On Fri, 15 Jul 1994, Joe Morlan wrote:

> As I understand it, there are two main models which help to
> explain biodiversity; Dispersal and Vicariance.
>
> Dispersal is what happens when a small population moves away from
> its parental group and founds a new isolated population. This
> happens a lot in archipelagos and on mountain tops. This model
> of adaptive radiation helps explain the high rate of endemism in
> island ecosystems. It is often called the "Founder Principle."

Well, yes and no. Joe is right in saying that there are two explanations
for disjunct populations. A dispersal explanation assumes that a
population originally confined to one area dispersed across preexisting
barriers (to already separated islands, for instance), whereas a
vicariance explanation assumes that the original population was split
by the formation of barriers (such as glaciers or mountain ranges, or
separation by continental drift). However, the Founder Principle is a
special case of genetic drift, or what has been called the Sewall Wright
effect, which Joe refers to later:

> Either way, small isolated populations tend to evolve more
> quickly than large continental populations because favorable
> traits multiply as a percentage of the total gene pool much
> faster in small populations than they do in large ones.

Let me try to put this in perspective. We can talk about a kind of
genetic ideal, a large natural population with random mating, which tends
to achieve a certain equilibrium frequency of the various alleles at each
gene locus, a so-called Hardy-Weinberg equilibrium. This assumes there is
no selective pressure or mutational pressure on the gene, which of course
is very unrealistic, but leave that issue aside for a moment. The point
is that in a large population the genetic coin is being flipped so often,
with so many matings going on, that allele frequencies remain constant,
just by chance. Genetic drift, however, can occur--just by chance--in a
_small_ population because the genetic coin is only being flipped a few
times, so one can get, in effect, a run of "heads" or "tails," and thus a
rapid change and an unusual distribution of alleles. It isn't just
favorable alleles that get accentuated in this way. Traits that have
negative selective value obviously can't be preserved, but genetic drift
presumably happens largely with traits that are selectively more or less
neutral. Genetic drift probably occurs in any small, isolated population
and accounts for a lot of the differences in plumage between closely
related species. For instance, it probably accounts for the more eastern
populations of Northern Oriole having the Baltimore traits and the more
western populations having the Bullocks traits; there probably is no
particular selective advantage to either plumage, and alleles producing
one type or the other were likely at equilibrium in the parent population,
before it got split (by glaciers, by grasslands, by ??).

The founder effect is a special case of this situation, where the small
population that becomes isolated is not a random sample of the main
population. This is most likely to happen during dispersal, as when a
small population moving off to a nearby island carries an unusual set of
genes to start with. But it could be a factor in speciation by any
means. Although the Northern Flicker presumably got split by vicariance
factors, maybe the more eastern population accidentally started with more
alleles for yellow pigment and the western population with more for red
pigment.

Burt Guttman guttmanb at elwha.evergreen.edu
The Evergreen State College Voice: 206-866-6000, x. 6755
Olympia, WA 98505 FAX: 206-866-6794