Def of evol: the phenotype is not an allele

Stewart Schultz schultz at unixg.ubc.ca
Sat Nov 2 17:41:41 EST 1991


In article <1991Nov1.231131.1842 at ac.dal.ca> arlin at ac.dal.ca writes:

>>>However, the process of replacing 100 "A" beads at a locus (in
>>>a population of 100 necklaces) with a 100 "B" beads at the same locus
>>>will never by itself split 100 necklaces into two populations of 50
>>>necklaces each.  One must invoke a conceptually distinct process of
>>>isolation.
>>
>>Let's say each individual produces one of two possible mating pheromones
>>(coded at another locus), and the "A" allele says "mate without regard 
>>to pheromone" while the "B" allele says "mate only with same-pheromone
>>individuals."  You start out with 100% A and random mating, and end up 
>>with 100% B and two isolated populations.

>If B individuals only mate with B individuals, then who does the first
>B individual mate with? Unless two B individuals arise independently
>in the same generation, the B allele will not be propagated, but will
>die out each time it arises.

You misunderstand the example.  The phenotype of B is not "mate only 
with other Bs," it's "mate only with same-pheromone individuals," the 
pheromone coded at another locus.  Hence the first B has no difficulty; 
it just mates with an A that produces the same pheromone.  B individuals 
may arise in both pheromone groups by mutations, or by occasional 
"forbidden" matings.  But how and why B increases to fixation is 
irrelevant; your claim was that such fixation, regardless of cause, is 
physically incapable of splitting a population.  I can't imagine a 
clearer, simpler refutation of this assertion.

...
>My argument is not that sympatric speciation cannot occur, but that it
>cannot occur by allele replacements alone. Given this, it is odd that
>Mr. Schultz omits from the section that follows the final sentence of
>my paragraph-- whichout which the argument is incomplete-- and
>thus attacks a strawman:
>
>>>To reiterate this point one more time (since it is indeed conceptually
>>>difficult) : if you wrote a computer program that carried out allele
>>>replacements in a population and set it running, you would never come
>>>back to your computer and notice that two populations or more were in
>>>existence.
>>
>>In fact this can and has been done.  You start out with a panmictic
>>population, introduce an allele whose phenotype is some sort of 
>>assortative mating (e.g. "mate only with other carriers of this allele," 
>>or "mate only with individuals who share an allele at some other 
>>locus"), introduce a mammoth selective force, and eventually you get
>>a "population" of two non-interbreeding subpopulations.  The splitting is
>>gradual as it accompanies the spread of your assortative mating allele.  
>
>The complete paragraph reads as follows:

...
>>In order for this to happen, you would have to add a new
>>mechanism to the program's repertoire, with an instruction such as 
>>"when the allele at locus 25 changes from *A* to *F* in a single 
>>individual, separate the population into two populations, one with 
>>all *A* at locus 25, and one with the *F* organism."

??? This adds nothing whatsoever.  The point is that the splitting 
occurs automatically, as an inevitable consequence of the spread (by 
whatever mechanism) of assortative mating alleles.  You write the 
program, specify the mating requirements of the alleles, set it in 
motion, and the population automatically splits, gradually with the 
spread of the alleles.  No eleventh-hour rewrites need be added to the 
program.

The point is that simple fixation (or even increase) of a point 
mutation, of appropriate phenotype, can block gene flow within or 
between populations.  Your personal definition of the population may 
change as the dynamic progresses, but the biological process is the same 
regardless of what you call it.

>To the claim that reproductive isolation is distinct from a genetic
>allele replacement, Mr. Schultz argues that:
>
>>Isolation can be "included" as the result of an external force 
>>(random or not), or as a phenotype of an assortative mating 
>>allele.  The former is the rule in nature, and I know of no definitions
>>of evolution that exclude the operation of external forces . . . 
>
>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.

>In general, if "allele replacement"
>is such a fuzzy concept that it can be expanded to include such
>diverse processes as 1) symbiotic association (see L. Moran's posting
>of 24 October); 2) isolation resulting from phenotypic expression of a
>variant allele (above);

Again, you're confusing logical equivalence with physical causation.

>and 3) changes due solely to mutation and not
>cumulative differential reproduction (when a macromutation creates a
>new species represented by a single individual, as described in the
>previous round of postings by Schultz and myself),

Apparently you misunderstood my previous statement.  A macromutation 
such as tetraploidy is by no means a point mutation.  My claim was that 
the same mating restrictions caused by tetraploidy could occur easily as 
a result of a point mutation of appropriate phenotype.

>then I would
>suggest that "allele-replacements" can be expanded to encompass
>everything including the kitchen sink, and that it is a foregone
>conclusion that "allele-replacements" will subsume all of organic
>evolution!

Perhaps you can point us to an evolutionary biologist who sees all of 
organic evolution as "allele-replacements"?

...
>The resistance to these simple concepts befuddles me, since it is easy
>enough to correct the allele-replacements definition with a more
>complete definition that refers to isolation (which allows speciation)
>and to "changes in the genetic composition of a population" (which
>allows non-allelic changes).  It would indeed be nice if evolution
>boiled down to some rigorous singular kernel like "allele replacement"
> . . but it doesn't.

I agree (and I know of no biologist who disagrees, except for maybe a 
straw man or two :-) that there are genetic mutations other than point 
mutations.  But I know of nobody who shares your curious viewpoint that 
the definition of evolution must "include" isolation.  I think this 
misconception is due again to confounding of logical and empirical 
truth.  Let's say my definition makes it a logical truth that all 
examples of evolution are examples of changes in the genetic composition 
of a population.  This doesn't contradict the empirical fact that such 
changes may be causes or effects of reproductive isolation, even though 
"evolution" and "reproductive isolation" are logically or conceptually 
distinct.  A definition of a process, to be useful and meaningful, need 
not "refer to" important causes (or effects) of the process.


--
Stewart Schultz
Botany Department
University of British Columbia
schultz at unixg.ubc.ca



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