Agreement Finally!!? (was Paraphyly)

[Curtis Clark]

At 05:43 PM 1/17/02, Richard Pyle wrote:
>But what about the A-C complex?  We could restrict the
>definition of the sub-grouping name to include only those descendants of the
>most recent common ancestor of "A" and "C", in which case the name would be
>"Six" -- but that would leave "X" out hanging in the paraphyletic breeze.

Why would X be paraphyletic?

>2. To make things interesting, let's say the really "profound" morphological
>divergence (e.g., feathers+flight) happened between nodes 4 & 6, such that
>"X" is much more closely associated with "D" and "E" in all aspects of
>morphology/ecology/behavior -- except for a tiny, trivial divergence (e.g.,
>a change in eye color from turquoise to light blue) that occurred between
>nodes 3 & 4. Would it be justifiable to then use the name "Six" for the
>group consisting of species A-C, and the name "ThreetoFour" to refer to
>species X, D, and E? (i.e., using Ken's example of defining a paraphyletic
>group in terms of its "start point" and "end point")

X is more closely related to A, B, and C than it is to D and E. Among
actual organisms, we would not expect that only eye color would link it to
those species, but at the *very next node* all hell would break loose. If
that indeed were the case, we are better off having the extraordinary
evolutionary occurrence brought to our attention than we are lumping X with
D and E on the basis of plesiomorphies.

Crocodilians, for example, are not lizards. They share important biological
similarities with birds, far more than just eye color.

>3. What name would you apply to the organisms that represent the generations
>spanning the gap between nodes 3 and 7?  Would they also fall under "Seven"?
>Or, would "Seven" be defined as only those critters from node 7 and forward?

A clade begins with divergence. Calling the node 7 is a convention, but the
lineage began with the split of 3.

>4. To throw the problem of ranks into this, [...] Which is the best way to
>assign the names?

Phylocode! <snicker> This is a continuing point of contention among
cladists, but as I contemplate Ginkgo biloba, the sole extant member of
Ginkgo, the Ginkgoaceae, Ginkgoales, Ginkgopsida, and Ginkgophyta, I am
persuaded that we can make do with a variety of solutions.


--
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu