Definition of evolution

arlin at ac.dal.ca arlin at ac.dal.ca
Fri Oct 25 21:48:11 EST 1991


In a previous posting I suggested that two major probems with the
"allele-replacements" definition of evolution are that it does not
encompass speciation, nor does it circumscribe genomic changes such as
(for example) "molecular drive."  Just to make sure that we understand
each other, an allele replacement refers to a process involving the
following components:

a) origin of a new allele by mutation of a pre-existing allele at a
locus; b) (optional) transient or long-term maintenance of the new
allele by selection or drift; c) increase in frequency and ultimate
fixation (reaching a frequency of one) of the new allele (by selection
or drift), replacing the old one.

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.  The bases of my two initial objections to the
allele-replacements definition are fairly straightforward and can be
reiterated (refer to my previous posting for  a simple example):

First of all, with regard to *speciation*: one population is not the
same as two populations, and one population cannot split into two
populations by replacing an allele at a locus with another allele at
the same locus.  Isolating mechanisms (genetic, behavioral,
geographic) are necessary for a population to be split into two.
Geographic isolation and other isolating mechanisms (see discussions
elsewhere in this newsgroup) are not encompassed in the process of
allele replacement, and thus a definition of evolution based solely on
allele replacements would not seem to address the phenomenon of
speciation, which is of paramount importance in evolution.

Secondly, with regard to *molecular drive and other results of
non-allelic genomic change*: I hope readers will agree that the
propagation of transposable elements in genomes and the general
changes in genome size, form, and chromosome number (more things could
be listed here) all constitute evolutionary phenomena.  Very simply
put, the propagation of a transposable element (for instance) cannot
be modelled as an allele replacement.  Allele replacements involve a
single pre-defined locus while transpositions (for instance) do not.
Therefore, a definition of evolution based solely on allele
replacements does not encompass a particular large (and possibly very
important) class of evolutionary phenomena.

I can easily address the responses of Larry Moran to these objections.
 
1.  In response to the problem of speciation, Larry suggested that he
was attempting to build a "minimal" definition of evolution, and that
it did not have to encompass the entire biological universe.  I agree
with this sentiment.  However, my response to its application here is
that speciation is of sufficient importance to be addressed in any
definition of evolution.  Many paleontologists would suggest that most
major evolutionary transformations are temporally associated with
speciation, and that anagenetic changes, taking place in a stable
population, play only a minor role.

2.  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."


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.  Its 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.

Second, at least pay some lip service to the interesting discussions
about speciation, incompatibility, and rearrangements going on
elsewhere in this newsgroup (there's alot of fascinating observational
and experimental evolutionary genetics by the Dobzhansky group on this
subject) and make some mention of speciation and isolating mechanisms.

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.

Arlin Stoltzfus
Arlin at ac.dal.ca
usual disclaimer



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