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Re: Taxa nomy (and intro)



On 11/3/04 2:05 pm, "T. Michael Keesey" <mightyodinn@yahoo.com> wrote:

> --- Christopher Taylor <ck.taylor@auckland.ac.nz> wrote:
>>     It's been my general impression that most people are sort of moving
>> towards a compromise system - the major ranks (family, order, class, phylum,
>> etc.) are still used, but most of the intermediary taxa are treated as
>> unranked.
> 
> Not quite. In cladistic taxonomy you'll often have taxa that are expanded to
> be
> monophyletic, and they may include other taxa that are traditionally of the
> same rank, or even great. An example near and dear to our hearts would be
> _Theropoda_ (traditionally a suborder) including _Aves_ (traditionally a
> class).
> 
> So you can't really have a compromised system.
> 
The general point of reference seems to be the traditional taxa of _living_
organisms - as you've just pointed out, though, this rather falls apart
when, as with Aves and Reptilia, one taxon is paraphyletic with regard to
another (I could add something on acceptance vs. non-acceptance of
paraphyly, but that's a whole new topic). So, for instance with regard to
animal phyla, they seem to be largely demarcated by "Porifera, Coelenterata,
Bryozoa, Brachiopoda, Mollusca, Annelida, Arthropoda, Echinodermata and
Chordata are phyla; the most inclusive living taxa that do not include any
of these groups are other phyla". Where extinct clades not covered by this
stand is not defined - everyone kind of looks sideways and changes the
subject :)
    Personally, I actually prefer an unranked system, but it does sacrifice
a few communication points.


>> After all, the main disadvantage of a totally unranked system is
>> that it gives nowhere to hang one's hat - people still want to be able to
>> say that x number of families can be found in an area, for example.
> 
> Such a statement is essentially meaningless. This is actually a harmful
> byproduct of the Linnaean system, IMHO; the perception that the ranks do
> correspond to something real.
>
This is certainly true in comparing ranks in widely separated taxa - for
instance, one can't help noticing that the size of an average avian "family"
would probably correspond to a small to middling "genus" in angiosperms. But
is this the case at smaller levels? Is there any validity in comparing avian
families with each other as families? Once again, fossil taxa have a habit
of making things difficult for the pedantic neontologist,


>>     I should also note that I personally don't like the idea of making
>> phylogeny our sole criterion for taxonomy because so many organisms
>> (probably even the majority) are still so poorly known phylogenetically.
> 
> What other criteria do you advocate?
>
Ha ha, got me :P Here we see the flaw in empty rhetoric - especially when as
badly phrased as that was. But I would reiterate that phylogenetic taxonomy
seems to have no place for those taxa which group together organisms that
share features useful for identification, but which are phylogenetically
unplaced or uncertain - for instance, many yeast genera in fungi. Remember,
the flipside to a solely phylogenetic taxonomy is that erection of a new
taxon then automatically implies that said taxon is monophyletic. And many
current taxa have not actually been explicitly tested for monophyly.

On the whole, I support most aspects of phylogenetic taxonomy, but I am
concerned that it loses something in the way of flexibility and
communication, especially to people who aren't primarily taxonomists.


Christopher Taylor
ck.taylor@auckland.ac.nz