Model Organisms

Arlin Stoltzfus arlin at is.dal.ca
Tue Apr 22 08:21:10 EST 1997


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.

It is true that biologists often make absurd or meaningless 
statements using "primitive", "advanced", and similar terms (e.g.,
"Drosophila is the antithesis of a degenerate insect", Adrian Bird, 
1995 paper in TIG).  However, its also true that children play 
with fire and get burned.  This doesn't mean that fire is bad, 
or that it cannot be a useful tool in the hands of a trained expert.  

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.

It is less straightfoward to apply these terms to whole organisms, 
because any organism will have both primitive and derived 
characters.  We might be tempted to say that a frog is more 
primitive than a rat, because the frog-rat ancestor is thought 
to be an amphibian, which would mean that it shares many aspects 
of physiology with the frog.  However, the frog has many derived 
features relative to that ancestor.  Is this valid, or not?

Whether or not it is meaningful to apply "primitive" and "derived" 
to whole organisms would depend on whether there is some sort 
of unity or consistency that would apply somewhat comprehensively 
to a gene pool through a long period of evolution.  This issue
hinges on many very old questions in evolutionary biology, such 
as whether 'bauplane' exist, whether evolutionary rates 
are dominated by mutation rates (if so, then lineages that 
have experienced different overall mutation rates will have 
consistently different rates of evolution for all characters), 
and whether there is evolutionary stasis.  Are sharks and 
horseshoe crabs morphologically (and probably physiologically) 
primitive merely by virtue of being extreme random deviates 
(i.e., they just happen to be a few standard deviations from 
the mean proportion of retained ancestral characters), or 
have they actually experienced a quantitatively or qualitatively
different type of evolution?

> 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.
	
This is not the notion that was discredited.  Since all organisms 
have a common ancestor, then *if* (according to above conditions) it 
is meaningful to apply "primitive" and "derived" to whole organisms, 
then there is nothing wrong, in principle, with putting all life on 
a scale or primitiveness, supposing that a very general metric could 
be devised.  A ranking on primitiveness would probably be different 
from a ranking on derivedness.  Although valid in principle, this 
excercise would probably be meaningless in practice because a 
multi-dimensional measure of divergence is much more useful.

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".

Arlin
-- 
Arlin Stoltzfus
Department of Biochemistry
Dalhousie University
Halifax, Nova Scotia B3H 4H7 CANADA
(email) arlin at is.dal.ca 
(phone) 902-494-3569 
(fax) 902-494-1355



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