Re: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)

["Jerry D. Harris" <dinogami@hotmail.com>]

> I would also appreciate understanding why descriptive purposes should be
> rejected when such would not create a false picture of either the animals 
> or
> evolution.  (I suppose I'm working to be comfortable with the idea that
> calling birds a group would be 'unnatural'.  It sure is intuitive.)

  If all you are looking at are end-members of the evolutionary spectrum -- 
that is, very primitive members and very advanced members -- then yes, 
intuition tends to want to put them in separate categories -- let's call 
them Group A and Group B (B being the very advanced end members).  And 
description-based (character-based) taxonomies work well for this; it 
certainly did a great job with extant organisms as Linnaeus and others used 
them.

  The problems arise when you start putting intermediate taxa into that 
spectrum, particularly ones close to "transitions" (that is, beginning to 
show one particularly "intuitive" diagnostic character, such as feathers, 
but lacking most or all of the other traits). For our purposes here, let's 
use _Herrerasaurus_ and _Gallus_ (the chicken) as our Group A and Group B.  
"Intuition" would put them in separate groups, easily -- they are very 
distinct (presuming _Herrerasaurus_ didn't have feathers).  Now, where do 
you put _Archaeopteryx_?  Into Group A or Group B?  It has one of those 
really distinctive traits -- feathers -- but otherwise looks a lot more like 
_Herrerasaurus_ in the rest of its body than it does _Gallus_.

  So, using intuition, we have 3 options:

(1) Put the intermediate into Group A and reserve Group B for the _really_ 
distinctive forms;

(2) Put the intermediate into Group B and ignore the fact that the only 
thing that is being based on is the presence of that one character;

(3) Create another new group, Group X, for the intermediary.

  Option 3 has been used often in Linnean taxonomy, but tells us nothing 
about the closeness of the intermediate form to either end-member, and 
certainly tells us nothing about the progression of evolution, so has been 
discarded in modern taxonomy. Options 1 and 2 are weak because it openly 
acknowledges that we are being held rigid by our bipartite grouping -- we 
admit _a priori_ that only Groups A and B exist, and everything must be in 
one or the other group, regardless of how much a mosaic, or how close it is 
evolutionarily, to either end member. Options 1 and 2 have also been used on 
occasion, but problems similar to the one introduced by finding a single 
intermediary are compounded when more intermediaries, filling in the new but 
ever shrinking gaps in the spectrum are found. The more intermediaries you 
find, the harder it is to shoehorn them into existing taxa when those taxa 
are _not_ nested (as Chris Brochu mentioned).

  "Intuition" is great for initial analyses and basing initial hypotheses 
on.  In this case, character-based (e.g., Linnaean) taxonomies can be viewed 
as an "initial hypothesis" that has broken down because of the discoveries 
of numerous "intermediate" forms. Every time one of those intermediates came 
up, it became harder and harder to shoehorn them into existing groups, and 
finding those "distinctive," "intuitive" separating characters became harder 
and harder.

  However, the nested groups concept of modern phylogenetic theory 
alleviates those problems _because_ it isn't dependent on the characters to 
define groups; it uses the evolutionary history instead (albeit that that 
history is inferred from the characters).  This way, an infinite amount of 
room is created for intermediates without necessarily shoehorning them into 
one group or another -- and, at the same time, delineates the evolutionary 
progression from one taxon to another.

  This explanation has, of course, been greatly oversimplified and 
overshortened in the interest of bandwidth, but I hope the explanation 
suffices...



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 898-5630
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu
and     dinogami@hotmail.com
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