Definition of evolution
lamoran at gpu.utcs.utoronto.ca
Fri Nov 1 12:16:12 EST 1991
This thread is becoming very complex because several different issues are
being debated at the same time. The main theme is the definition of evolution.
I have defended a common textbook definition which is;
"Biological evolution is a change in the frequency of alleles
in a population."
Arlin Stoltzfus objects to definitions like this on several grounds and I
hope to address all of his points in subsequent postings. However, I would
first like to clear up an important misunderstanding. Arlin says,
"If you start to think about evolution in an open-minded way, you
should be able to think of more phenomena that cannot be explained
by allele replacements. This is especially true if you are not one
of those molecular biology types who can only think of DNA when
asked to think about evolution."
He also said,
"Larry and many others define evolution in terms of successive
allele replacements, and Larry goes a bit further in suggesting
that a single shift in the frequencies of alleles at a locus
constitutes evolution. I do not disagree that a succession of
allele replacements constitutes evolution, but rather that
evolution comprises more than allele replacements."
I don't believe that I have EVER said that evolution can be defined solely
in terms of allele REPLACEMENTS and I don't know of any evolutionary
biologist who makes such a claim. I always understood the common definition
to include gene duplications, recombinations, deletions, etc. Arlin seems
to recognize that many of us take this broad view of "a change in the
frequency of alleles" but he persists in putting the words "allele
replacements" in my mouth. He says,
"With regard to the phenomena of non-allelic genomic change,
Larry's response was that these could be considered to be the
results of allele replacements. Since Larry and I seem to make
contradictory statements about the same thing, I suspect that
we will have to go into a few detailed examples in order to
determine whether or not molecular drive (for instance) can be
modelled by allelic changes. However, it seems clear that Larry's
view of allele replacements is a bit too broad at present. In
response to my contention that the symbiotic association of an
urkaryote and a bacterial proto-mitochondrion was a significant
evolutionary event but not in any rigorous sense an allele
replacement, Larry again suggested that this did not 'clearly fall
outside of the [allele-replacements] definition.'"
In an attempt to improve the definition of evolution to cover all examples
of changes in the genome, Arlin makes the following suggestion;
"Larry and other readers interested in salvaging genetic definitions
of evolution would do well to make the following corrections:
First, avoid limiting the circumscribed phenomena to allele
replacements. It's much better to talk about "statistical
fluctuations in the genetic composition of populations" (Sewall
Wright's definition), since this will include allele replacements,
molecular drive, etc., which are legitimate evolutionary phenomena."
I don't think that the textbook definition of evolution that I quoted ever
intended to limit evolution to allele REPLACEMENTS but nevertheless Arlin's
point is well taken. Since there is some confusion on this issue it would
be preferable to choose a definition that was more general. I found the
following examples in introductory biology textbooks;
"Evolution [is] a change in gene frequencies."
Wessels and Hopson, BIOLOGY, 1988; Random House
"Evolution: genetic change in a population of organisms over time."
Raven and Johnson, BIOLOGY, 1989; Times Mirror/Mosby
"Evolution [is a] change in the genetic makeup of a population."
Keeton and Gould, BIOLOGICAL SCIENCE, 1986; W.W.Norton
Taking these ideas into account I propose the following definition;
Biological evolution is the process of change in the genetic
makeup of a population.
This definition is necessary and sufficient (IMHO).
Laurence A. Moran (Larry)
Dept. of Biochemistry
University of Toronto
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