Model Organisms

L.A. Moran lamoran at gpu.utcc.utoronto.ca
Thu Apr 24 14:34:35 EST 1997

In article <335CBB46.1DF5 at is.dal.ca>, Arlin Stoltzfus  <arlin at is.dal.ca> wrote:
>L.A. Moran wrote:
>> There is no such thing as a living organsim that is more "primitive" than
>> Drosophila and more "advanced" than C. elegans.

>Among trained experts, the 
>word "primitive" means "retaining an ancestral state".  The 
>contrary of "primitive" is "derived".  It is a straightforward 
>matter to apply these terms to individual characters in cases 
>where ancestral states can be inferred.


I appreciate the point that you are trying to make but perhaps it would be
best to remember the context. Someone wanted to knw if there were any
organisms that were more primitive than Drosphila but less primitive than
C. elegans. Why? Because he was hoping to isolate a C. elegans gene using
a human probe and was looking for an "intermediate" model organism. Do you
think that this is a reasonable experiment? Is it consistant with your
understanding of evolution. (I sure hope not or I have seriously misjudged
you!)  (-: 

>> Modern living species cannot be arranged on a evolutionary ladder from
>> primitive to advanced. This is a false notion of evolution that was
>> discredited more than one hundred years ago.

>The problem with the "ladder of life" idea was not that it is 
>necessarily invalid to apply the same primitiveness metric to all 
>organisms relative to a single common ancestor, but the idea 
>that each extant organism arose *by descent* from the extant 
>organism on the previous rung of the ladder.  We can have 
>rat-dog-cat-drosophila on our scale of primitiveness, but 
>a modern biologist would not take this to mean that fruit flies 
>evolve from cats, cats from dogs, and dogs from rats.  This is 
>the "false notion of evolution that was discredited more than 
>one hundred years ago", not the concepts of "primitive" and "derived".

I accept your point. The original questioner clearly implied that there
was a ladder of life as well as a recognizable scale of primitve to 
advanced. One could successfully quibble (as you have) that the two
concepts are logically separable. (Note that the original question
implied that humans were more "advanced" than Drosophila and all of
the responses referred to organisms that would easily have fit onto
the nineteenth century ladder of life. This is not a coincidence. 
I believe that there are many molecular biologists who still cling to
a false "ladder-of-life" notion of evolution. Here is a case where this
false understanding of evolution is affecting the ability to design
meaningful experiments.) 

Do you think that there is an organism containing genes that are 
intermediate between the similarities of the human and C. elegans genes?
(i.e. assuming that particular human and C. elegans genes are 50%
identical, is there an organism with genes that show 75% identity to humans
*and* 75% identity to C. elegans?) I maintain that in molecular terms there
is no such thing as primitive and advanced living organisms. If you want
to debate this particular point then I would be happy to oblige - please
point me to examples in the literature.

Larry Moran


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