Def. of evol: isolation is necessary

arlin at ac.dal.ca arlin at ac.dal.ca
Wed Oct 30 00:56:30 EST 1991


I can't succinctly respond to the several excellent points made by
Stewart Shultz in a single moderately-sized posting, so I will devote
the following words to the subject of *isolation*, to be followed
by another posting on the subject of *non-allelic changes.*

Stewart Schultz writes: 

> Models of sympatric speciation, for example, provide a clear 
> demonstration of speciation through simple allele replacement.  Of 
> course the conditions necessary for sympatric speciation (or 
> reinforcement) are stringent, but the models have done more than 
> rhetorical argument in elucidating them.  Far from being some sort of 
> paradigm shift, the splitting of a population is easily addressable by 
> population genetic theory.  Alleles that would tend to split a 
> population include those predisposing individuals to colonize new 
> environments, or those promoting assortative mating.
> 

I am in no way suggesting something as revolutionary as a "paradigm
shift" in evolutionary genetics when I contend that the isolation
necessary for separating one population into two is conceptually and
mechanistically distinct from simple allele replacement.  On the
contrary, this point was made roughly half a century ago by Dobzhansky
and Mayr, among others.   The argument is as follows:

As Mr. Schultz so eloquently states, the "tremendous advances made as
a result of the allele's-eye-view of population genetics are
undeniable."  The strength of the allele replacement paradigm is in
the simplicity and crystalline rigor that have allowed the development
of mathematical population genetics models.  For a population of
necklaces (genomes), each with many beads (alleles) at particular
positions (loci) on a string (chromosome), an allele replacement can
be conceived of rigorously as replacing one set of beads (representing
the old allele) at a particular position in a population of necklaces,
with a different set of beads (representing a new allele) at that same
position.  Thus, the frequency of one type of bead at a locus goes
from 1 to 0, while that of a different type of bead goes from 0 to 1.
A succession of bead replacements (motivated by whatever force-- it
doesn't matter) constitutes an evolutionary change in a single
population of necklaces strung around your neck, as we would all
agree.  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.

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.

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 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* allele."

In the case of allopatric speciation, it is even more obvious that the
isolation involved (geographic isolation) is conceptually and
mechanistically distinct from the process of replacing alleles in a
single population.  Of course, if you just took your population of 100
necklaces and split them into two populations of 50 each (pretend that
the isthmus of Panama has just risen from the caribeo-pacific ocean and
split your favorite population of sea urchins into two populations),
this would not constitute speciation until the two populations of
necklaces had diverged by a process of allele replacement.   However,
although the allele replacements are part of the process, the equally
necessary process of splitting the population (with the isthmus of
Panama, for instance) is in no sense an allele replacement.

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.


epilogue: parapatric and sympatric speciation:
 
Evolutionary biologists tend to get into disputes where two camps
each stake out unreasonable extreme positions (examples: mutationism
and gradualism; pan-selectionism and pan-neutralism).   The advantage
of taking extreme positions is that they are more easily disproved,
the falsification of hypotheses being an important step in the
development of systematic knowledge.

The extreme case asserting that all speciation is allopatric and the
extreme case asserting that all speciation is sympatric have each been
disproved to the satisfaction of most biologists.  It is highly
unlikely that all speciation is allopatric, since single mutations 
such as polyploidization in plants can create new sympatric species in a
single step, and many closely related plant species differ in just
that way (as someone has recently pointed out in this newsgroup).  The
argument for the occurrence of allopatric speciation is based on
inferences from ecology and biogeography, among them the existence of
ring species, in which two distinct but closely related species
co-exist without mixing at one locale (they are good "biological
species"), but are connected by a bridge of interbreeding subspecies
that constitute a geographic ring.

Thus, both sympatric and allopatric speciation can be inferred to have
occurred,  and the question remains as to their relative importance.
Traditionally, systematists and evolutionary ecologists place greater
reliance on allopatric speciation, due to the empirical observation
that most well studied natural populations are highly subdivided
geographically, while relatively fewer species are self-fertile and
likely to undergo the sort of mutations necessary for sympatric
isolation.  On the other hand, these opinions are based mainly on
studies of animals and fewer plants: micro-organisms as a group tend
to interbreed less and self-produce more, so that the relative
contribution of sympatric speciation may be underemphasized in the
literature.
 
Arlin Stoltzfus
Department of Biochemistry
Dalhousie University

Arlin at ac.dal.ca



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