Definition of evolution

L.A. Moran lamoran at gpu.utcs.utoronto.ca
Thu Oct 24 15:32:12 EST 1991

Gary Stormo writes:

     "Some time ago there was a discussion about the definition of
      evolution. While a consensus did not emerge there was considerable
      support for any change in allelic frequencies."

It is obvioulsy useful to agree on a definition of evolution if we are going
to argue about it! There is usually discussion on this topic under way in 
talk.origins for those of you who want to pursue it. The most popular
definition among evolutionary biologists seems to be something along the 
lines of "evolution is a change in the frequency of alleles in a population".
Here is what Futuyma has to say,

     "Biological evolution ... is change in the properites of populations
      of organisms that transcend the lifetime of a single individual....
      The changes in populations that are considered evolutionary are 
      those that are heritable via the genetic material from one generation 
      to the next. Biological evolution may be slight or substantial;
      it embraces everything from slight changes in the proportion of 
      different alleles within a population (such as those determining
      blood types) to the successive alterations that led from the
      earliest protoorganism to snails, bees, giraffes, and dandelions."

                                              Futuyma, D. J. in
                                              EVOLUTIONARY BIOLOGY
                                              Sinauer Associates 1986

Just like Futuyma I happen to think that slight changes in the proportion
of different alleles in a populations is an example of evolution. Please note
that if we DEFINE evolution as "a change in the frequency of alleles in a 
population" we are creating a MINIMAL definition. We are not trying to 
describe every single process and example of evolution in the definition.

The value of a definition is that it prevents people from arguing at cross-
purposes but obviously everyone must agree to the definition. I like this
simple definition because it defines the minimum amount of change that is
necessary to be called evolution but it does not exclude macroevolution.
I do not know of any examples of evolution that clearly fall outside of the
definition. This includes the symbiotic association of bacteria with
primitive eukaryotic cells.

One the other hand, I have yet to see a simple definition of evolution that
is any better than (or even as good as) the one which seems to be accepted
by evolutionary biologists. Remember that definitions of evolution have to
exclude cases such as the average increase in height among Europeans in the
past several hundred years. It would also be desirable to avoid definitions
that describe MECHANISMS such as natural selection, genetic drift, or
molecular drive since our understanding of the process of evolution is 

In an earlier posting Tom Schneider said,

     "...these so called molecular clocks are only an approximation. 
      Besides, what they measure is the changes which DON'T matter!
      The changes in cytochrome C are equivalent to the rearrangements 
      of water molecules in a glass of water. I wouldn't call that 
      evolution, but in the perverted way people think these days, 
      it is.  (Those who want to defend this idiocy, flame away!)
      The really interesting evolution is harder to capture (eg the
      creation of a whole new gene) so people tend to ignore it!"

There are readers of these newsgroups who share the opinion of Tom Schneider
that a simple change in the frequency of alleles in a population is NOT
evolution. I believe that the onus is on them to convince the rest of the
biological community that evolution should be redefined to exclude this
case. As far as I can determine the majority of experts in the field seem
to think in the "perverted" way that Tom maligns.

There are many other readers who believe that evolutionary biologists are
being too molecular and have relied too heavily on the population biologists
in making up a minimal definition. Ernst Mayr falls into this category, as
does Arlin Stoltzfus when he says,

     "I have not seen the earlier parts of this thread, but I can tell 
      you that the definition of evolution is most definitely not "shifting
      allele frequencies."  This canard is propagated in introductory
      textbooks, but a brief look at facts familiar to us all will show 
      that the vast panorama of evolution cannot adequately be explained 
      solely by the process of replacing one allele with another."

I think that Arlin's difficulty is that he is asking too much of a simple
definition. When we say that evolution is "a change in the frequency of
alleles in a population" we mean that this is necessary and sufficient
in order to define evolution. We do not mean that there is nothing more to
evolution. The definition does not mean to exclude gene duplication or
chromosome rearrangements - both of these events can be considered changes
in the frequency of alleles although I agree that "change in the composition
of the genomes of a population" might be a bit better as a definition.

Arlin also says,

     "Ernst Mayr argued all of his life for the importance of non-
      biological ("geographic") isolating mechanisms in evolution.  
      Can anyone conceive of a mechanism of splitting one panmictic 
      gene pool into two by a process of replacing one allele with 
      another (remember to justify all intermediate steps)?"

Here again Arlin is asking too much of a simple definition (IMHO). The
definition does not include mechanisms of evolution. In other words we
may not know HOW evolution happened in order to define WHETHER it happened! 
If we examine two separated populations and determine that the frequencies
of alleles in the two populations differ then we can conclude that evolution
has occurred. It is not important to describe the mechanism in order to
reach this conclusion. 

Incidently, the value of the definition is that we may determine that the
two separated populations do NOT differ in the frequency of alleles in
which case we can conclude that evolution has NOT occurred.

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

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