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Definition of evolution

john bishop john at lupine.botany.washington.edu
Sun Nov 3 13:10:55 EST 1991

> Consider the molecules in a glass of water.  Over time they rearrange
> themselves (if the water is not frozen).  This is similar to random mutation
> and drift in a population.  So your definition includes changes which have no
> phenotypic result at all.  Surely the rearrangements of molecules in a glass  
> water are not an interesting kind of "evolution".  Why not simply call the
> genetic changes 'mutations' and be precise about it?  Shouldn't the term
> 'evolution' be reserved for something more important?

You take your analogy too far.  The random rearrangement of water molecules may  
have no phenotypic effect in a glass of water, but stochastic evolutionary  
processes certainly can have important effects on individuals and populations.   
Even if they did not, why is this not evolution?  You may or may not think  
random changes are interesting, but that has no bearing. The definition which  
you attack is simple, and defines what biologists call evolution.  Which kinds  
of evolution are most interesting and which processes are the most important  
causes of evolution is debate and study which can only occur once we have  
defined evolution.  Defining evolution as "any change in the genetic  
composition of a population" allows this. 
	I would also argue that stochastic evolutionary processes can be  
extremely interesting and important. Mutations, whether they be single  
base-pair changes, non-reduction of gametes, deletions, insertions, etc., play  
a special role in evolution, since only mutation introduces new genetic  
variation, without which other processes of evolution cannot occur.  Mutations  
provide the raw material of evolution.  The other stochastic evolutionary  
process is genetic drift. This refers to random fluctuations in gene  
frequencies due to randomness in Mendelian segregation and random changes in  
offspring number. In a small population, this process can cause the fixation or  
loss of an allele regardless of its effect on phenotype.  It can bring together  
new allelic combinations which might not occur otherwise.  These effects can be  
important in later adaptive evolution.  Some of the most important theories of  
how evolutionary change occurs, like Sewall Wright's shifting balance theory  
and Ernst Mayr's founder effect speciation,  attribute important roles to  
drift.  Its actual importance in real populations still is not well known.
>   Tom Schneider
>   National Cancer Institute
>   Laboratory of Mathematical Biology
>   Frederick, Maryland  21702-1201
>   toms at ncifcrf.gov

John Bishop,  Department of Botany, University of Washington, Seattle, WA

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