[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

sister groups do not exist

Very well stated. Given the continuous nature of evolution, dividing the tree of life is unavoidably arbitrary (although some cuts are more arbitrary than others). You can minimize arbitrariness, but cannot eliminate it.
Extinction and the relative rarity of fossilization is good news in that the gaps make cutting the tree easier, but at the same time bad news since it erases a lot of information. Extinction is what makes cladistic analysis possible and useful, and with a continuous fossil record, strict cladists would see that virtually all cuts create paraphyletic groups (there may be rare exceptions).
Speciation is a paraphyletic mother-daughter "budding" process. Sister species (and therefore sister groups) are a convention invented by Hennig to make cladistic analysis easier, but I serious doubt that he actually believed they reflect how evolution actually works.
When I say two taxa are sister groups, I realize that with a complete record, one group is actually paraphyletic with respect to the other. But extinction has wiped away the evidence of this paraphyly, so the illusion (convention) that they are sister groups is a very useful way to view and analyze such groups.
Sometimes I wonder how many strict cladists realize this, and how many think sister species and sister groups are based in reality rather than convention.
---------Ken Kinman
From: "Jeffrey Martz" <jeffmartz@earthlink.net>
Reply-To: "Jeffrey Martz" <jeffmartz@earthlink.net>
To: <twilliams_alpha@hotmail.com>, <kinman@hotmail.com>, <dinosaur@usc.edu>
Subject: Re: warm-bloodedness
Date: Wed, 21 Jun 2000 21:25:05 -0000

Timothy Williams wrote

>So we're talking Linnaean hierarchies, yes? One nice thing about cladistics
>is that you can dispense with these ranks. "Mammals, the grades are just
>in, and you achieved an A+ in Morphological Diversity, so you get to have
>your own Class. Congrats! Crocs - oh, bad news I'm afraid. Unfortunately
>that spurt of evolution in the Triassic wasn't quite enough. You get a D
>for Diversity - I'm afraid you'll have to stay in the Reptilia. Better luck
>next time!"

Cladistics hasn't dispensed with ranks, it still uses genera and species. An arbritary decision to divide organisms into units always has to be made somewehere. If we could examine every individiual organism that ever lived, the distinctiveness of individual species, and certainly genera, would dissappear in a hurry. It would be clear that divying up species and genera is really no less arbitrary then higher level divisions like families and classes. Fortunately for taxonomy, evolution has conveniently killed off most of the ancestors and intermediaries between living and extinct organisms with a patchy fossil record, making what Ken would call "phenetic gaps" that lets them appear distinct enough to divide.
Compare two sister taxa on a cladogram, particularly if they are different genera or species. Why are they not lumped into the same genus
and species? Why are ANY of the taxa on the cladogram distinct from one another? Why not just lump all the taxa on a cladogram into a single
species? A decision has been made to keep them seperate on morphological
grounds having nothing to do with common descent. This is a decision no
less arbitrary or artificial then deciding where to cap off a paraphyletic
group, and like defining paraphyletic groupings, is done for the sake of convenience.

LN Jeff

You have to study a great deal to know a little.
-Baron de Montesquieu

They may forget what you said, but they will never forget the way you made
them feel.
-Carl W. Buehner
Jeffrey W. Martz
3002 4th St. #C26
Lubbock, TX
(806) 747-7910

Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com