In article <1991Oct30.015630.1801 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.
Of course, the big question is how and why does B spread? This is the
precisely the sort of problem confronted by advocates of (non-
chromosomal) sympatric speciation. My point is not that this mode of
speciation is common, but that isolation can, physically, result from
single-locus allele replacements. Isolation is not conceptually
distinct, but can be considered as a phenotype of the allele.
Contrary to appearances, these scenarios do have biological relevance.
For example, a nereid worm occurs down the length of the Pacific coast
from Alaska to Central America. Taxonomists recognize it as a single
species, and no ecological variation has been found. Yet individuals
separated by 200 miles fail to mate with each other in vitro, due
apparently to pheromonal differences. We can easily interpret this
situation as a single locus allele replacement: different mutations at
the pheromonal locus have been fixed along the worm's distribution. The
reason this is probably not an example of (non-chromosomal) sympatric
speciation is not any difficulty with the mechanism of allele
replacement, but rather the lack of any reasonable selective force to
produce the pheromonal variation in sympatry.
>This process of isolation may indeed result from a change in genes.
>For instance, it has long been recognized that one population of
>self-fertile plants can turn into two populations if a single
>polyploid individual arises that is reproductively incompatible with
>the remaining individuals (an example of "sympatric" speciation).
>Speaking precisely, there is no way to describe this event as an
>allele replacement. The number of individuals in the parent
>population is decreased by one, while a single individual with a
>genetic difference (for example) at locus X constitutes a new
>population.
Or you can call it the replacement of an allele whose phenotype is the
inability to mate with individuals containing some other allele at locus
X, within a single non-panmictic population.
>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.
...
>Thus, both divergence (by the mechanism of allele replacements or by
>some other mechanism) and isolation (geographically, temporally,
>behaviorally, etc) are necessary for speciation to occur, under either
>model of speciation. Any definition of evolution or any statement
>that attempts to define the mechanistic basis of evolution must
>include isolation. Otherwise, it will have failed in its task, since
>speciation (the origin of species) is of paramount importance in
>evolution.
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 (doesn't mean
there aren't any :-). It's easy to address external barriers in simple
allelic models: assume two populations, and vary the migration
parameter. At one extreme, you have the equivalent of a single
panmictic population, and at the other full isolation. In fact, any
allelic model implicitly assumes some degree of isolation by bounding
the population in question, or by setting the migration parameter.
I agree that homoploid sympatric speciation via allele replacement is
undoubtedly rare, but the reason is not that such replacements are
physically incapable of splitting a population, but rather that there
are few if any selective forces strong enough to produce the genetic
correlations required if the initial population is panmictic.
--
Stewart Schultz
Botany Department, U. British Columbia
schultz at unixg.ubc.ca
