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




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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