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my piece on clades



I can't see much in this ontological debate over clades.  A monophyletic
group exists in precisely the sense you'd expect it to, or else it doesn't 
exist at all.  Exactly the same can be said for paraphyletic and polyphyletic 
groups.

I see two advantages in using clades - firstly they establish ground rules
for taxonomy, which is a rather finnicky business where precision matters
more than imagination.  Secondly, they encourage the right trains of thought.  
For example, if I'd never heard of cladism and you asked me what the
integument of Dimetrodon was like, I'd have said something like:

  "Dimetrodon is a mammal-like reptile.  And one of the features of all
  reptiles is a scaly, dry skin.  It didn't have hair or sweat glands"

But with the benefit of the cladistic approach, I'd say:

  "Dimetrodon is a synapsid.  Living synapsids (mammals) all have hairy,
  glandular skin.  The closest outgroup we know enough about is the Diapsida,
  with scales (sometimes feathers too) and no glands.  The next outgroups are
  amphibians and lungfish, both of which have glandular skin.  Amphibians
  have no scales, I don't know about lungfish.  So from what I know, it's
  most likely that secretory skin is the primitive condition, retained in
  Dimetrodon.  There is no phylogenetic evidence either way on the hair." 

(I am assuming Testudomorpha is a subclade of Diapsida.)

> = Jeffrey Martz
>> = Tom Holtz as I recall

>>      Look at it another way: To recognize a monophyletic group, The
>> Watcher, The Beyonder, or The Maximortal merely wades into the river of
>> time and prunes a clade off at the root. However, to recognize a
>> parpahyletic group, the supreme entity in question must first prune a
>> clade, then prune one or more clades from that clade, and accept what is
>> left over. 
>     This is assuming that the clade was there to begin with, something I
>am arguing against.  You have to think it up before you can prune it.  

What this is assuming is simply that there is a 'tree of life', in which all 
organisms are united by common descent, and that once brances separate they 
never rejoin.  Then one single cut with the pruning hook always grabs a 
monophyletic group.  A paraphyletic group requires several cuts, and a 
polyphyletic one requires both cutting and grafting.  But so much for analogy.

>> A paraphyletic group cannot be taken from the tree of life as a
>> unit...

>     First of all, no one is proposing that taxa be based exclusively on
>morphology; any paraphyletic group can be considered as a subset of a
>monophyletic group, so in essence is still exists due to the existance of
>a larger monophyletic group.

Any set of organisms from the planet Earth is a subset of a monphyletic group.

>    Second of all, the allegation that morphology is a result of
>phylogeny is a chicken and egg question

So which did come first?  The domestication of the jungle fowl _Gallus_, or 
anisogamy?  :)

>      Even cladists refer to "basal archosaurs" and "non-avian dinosaurs".
>Is using these words REALLY muddying the waters less then abbreviating to
>"Thecodont" and "Dinosaur"?  The groups are still being referred
>to and therefore are given a place conceptually, so what has been
>simplified? Everytime you define a clade, you are also recognizing a
>paraphyletic group; if you were to define Aves as the most recent common
>ancestor of modern birds and _Archaeopteryx_ and all its descendants for
>example, you are automatically making all dinosaurs not belonging to that
>clade into "non-avian dinosaurs", regardless of if you give them a
>capitalized name or not.

If you want my opinion, taxonomists all groups with an initial capital should 
be clades.  This includes genera and all higher Linnaean taxa.  So 'Thecodont' 
is wrong.  But 'thecodont', 'herbivore', 'female', etc. are all to my mind 
perfectly legitimate terms to describe organisms.  If you see a capital letter, 
you know the term is expressing a theory about evolutionary history and not 
about any other aspect of the creature's biology. 

>     Incidently, as intristic as cladistics is to discussions on this
>list, is it really so innaproprite to discuss it, even if dinosaurs aren't
>always included in the examples?

I think it's appropriate, and I have more to say on the subject.  But if a 
groundswell of opinion is rising against cladobabble, now is the time to say 
'enough!'