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Cladistic Stuff [Part 2]



Here we go again...
----------------Nick L. wrote:
>       I'm asking about species A and species B which come from a common 
>ancestor. Then they produce species C:

        A dendrogram describing decent and two cladograms:

           "1"                   2                       3
    ?       C       ?    A'      C       B'      A'      C      B'
     \     / \     /      \      |      /         \      |      /
      \   /   \   /        \     |     /           \     |     /
       \ /     \ /          \    |    /             \    |    /
        A       B            \   |   /               \   |   /
         \     /              \  |  /            "A"  \  |  /  "B"
          \   /                \ | /                   \ | /
           \ /                  \|/                     \|/
            Y                    Y                A------Y------B
            |                    |                       |

Recall that a cladogram is a diagram of recent common ancestry, not a
tracing of decent patterns.
        Your taxa A and B are paraphyletic, as they do not include those
members of the taxon which contributed to Taxon C ("?" on fig 1).  If all
living members of A and B contribute to C ("?"s on fig 1 do not exist), then
A and B are in a trichotomy (figure 2), because all members of C are equally
closely related to A and B.
        If one excludes parpyletic taxa, "A" and "B" no longer exist (fig 3)
and the members of "A" and "B" which were spawned after C was created are
their own taxa, A' and B'.  Those that existed before C was spawned are in a
trichotomy with C (fig3).

>       Dawkins mentions a fact in one of his books (Blind Watchmaker) 
>that peas have hemoglobin, of all things. How in the heck did that 
>happen? Maybe the transfer of that sequence was through a bacterial, 
>viral or other kind of vector. How does this fit into a cladogram?

        I don't know.  I work with vertebrates.  Dr Holtz?

>       To use Kuhn's vastly overused and kind of annoying word paradigm, 
>cladistics and branching phylogeny is a paradigm. It is one that does not 
>fit all the evidence in front of us

        If we cannot use common ancestry as a basis of evolution, what can
we use?

>        There is evidence that viable species of animals (to say
>nothing of plants) can be created from hybrids. That either argues
>that we need to revamp our phylogenetic system or our concept of
>species.

        This is being worked on...

>The intimate symbiosis of the eukaryotic cell cannot be
>represented in a cladogram, as far as I know.

        Is it necessary?  A eukaryote has a common ancestor with many other
animals.  We would, in this case, use the host cell as the unit of
phylogeny, if I am not mistaken.

>       I am not very well educated in cladistics, but none of what I
>have seen suggests to me that these relationships can be elucidated or
>even described by the cladistic method.

        Som eof them no doubt can be, probably some of them cannot be.  If
the ones that cannot be are really going to interfere with out ability to
describe phylogney, someone will have to come up with an answer.  Is any
other taxonomic system doing a better job of addressing these points?

        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             |
+--------------------------------------------------------------------------+