Definition of evolution

L.A. Moran lamoran at gpu.utcs.utoronto.ca
Fri Nov 1 15:43:20 EST 1991


     I am looking for a working definition of evolution that will guide us in
discussions and debates. The standard textbook definition is often given as
"change in the frequencies of alleles in a population" but this turns out to
be somewhat misleading because it can easily be misinterpreted to mean that
evolution is the same as allele replacements. I suggest the following;

      Biological evolution is the process of change in the genetic
      makeup of a population.

I submit that this definition is necessary and sufficient to define evolution.
It is necessary because there are no known exceptions to the definition. That
is there are no cases that we would all agree to label as evolution where we
know for a fact that there has been NO change in the genetic makeup of a
population. It is sufficient because any change in the genetic makeup of a
population is recognized as an example of evolution. What more can one ask?

Arlin Stolzfus takes exception to my attempt to promote a minimal definition
such as that seen in the textbooks. He says,

     "Additionally, I would suggest to Larry that he mispoke in 
      suggesting that allelic changes are "necessary and sufficient" 
      to define evolution. N and S are the keywords used by physical
      scientists to wrestle with *causation*, and elsewhere you 
      expressed a more reasonable desire to talk about definitions, 
      rather than causal mechanisms. The statement "ABC is necessary 
      and sufficient to define evolution" refers to *causing a definition*
      and is a nonsense statement. A definition of X is something that
      identifies X for us by listing its salient properties and
      distinguishing X from similar non-X things with which we might
      otherwise confuse it."

I don't follow this argument. The definition that I defend simply lists a
requirement of evolution, namely that change must be heritable. It says 
nothing about what caused the change (ie. mutation, recombination, natural
selection, genetic drift, molecular drive etc). Arlin is familiar with my
discussions in talk.origins on the difference between the FACT of evolution
and the MECHANISM (or cause) of evolution so he knows that I would not
deliberately choose a definition that included a mechanism.

Perhaps Arlin could explain what he means here, and at the same time explain
why he thinks that the textbook definition of evolution describes a CAUSE of
evolution as he wrote last week;

     "...it is time to state that the basic problem with the "allele
      replacements" definition of evolution is that it represents a
      failed attempt at reducing a set of phenomena to a single mechanism.
      I have no quarrel with reductionism: if we can reduce the vast
      panorama of evolution to a set of precisely defined mechanistic
      processes, I will applaud the effort (it should be noted that 
      evolution will never be reduced to a *single* mechanism, for the
      same reason that there will never be a unified field theory in
      physics). At present, however, we cannot. Therefore it is best to
      define evolution as a phenomenon, rather than a mechanism. This
      definition would be verbose, and would refer to anagenetic changes,
      speciation, adaptation, interactions of organisms with each other 
      and with the environment, evolutionary trends and other important
      phenomena in the history of life."

It seems to me that Arlin's argument is way off base. Many of the things
that he describes as phenomena are actually mechansims (ie. adaptation) and 
that which he criticizes as a mechanism is actually a phenomenon or
observation. Is this just silly semantics? Can anyone help me here?

Laurence A. Moran (Larry)
Dept. of Biochemistry
University of Toronto



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