[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: holophyletic groups only & the future
These are my last posts on these for a while - I actually have things to
do. I am responding to several people here.
I really do think phylogenetic taxonomy is the way taxonomy is going. It
seems to be the method of choice among nearly all systematists I work with,
and there is a growing understanding that paraphyletic assemblages should
not be recognized as taxa.
>If I'm following, creating a named group would exclude species which should
>evolutionarily be included based on some further rules for inclusion.
??????? not sure I understand, but I'd say no - a monophyletic group
includes an ancestor and ALL of its descendents (irrespective of Ashlock's
terminology, which very few people use). This is true whether the group
has a name or not.
>group identified by anyone will exclude direct ancestors which are part of
>another group, as you're saying. However, if 'real' (evolutionary) group
>were to equal 'named' group, wouldn't that be acceptable (natural)? I think
>you'd answer that because the group is wholly enclosed within another group,
>separating it does not correspond to an <important> evolutionary dividing
>line, but I want to be sure.
"importance" is a subjective human thing, and has nothing to do with
>I would also appreciate understanding why descriptive purposes should be
>rejected when such would not create a false picture of either the animals or
Precisely because it usually creates a false picture of either the animals
or evolution. Have a look at some of the references posted in earlier
threads on this subject for examples.
(I suppose I'm working to be comfortable with the idea that
>calling birds a group would be 'unnatural'.
Ah, but as long as "bird" corresponds to a monophyletic group, it is
natural. Had we defined Aves as "all feathered animals except blue jays
and cormorants," we would be making it unnatural.
It sure is intuitive.)
>These are different terms of discussion from prior ones, so I'm trying to
>establish the connections.
> That is the problem with strictly cladistic classifications, they ARE
>restricted to monophyletic (holophyletic) groups. But you are failing to
>recognize that paraphyletic groups can be natural too (especially if
>semi-paraphyletic), and are only considered unnatural by strict cladists.
Paraphyletic assemblages can never be individuals the way a monophyletic
(or "holophyletic," to use Ashlock's terms) group can be. For this reason,
no supraspecific paraphyletic assemblage can ever be a natural taxon, and
such assemblages are not taxa at all. This is because monophyletic taxa
can be united on ancestry alone (a natural process), but paraphyletic
assemblages must be delimited to some extent on the basis of "significant"
or "important" characters, the significance or importance of which is a
human construct only. As soon as you base groups on characters and not
ancestry or descent, the groups cease being individuals and become classes.
ciao for a while,
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
1400 S. Lake Shore Drive
Chicago, IL 60605