CL wrote:
> >This seems a challenge to a rather fundamental point. Can someone explain
> >in non-mathematical terms why an advantageous mutation could not spread
> >within a population? Given of course, that it is not hopelessly
> linked with
> >disadvantageous genes.
AC replied:
> Put simply, "Hardy Weinberg makes evolution impossible."
> 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. Only dominant traits that have a sufficiently high positive
> effect can become fixed by Hardy Weinberg pathways. 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 can agree with Art's first sentence (above). The rest is interesting
since in all the biology, genetics, and evolution books I've seen, and in
the classes I've taken (and taught), selection has never been part of the
Hardy-Weinberg equilibrium model. Quite the contrary.
The Hardy-Weinberg equilibrium model is an algebraic extrapolation of
Mendelian inheritance as it was understood in 1908. It applies to
theoretical populations, and ignores certain aspects of real populations. It
is frequently used in the classroom to introduce population dynamics that
lead to evolution by contrasting its claims with the restrictions on real
populations that allow the model to apply.
It states that a population with two alleles for a locus will rapidly reach
a state of equilibrium, and that once this equilibrium is reached it will be
maintained. Various biologists have pointed out that there are many
restrictions for the Hardy-Weinberg equilibrium to apply. The most commonly
stated are:
1) The population must be large (infinitely large in theory, large enough to
avoid sampling effects in practice)
2) No selection occurs. (Contra Art, above)
3) No mutations occur.
4) No immigration occurs.
5) Breeding is completely random.
The usual practice in classes is to go through these limitations and show
that they do not apply to real populations, and that the violations explain
how evolution within populations occurs.
It is interesting that Art recently brought up peppered moths again, since
they are a widely used example of a change that demonstrates the
restrictions of the "Hardy-Weinberg Law".
Don Frack