>By cladistic point of view, I should have said a cladistic
>point of view on higher level taxonomy... i.e., that only
>monophyletic taxa deserve a formal name. And by monphyletic, I mean
>those that comprise ALL descendants (to avoid the monophyly holophyly
>paraphyly business). The ancestor of all protozoa is also an ancestor
>of elephants.
Mark,
I disagree with your usage of monophyly. I have noticed that the
Hennigian definition of monophyly is usually insisted upon by
those cladistically
inclined, eventhough it is not a necessary component of the
cladist
viewpoint. As Ashlock pointed out in 1971 (Syst. Zool., 20:63-69),
the word monophyly has a root meaning of "tribe of one".
A monophyletic
group is one derived from a single ancestor with the group
characteristics.
The alternative to "mono", is polyphyletic...a tribe derived from at least
two independant ancestors and whose most recent common ancestor
was, therefore, not a member of the group. These two terms form a
nicely exhaustive set of alternatives- one, or more than one.
And the difference between the two is solely one concerned with
the number of common ancestors which give rise to the group. Hennig
invented the term
paraphyletic, whose definition depends not on the nature of the
ancestors but on
whether or not all the descendants are included in the group and ranked
it equally with mono and polyphyletic. He redefined monophyletic
in a more restricted sense as a group comprised of all the descendants of
a single biological species. This is confusing because a
paraphyletic group is still a tribe derived from one ancestor
and is therefore sort of
monophyletic in the earlier but non-Hennigian sense of the word.
To rectify the confusion that redefining monophyletic caused,
Ashlock
cleverly preserved the distinction between mono and polyphyletic
(which is concerned with the nature ancestors of the groups) and
subdivided monophyletic into holophyletic (containing all descendants of
the single most recent ancestor) and paraphyletic
(not containing all descendants of the most recent ancestor).
Do you agree with the logic of these definitions? A cladist could accept
these terms and still argue that only holophyletic taxa deserve formal
names.....Clarity and agreement in terms would certainly make the arguments
about the latter point easier to follow.
Andrew Roger
aroger at ac.dal.ca
Dept. of Biochem.
Dalhousie University
Halifax, N.S.
