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Re: Cladistics and extinction



At 06:30 PM 11/22/96 -0500, Jeff Martz wrote (and then tucked tail and ran,
I believe :) :

>                           My point is that paraphyletic concepts are 
>useful in discussions of evolution, even if you don't want to include 
>them in the formal taxonomy.

        Now this I can agree with.

>If you are trying to figure out why one 
>group went extinct [..] morphological differences 
>(indicators of lifestyle) are probably at least as important as common 
>descent.

        True enough, but this does not justify erecting formal (and
paraphyletic) taxa, just to emphasize these differences (which, I must say
again, are frequently obvious to a child, and do not need reinforcing throgh
taxonomy).

>     For another example, how could you discuss the evolution of 
>endothermy in birds and mammals without the concept of "reptile?"

        Easy!  As I was trying to make clear in my last post, the
characteristics of "reptiles" as you define them are symplesiomorphies of
the Amniota.  This makes it a *LOT* easier to understand the phylogeny and
evolutionary features in the developments of the lines leading to birds and
mammals.  It takes the emphasis off the morphological similarity between the
wo groups of basal amniotes (which is accounted for by their common
ancestry), and places the emphasis on their differences.  This highlights
the most important point of the development of endothermy in synapsid and
sauropsid line, that it was a convergence.
        The traditional way of describing this, the "reptiles" crawling out
of the gutter to achieve endorthermy and superiority over all beasts save
man, gives a misleading impression that "reptiles" were an homogenous mass
of critters, a few of whome bettered their bloodlines by becoming more like
us.  Is this discussing evolution?
        If we are to discuss evolution, we must discuss the derivation of
animals through pylogenetic propinquity, and the character state
transformations which occurred.  I fail to see how lumping two seperate
dichotmous groups of basal amniotes into one, just because their later
decendants underwent a similare transformation, makes this easier.

>if the LOSS of a certain trait ultimately results in a groups demise?  

        That is synapomorphic, and will more likely be discerned through the
study of monophyletic groups.

>Lets pretend for a moment that birds and mammals evolved endothermy to 
>exploit a small niche which then dried up, causing them to go extinct.  
>What if the loss of more primitve reptilian characteristics ultimately 
>caused the EXTINCTION of birds and mammals?  Wouldn't the concept of 
>"reptile" be useful then?  Not amniote- REPTILE.

        No, because this loss is autapomorphic for those two groups.  It is
the differences in the derived groups which cause their survival or
extinction (presumably, let's not argue this one, ok?), not the the
ordinariness of the more inclusive group. We must concentrate on the group
that did things differently, not the ones that carried on business as usual.
Look at it this way:  When a detective wishes to solve a murder case, does
he go around asking if everyone was acting normally the night of the murder?
No, he asks if they were doing anything suspicious.  He is looking for
things that are out of the ordinary, not things that are ordinary (Dirk
Gently perhaps being the exception).
        I sense also that you are still confused on one point:  "loss of a
trait" (secondary reversion) *can* be synapomorphic.  It is frequently a
supporting character for clades in phylogenetic analysis.

>     You are assuming that it is always the DERIVED characters that will 
>always be the key to survival.  If the underived condition ultimately 
>turns out to be more advantageous, then the surviving group would be 
>paraphyletic.

        No. the derived characters may be the key to the groups extinction.
But when a more inclusive group goes extinct, and one of it's more exclusive
clades survives, we must look at why the latter survived in terms of it's
derived characters.  If the more inclusive group survives, and the exclusive
group goes extinct, we must look at the derived characters of the exclusive
group, only this time to see why it died out.
        You see, the differences are in the derived groups.  That's why they
are derived.  If you want to know why insects colonised land before the
vertebrates, you don't go looking for flaws in the Animalia which kept them
back, you look at what about the Insecta got them there first.

        Happy Turkey-Day all!
        :)
        Wagner
+--------------------------------------------------------------------------+
| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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