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Re: courses

On Thu, 5 Dec 2002 18:36:45   
 Jaime A. Headden wrote:
>Steve  Brusatte (dinoland@lycos.com) wrote:
><Technically it is possible for a coin to come up tails 50000 times >in a row. 
> But, it's not probable. I think this is what George is >saying. There are 
>probable paths ("easy") and very improbable paths >("hard"). The improbable 
>paths are possible, but very rarely will >they be seen, because nature is so 
>complex, as Dan said. So, >perhaps "probable" and "improbable" are better 
>terms for "easy" and "hard."
>It goes beyond this, I think. Minimal evolution requires the idea >that to get 
>to any one product, the course will have as few >reversals, duplicate 
>scenarios, revisions, etc. in the genome. If >there is one ...well the idea is 
>that there isn't. If nature is >easy, or minimal in evolutionary expression, 
>then reversals >shouldn't happen. And like I said, the idea is that is doesn't 
>>happen. This means a minimum of support to every two taxa, not a multitude 
>... simplicity in all modes.
>I think this is what George is trying to say. I won't argue against >it, 
>that's not my place.

Yeah, I won't put words in George's mouth either (it's foolish to try to assume 
the words of somebody more knowledgeable on a subject than yourself).  However, 
I wasn't necessarily referring to parsimony.  We all know that parsimony is not 
always correct, but, as John Conway just discussed, is a necessary assumption 
to make.  Otherwise, we'll have a mess of characters and trees that cannot be 

I was referring not to simple reversals or duplications, but rather to overall 
evolutionary paths.  With a random process like evolution there are seemingly 
numerous possible paths.  Describing any path as "easy" or "hard" is too 
simplistic, but some paths will most certainly be more probable than others.  
The coin analogy is the simplest analogy to make concering probability and 
chance, but, yes, realistically the processes of evolution cannot be described 
by a heads/tails dichotomy, because there are so many variables.

*However*, this just means that the model is harder to create.  Parsimony, and 
the trees down/ground up debate, is something entirely different (to me 
anyway).  Parsimony is an assumption made in order to not just simplify, but 
enable an evolutionary analysis that limits subjectivity (not eliminates, but 
limits).  The evolution of flight debate just describes two evolutionary paths, 
both of which are certainly probable.  Mathematical and physics-based models 
show that both are possible, and the suite of dinosaurs closely related to 
birds with both terrestrial and arboreal adaptations means that neither 
hypothesis disagrees greatly with the data.  There is no way to say that either 
one of the paths is more probable than the other.  Evolving flight with the 
assistance of gravity might seem easier than evolving flight against gravity, 
and it might very well be, but neither is by any means improbable.  I would 
say, however, that evolving flight as a mechanism to leap through und!
round burrows is highly improbable (but, who knows).

Therefore, we should refrain from using words like "hard" and "easy" to 
describe evolutionary paths.  Evolution is random (although environment plays a 
large role and might limit some randomness), and selection acts on whatever 
characters can give the smallest advantage.  Evolution is also probable, 
however.  Nature cannot select upon something that, by probability, isn't going 
to happen, just like a glass that falls to the ground and breaks isn't going to 
spontaneously gather back into a glass.


Stephen Brusatte
Geophysical Sciences
University of Chicago
Dino Land Paleontology-http://www.geocities.com/stegob

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