Brett Aubrey  wrote in
news:ci4jti$12qd$1{at}darwin.ediacara.org: 

> "r norman"  wrote in message
> news:ci0foo$2p7i$1{at}darwin.ediacara.org...
>> On Sat, 11 Sep 2004 20:34:10 +0000 (UTC), Brett Aubrey
>>  wrote:
>> >Merriam-Webster Online has the following definitions:
>> >
>>
>> There are several things at work.  You may be missing something and
>> the definitions are definitely somewhat flawed.
>>
>> Evolution is generally defined as something like "a change in the
>> genetic composition of the population from generation to generation."
>> There are several aspects to the "genetic composition" that can
>> change.  Ordinarily there is some variability in the specific DNA
>> sequence from individual to individual.  That is, each gene can exist
>> in several different forms or alleles.  The "genetic composition"
>> really refers to the relative abundance or frequency of each allele
>> for each gene in the population.
>>
>> In a small population, it is quite possible (even very likely) that
>> the abundance of a particular allele might change just be chance over
>> generations.  For example, those individuals with big noses just
>> happened to have more babies with big noses than expected.  It is
>> sort of like flipping a coin twice. You expect one head, one tail,
>> but sometimes you end up with two heads. That is the definition of
>> genetic drift: a change in the genetic composition of the population
>> (in the allele frequencies).  If the change is drastic enough, it can
>> in fact lead to the elimination or the fixation of an allele: so few
>> people with small noses happen to have small-nosed babies that the
>> small-nose allele simply disappears and the big-nose allele is then
>> "fixed" in the population.  The flaw in the definition
is that drift
>> is any random change in gene frequency whether or not it leads to
>> "preservation or extinction" of a gene.
> 
> Thanks muchly.  This is now making sense.  Is there any quanification
> (or guidelines) of what constitutes a "small population" with regards
> to genetic drift?

The amount of drift will vary inversely with population size - but if you 
want an arbitrary figure, a "small population" is on the order of 100 
individuals. This may seem very small for many species, but there are a 
number of factors, including inbreeding and the percentage of the total 
population that is actively breeding at a given point in time, that keep 
the "effective population size" much lower than the total population 
size. Wilson in "Sociobiology", which should be available at even a 
mediocre library or bookstore,  gives a good review of this question. If 
you have access to a good library or bookstore  you can probably find 
much better and more recent references, and I am sure there are good 
references available on the web with some searching.

Yours,

Bill Morse
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