Definition of evolution

Stewart Schultz schultz at unixg.ubc.ca
Mon Nov 4 17:53:23 EST 1991


In article <1991Nov2.174903.1852 at ac.dal.ca> arlin at ac.dal.ca writes:

>In the case of introductory textbooks
>that define evolution as shifting allele frequencies or some such
>simple formulation using "allele", it is a matter for debate by
>literary psychoanalysts what the authors intend, or whether they can
>be convinced to change their minds when the meaning of their statement
>is explained to them.  Whether or not they actually mean what they
>say, the formulation is wrong.  When I see incorrect statements in
>textbooks, I cross them out with a big red pen.

So it's oversimplified statements in introductory textbooks that you're 
objecting to.  Perhaps we should consider that introductory textbooks 
are simple for a reason, and an arguably good reason.  And perhaps their 
authors' ideas are substantially more multidimensional than the simple 
heuristic models and definitions they present solely for the pedagogic 
benefit of beginning students in an enormously complex field.  Such 
texts are to be judged by a standard of teaching, certainly not by a 
standard of logical completeness expected e.g. from a treatise in the 
philosophy of science; this is simply bullying an easy target.  

I think we should accept the fact that introductory textbooks are not 
intended as the final word on evolution...

>If accepted, this is a BREAKTHROUGH in our current discussion.

...  
>Stewart Schultz has recently 
>argued that molecular drive and the origin of new loci could
>be modelled as allele replacements (if he still holds this position,
>he should speak up soon).

Why?  Is your cousin Carmine yanking my net access on Tuesday?  :-)
 
Seriously, regarding molecular drive: if you are referring to the 
process of concerted evolution of a gene family via a biased conversion 
rate, I suggest you read J.B. Walsh, 1985, Interaction of selection and 
biased gene conversion in a multigene family, PNAS 82: 153-157.  This is 
a theoretical treatment of the rate of fixation of the preferred allele 
at each of several loci, in the face of random drift, selection, and 
biased mutation.  Although the mechanism of mutation creates a bias in 
one direction, at each locus the spread of the favored allele to 
fixation in the population occurs just as in any nuclear locus, gene 
family or not.  Whether this conforms to your personal concept of 
"allele-replacement" I suppose is for you to decide.

As for the second question: of course a new locus is not physically the 
same as a new allele at a pre-existing locus.  What's the point?  My 
claim is that the process of fixation of a new locus in a population is 
modelled by the exact same equation that describes the fixation of a 
dominant allele.  This is fairly simple to see: a new locus acts 
physiologically like a dominant allele, and is passed to offspring as a 
dominant allele would be.  The process may be physically distinct, but 
the evolutionary dynamic in response to selection, drift, migration, or 
mutation is the same [disclaimer: a new locus may be more likely to 
encounter physical problems in expression or inheritance than a dominant 
allele at a pre-existing locus, but this is a minor point.]

...
>1. Isolation.  Must a definition of evolution encompass speciation?
>Must speciation involve isolation?  Is isolation of one population
>into two something distinct from genetic change in one population?

Isolation is necessary for speciation.  But isolation is not sufficient 
for speciation, because evolution is necessary for speciation, and 
isolation is not always evolution.  Now, when does the train arrive in 
Philadelphia? :-)

--
Stewart Schultz
Botany Department
University of British Columbia
schultz at unixg.ubc.ca



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