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Re: Paraphyly and sister taxa



Jeff Martz wrote:

<Paraphyly is implicit at EVERY NODE where the sister
taxa are not identical, which means at every node
except where individual specimens assigned to the same
genus and species are sister taxa. Unless we are
willing to place every single organism in the same
genus and species, there is no possible system of
taxonomy that can eliminate paraphyly. Taxonomy is
useless WITHOUT implicit paraphyly.>

  An end result is not a taxon from which stems
subsequent taxa; the forms recognized as taxa cannot
comprise groups of other taxa, for they are
biologically unique. Unless they're bacteria. :)

  End taxa represent units in a population, and per my
understanding, the following would hold that each
species cannot comprise another:

  sattleri---\
  hansemanni--|---* root population
  cazaui-----/

  This says the root population is paraphyletic, and
thus cannot comprise a unique biological entity [UBE],
the neccessity of recognizing binomina. We can look at
our own geneology as an example of this, where we are
not our mother or father, that they each comprise a
UBE, regardless of the system through which progeny
occur; bacteria fission off a duplicate, but which of
the two are the original form? Mutation in our lineage
and selective genetic copying, in which some genes are
expressed instead of others, identify that you, me, my
mother, and your father are four separate, UBEs.

  A population is therefore paraphyletic, and each
compromising individual is a UBE, distinguishable from
any other by its genetics. If a population can
intrebreed, they do not represent a UBE; but if they
cannot interbreed, they do represent a UBE, and this
is what scientists choose to put labels on -- or at
least try to. You, my brothers, my sisters, are all a
UBE as far as the extent to which one form of the
population cannot breed with another. All humans can
interbreed, but humans cannot breed with their closest
living relatives, and so a UBE is present, with
compromising individual forms, much as each flower on
a tree, the few trees of the type with which it can
pollinate, and so on. But genetically, neither of us
is a UBE, since we can interbreed.

  This is my understanding on it.

  The entity regarded as "sattleri," whether it shared
a more recent divergence from "hansemanni" or with
"cazaui," may all represent UBEs, but only the root
would be paraphyletic. Of course, to verify, we'd have
to find sufficient populations which could display
enough morphological variation from another population
as to establish the great likelihood of UBEs in these
two forms. If "sattleri" then belongs to
*Amargasaurus* as an entity sharing more recent
divergence [an event marked by the inability to
interbreed -- ?] from *Dicraeosaurus,* then the UBEs
labelled *A* and *D* are both monophyletic, neither a
natural population exlcuding each other from its root.
The common ancestor is a hypothetical member of this
root population, but any individual found to represent
this root would have to be classfied as a UBE unless
it could breed with any of its progenitous
populations. Take one that could, but not another, and
remove that form's UBE status, because it belongs to
the root population. The other two are safe, because
neither can breed with the root population. If then
"sattleri" is a UBE, does it represent a form that
diverged from *A* or *D* more recently? Whatever name
you give this, there are three populations that form a
group. You can name all five resulting groups, if you
so choose, and we already have them:

1--Dicraeosaurinae
    |--2--*Amargasaurus*
    |       |--3--*"D." sattleri
    |       `--4--*A. cazaui*
    `--5--*Dicraeosaurus hansemanni*

  We have five if S diverged more recently with D than
A, and also five as an unresolved divergence from
either, and may therefore be labeled with a unique
binomen until resolution occurs. We would have to
agree that it diverged more recently with one, than
with another, however, and we can apply a binomen to
each one anyway we want.

  My point is that a genus merely reflects a
morphological variation, and can be used to group
populations sharing ancestry; a species reflects a
UBE, an actual, natural event. A genus is then a name
only for the most restrictive group of UBEs (two?),
and all further taxa are merely counting up
inclusivness of UBEs. Chris Brochu (1998: SVP mem. 5)
named a group of species of *Crocodilus* that seems to
reflect this, intentional or not, Globidonta, that in
turn do not comprise several other *C.* species
[UBEs]. Kudos, I think it's cool.

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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