>This is like explaining relativity in non-mathematical terms by saying
>'It's Einsteinian!' I think people on this list have become inured to the
>tactic of throwing names around.
Sorry, Cliff. I didn't know you were not a biologist. A late nineteenth
-century mathematician, G. H. Hardy wrote a now famous letter to the editor
of Science in the early 1900's, pointing out a problem non-mathematically
trained biologists had with understanding the consequences of gene
distributions in a population. He showed that a simple binary expression
p^2 + 2pq + q^2 sufficed to describe the frequencies of two alleles in a
population over time, where p is the initial frequency of the dominant
allele, q is the frequency of the recessive allele, and 2pq is the
frequency of the heterozygote. Hardy concludes that for dominant alleles,
there is no tendency to spread over a population, and for a recessive
allele, there is no tendency to die out or to spread in a population. His
work was along the same lines as that of a German by the name of Weinberg,
and this fundamental principle of population genetics came to be known as
the Hardy-Weinberg Law. All of this was done with the evolutionary concept
in the background, but that was not explicit in Hardy's letter. Thus
natural selection was not mentioned as a contributing factor.
>
>>Unless the selective advantage of a trait is nearly perfect, a condition
>>that is unattainable by definition, it will not become fixed in the
>population.
>
>And why not? Hardy-Weinberg!
One can argue that a dominant allele in theory can become fixed in a
population (See the discussion in Wallace Arthur's book "The Origin of
Animal Body Plans" Cambridge, beginning around page 210.), if the mutation
is selectively advantageous enough. This would be an extraordinary case,
though.
>
>>Only dominant traits that have a sufficiently high positive effect can
>>become fixed by Hardy Weinberg pathways.
Recessive allele frequency remains constant more or less within the
population unless the same mutation is repeated enough times that the gene
frequency in the population becomes high enough to produce homozygous
recessives. At that point, and not before, recessive alleles can be
subjected to selection, and thus to evolutionary advantage. Some have
postulated small but selective effects for the recessive in heterozygous
condition (such effects are known for some genes), but this would be a rare
exception in any case. Another consideration not anticipated by Hardy is
genetic drift (whatever that is - it can be quantified, at least), which
could under the right circumstances, lead to an increase in recessive
allele frequency without evident selection.
>
>There must be some other pathways then, for recessive traits from blue
>eyes to sickle cell seem well established.
>
>>Of course, one can always posit that the whole population, except for the
>>one individual was annihilated, and that one individual (or small related
>>population) survived, and thus raised the frequency to 100%....
>
>I thought I might learn something by asking for a clarification. Well, I
>learned not to ask.
Sorry, Cliff. I hope this helps.
Art
http://geology.swau.edu