Genetic "def" is not a definition at all
arlin at ac.dal.ca
arlin at ac.dal.ca
Sun Nov 3 12:52:46 EST 1991
:-)Just a few days ago I heard sweeping statements to the effect that all
:-)evolutionary biologists defined evolution in terms of changing allele
:-)frequencies. Now I have it on good authority that all of the world's
:-)evolutionary biologists *couldn't possibly actually mean that,* given
:-)that the definition has been shown to be inadequate. I'm glad to hear
:-)that all the world's evolutionary biologists have admitted that
:-)non-allelic changes take place. I had no idea that my arguments would
:-)be so convincing, or that the net reached so many people so quickly!
:-)I guess we can expect that the allelic "definition" of evolution will
:-)be stricken from the 1992 textbooks, a much-needed clarification.
If things keep going as well, in a few days all the world's evolutionary
biologists will have conceded that they weren't really trying to
*define* or describe evolution with their genetic statements, but that
all along they were really trying to set up a mechanistic framework
for evolution, a *causal* theory telling us why evolution occurs. Here
are three brief arguments why the genetic "definition" is really a
1. Isolation is a distinct phenomenon from genetic change _per se_, so I
argued in previous postings that it should be included in the "definition."
However, Mr. Schultz has argued forcefully that one does not have to make
reference to isolation in sympatric speciation, given that the phenotype
leading to isolation is caused by the genetic change. He writes:
>>But the phenotype of an allele is not an allele, nor can behavioral
>>ramifications of expressing the phenotype be construed as an allele
>>replacement in any precise way.
>You're confusing cause and effect with semantics. Of course
>"reproductive isolation" is a concept distinct from "genetic allele
>replacement." That doesn't mean the latter can't cause the former.
>Obviously an allele is not its phenotype, but an allele certainly is
>responsible for its phenotype, no matter how behaviorally complex.
It is indeed true that isolation and genetic changes are distinct
concepts, and that the latter may "cause" the former, as Mr. Schultz
clearly states in the above passage. I agree completely-- in fact, I
said the same thing last week, after speaking of the isolation
necessary for speciation:
>This process of isolation may indeed result from a change in genes.
The problem is not that genes can't cause phenotypes that cause
isolation: ITS THAT WE WEREN'T SUPPOSED TO BE TALKING ABOUT CAUSATION!
However, if we start out with this genetic "definition" of
evolution, we will inevitably have to start explaining that the
genetic changes cause other phenomena that are a necessary part of
evolution. The reason is simple: the genetic "definition" is not a
definition at all, but a mechanistic hypothesis.
2. As I have stated before, a definition of X is a set of statements
that describe X (listing its salient properties), allowing us to
identify X and distinguish it from other non-X things with which we
might otherwise confuse it. The statements of the genetic
"definition" of evolution refer to changes in the genetic composition
of a population, and should allow us to identify evolution and
Some paleontologists spend their entire lives studying evolution
without ever seeing a gene. What they see are spatial distributions
(in rocks representing temporal sequences) of the fossilized remains
of organisms. From this they infer (by a type of inference based on
geologic principles, rather than any evolutionary theory) the
existence of species of organisms that changed in their properties
over time. These sequential transformations are evolution, as we
would all agree, but they are non-evolution by the genetic
"definition." Of course, if we came out in the open and admitted that
the genetic "definition" is not a definition at all, but the bare
bones of a mechanistic theory, we could make statements such as "we
believe that the sequential transformations, a kind of evolution seen
in the fossil record, is *caused* by genetic processes."
3. In general, the phenomena of evolutionary change can be described
in many ways. We can infer successive transformations and adaptive
radiations from the fossil record; we can infer that genes have arisen
and diversified to create gene families; we can describe the
relationships of organisms to their environment and describe
adaptedness in organisms.
What kind of choice are we making when we choose to give priority to
the evolutionary transformations of genetic material, as opposed to
(for instance) giving the genotype and phenotype equal representation?
In describing evolution, can we say that genotype must be given
priority because it is more "fundamental" or something like that?
What we are doing is obvious: we all know that both genotypes and
phenotypes undergo evolutionary changes, and if anyone tries to
"define" evolution in terms of genotypes alone, it is because they
think that the genotypic changes "cause" the phenotypic changes, and
that therefore the phenotypic transformations can be reduced to the
effects of the genotypic transformations.
Those of you who read my original posting know how I feel about this
attempt to reduce evolution to one or a few simple genetic mechanisms.
There is no genetic "definition" of evolution, only a halfway attempt
at mechanistic reduction of the phenomena of evolution to the effects
of genetic changes. But this is getting ahead of things: first I have
to undergo the inevitable flamings about how I'm trying to turn the
discussion on its head by saying there is no genetic definition of
evolution, though I'm sure this has been obvious to some all along.
Arlin at ac.dal.ca
:-) I only make semantic arguments. The reason, of course, is that my
opponents are so smart they only make semantic mistakes.
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