-based taxa), but it is most certainly a real enitity.
        You are confusing the hypothesised clades on a PHYLOGENETIC
HYPOTHESIS (cladogram) with PHYLOGENETIC TAXA (in a phylogenetic taxonomy).
In a cladogram (phylogenetic hypothesis), the clades hypothesized are not
real groups, they are hypothesized to indicate a real group, of which they
are a part.  For example (Rabbit, (Cow, Dog)) is a clade you may find in a
hypothesis.  This "clade" is not real, as there are certainly other animals
which decended from their common ancestor, and it does not include that
ancestor.  It indicates, if the hypothesis is correct, that there is a clade
which included these taxa but excludes all other taxa in the study.
        There is, however, a *real group*, or clade which consists of the
common ancestor of the Dog, Rabbit, and Cow.  Period.  Whether or not our
phyolgenetic hypothesis bears out our hypothesised clade, a phylogenetic
taxon erected on the basis of "the most recent common ancestor of the cow,
the rabbit, and the dog and all of its decendants" will *ALWAYS* be
monophyletic, and *always* be a real group.  
        This is the key to a phylogenetic taxonomy.  This results in the
very useful feature of providing a vocabulary for the discussion of
phylogenetic relationships.  Which statement is more useful for the study of
evolutionary relationships:
        "The synapsid reptiles, turtles, lepidosaurs, and ruling reptiles
all sprung independantly from different ancestors amongst the cotylosaurs,
which in turn find their ancestor amongst the Semourimorphs." (paraphrase of
Romer 1966)
        "The two major clades of the Amniota, the Synapsida (including the
mammals) and the Sauropsida (including turtles, lepidosaurs, and archosaurs)
diverged after the semouromorphs."

>    Cladistics cannot accurately represent some phylogenies. Period.

        This is a different point entirely.  To be perfectly honest, I think
you need to go back and reread Sereno 1990 (in _Dinosaur Systematics_)
before continuing.  Besides, even if it cannot, no other system can do any

>        This isn't an argument, it's just a dumb fact we should keep
> in mind to remind us of the difference between what is objective
> reality and human construct.

        While this is a very very good point, but it works against you're
origional intent of demonstrating subjectivity in phylogenetic taxonomy.
Tradional taxonomy, with its paraphyletic groups and diagnostic taxon
definitions, is the human construct.  Phylogenetic taxonomy is only human
construct with regards to which groups we choose to define, and how we name
them. The groups are *objectively* there, waiting to be named.

>Cladograms are the latter trying to fit the former as closely as possible.

        True enough.  All science is subjective construct trying to approach
objective reality.  Amusingly enough, so are Philosophy and Religion.  Is
this news?

        The Cladistic Assassin

        Y'know, this all seems to come down the the buring question of
Beverly Hills:  What's more important, what you look like, or who you're
related to?
| 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             |