"Jim McGinn"  wrote in message
> > I asked you the question about beans being selected a) by a
> > scoop and b) by a sieve. You replied that neither of these cases
> > approximated to Natural Selection.
>
> Right.  They are analogies.  (IMO, NS is something that
> does'nt become clear through analogy, it becomes more
> vague and more confusing.)
>
These are not analogies. If we plant the beans we have selected, and use the
rest for chili con carne, and if we repeat the process for many generations,
we have two evolutionary systems going.
>
> I've been expecting you to develop your "bean falling
> out of a sieve" argument.  And, supposedly, you were
> going to clarify your position on caustion.  Now it
> seems like you're backtracking and I don't know why.
>
OK. Now the argument goes that if we pick the beans out of the bucket using
a sieve, so that smaller beans tend to fall out, a small bean will have less
chance of being planted than a large bean. If bean size is a heritable
trait, over time the beans will respond by becoming bigger.

What will happen with the beans selected by the scoop? In this case, smaller
beans won't tend to fall out. There will be no tendency for beans to get
bigger. However you will get genetic drift effects - I might get 51% above
median size beans and 49% below median beans in the scoop on the first
selection. On the second selection, assuming heritability, median bean size
will have drifted up slightly. Bean size takes a random walk, until alleles
determining genetic variation are fixed.
>
> I'm sorry, but I'm lost again.  Are you equating
> phenotypic variation to genetic drift?
>
No. Genetic drift occurs independently of phenotypic variation. In fact,
since no two phenotypes will ever have prcisely the same fitness, you could
say that pure genetic drift only occurs when the phenotypes are identical.

An example would be a silent mutation that leaves the DNA codon still coding
for the same amino acid. Two separated populations could fix different
versions of this allele, and this could form the basis of a genetic test to
determine the origin of an animal.

Phenotypic variation is necessary for NS (as I use the term) to operate.
This is because the gene has got to contribute to its own chance of
survival, which it does by influencing the phenotype - making the organism
faster, or cleverer, or whatever is advantageous.
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