Cladistics and "Eclecticism"
Pierre.Deleporte at UNIV-RENNES1.FR
Tue Feb 5 16:18:10 CST 2002
At 10:44 05/02/2002 +0100, Peter Hovenkamp wrote:
>1. If a group of people (say, a mixed group of 100 men, women and children)
>embarked on a spaceship and were to settle on Mars - would that effectively
>split Homo sapiens into two species (assuming contact would be scarce and
>limited to raw materials, not frozen sperm etc.) the moment the spaceship
>took off? (...)
Hi Peter and taxacomers,
Maybe we could stick to the way systematics are performed, effectively ?
We don't deal with direct observation of populations history, but with
specimens carrying characters.
The population split evoked by Peter does not provide different characters,
thus there is no ground for classifying the two sets of individuals as
different terminal taxa, from a systematists point of view (different from
the ecological point of view, as reapetedly underlined by others in this
list: conservation ecology may be concerned by the two populations).
No difference in characters, no concern for classification.
A problem seems to arise if only one population evolves, and the other
remains exactly the same in all its characters: should we apply the
cladistic convention of giving them both terminal taxon status?
But in the real life, we did not observe the evolution of the populations.
Then we are facing two different taxa, because they carry different
character sets. One of them could be the strict copy of the "ancestor" of
the other one. Anyway it is not properly the ancestor, the two populations
Is there any problem in naming them as sister terminal taxa, that is
"closest contemporaneous relatives", identified as such by their
synapomorphies, and differing in a series of characters?
In the present case, some symplesiomorphies of the two taxa could be
mistakenly taken as autapomorphies of the "unchanged" terminal taxon.
Correct, but is there a remedy to such a mistake, given than we have no
time machine to go back in time and observe the evolution of the ancestral
population? There is no label "false autapomorphy" written on the character
of the specimen at stake. Thus, is this an issue at all for systematics?
Misleading data may mislead us, and what can we do to that?
I think that the key point is that two sister terminal taxa simply may, or
may not, be two "equally diverging" taxa; one of them may, or may not, be
the more or less strict copy of their exclusive common ancestor, and maybe
we have no way to know that. Where is the problem ? Just agreeing on the
full acception of the naming convention: a sister group, i.e. a
contemporaneous closest relative, may effectively be unchanged since a long
time in some characters. Coelacanths do exist, as a sister group nearly
unchanged in some morphological traits.
For a phylogenetic classification, the key feature for the two sister taxa
is their separation with no back crossing, not their differential
Identifying reticulate evolution is a different problem, as well as
conventions for naming taxa in case of reticulate evolution. We could have
a discussion focusing on this point specifically.
Identifying an ancestor taxon in fossils is still another problem, well
debated in the cladistic literature: there logically cannot be positive
evidence of being an ancestor in the characters of fossil specimens (only
lack of synapomorphies in the known characters), thus some specimens may be
placed either as terminal taxa, or pulled down at the internal node of the
cladogram as possible ancestors. If placed at the top of a branch, this
branch will carry no synapomorphies, but this graphic representation could
perhaps be worth translated in the naming, with a marker indicating this
ambiguous status "sister or ancestor".
But can't we separate the questions, and deal specifically with problems like:
- possible difficulties to infer the phylogeny (including possible
reticulation): not directly a problem of naming conventions;
- rules for naming taxa in case of doubtful phylogenetic resolution: should
we better urge the change in naming or name content, or wait and possibly
use a marker indicating doubtful phylogenetic status of a taxon?
- problems with possibly adjusting classic taxon names to new discoveries,
e.g.: where should "birds", or "Aves", begin now? And why not keep their
ancient acception ? Unless we would consider that an ideal pre-defined
"bird" taxon should be decifered in nature, changing the content of "Aves"
when new close relatives are discovered is perhaps not a necessity (think
of stability, Ken! Maybe it is not forbidden either, but then let's stop
arguing of stability).
- conventions for naming taxa in case of a well documented reticulate
phylogeny: botanists could provide nice real examples here. How should we
name such things? With markers?
- conventions for naming taxa in case of a well resolved, strictly
dichotomous phylogeny. Do eclecticists have a general criterion for
defining a relevant (or useful) paraphyletic group? (better answered by
eclecticists). If yes, does this criterion applies to non-bird dinosaurs
- is eclecticism (implementing simultaneously two or more incompatible
criteria) a problem in itself for a classification? For what practical
reasons? What do we gain, what do we loose, compared with a nested
hierarchy of taxa based on a unique criterion?
- if eclecticism is a problem, and if some paraphyletic groups are worth
naming anyway, why not call such groups "non-something" taxa (like
"non-bird dinosaurs" or "non-human primates"), and everybody is happy?
(exactly a complement to the Kinman system: names for holophyletic groups,
with "non-something markers" for paraphyly, while the Kinman system uses
names for some paraphyletic groups, with markers for providing the
corresponding phylogenetic information). Exactly the same information
content may be conveyed with and without eclectic definiton of names, hence
my insistence in discussing eclecticism in itself.
- the "too many names" issue: apparently nobody suggests to name all
clades. Not all possible paraphyletic groups either. Choosing which groups
to name seems a practical question, not "natural" or "scientific". But is
there any limit, besides imagining the names? Is this an issue at all?
- still another question is the subjective feelings facing some conventions
for some groups. Just a test:
* Humans are primates, and of course they are not non-human primates.
* Birds are dinosaurs, and of course they are not non-bird dinosaurs.
These formulations convey information about both monophyletic and
paraphyletic groups, and they do not hurt any of my non-scientists friends,
nor even most of my scientist colleagues. Given that the information
content is not ambiguous, why should they hurt some people ? (better to be
answered by people feeling hurt or uneasy with such conventions).
This series of questions is not limitative, I simply suggest that shifting
from one topic to the other is a very efficient way (intentional or not) to
empede a thorough debate on any given point.
Not to mention questions of cloud castles, pedantic head banging from top
of ivory towers, giving somebody a break, good ol' fashioned criteria,
rafts and rocks, and apocalyptic prophetia...
(sorry, I fear I'm getting somewhat trolled ;-)
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