Yes you DO get loss of genetic diversity with evolution Part 9

And here's Taz on that same thread:

Faith writes:
If you start with twenty alleles in a population for one gene and one of them becomes crucial for a particular environment and therefore gets selected, either rapidly or slowly depending on the selection pressure, you will lose the other nineteen alleles as the one selected comes to determine this particular trait.

But this isn't how evolution works. The other 19 alleles don't just disappear unless there are selective pressures against them.

I clearly defined the situation as having selective pressures. If the selection pressure isn't severe you'll get a reduction rather than a total elimination of the other alleles.

You're still thinking in black and white.

What happens is by some selective pressure, say environmental or predatory, begins to favor one trait out of the 20, we will begin to see a steady increase of that one trait in the population. But the other 19 still remain, perhaps in lower number than before.

For the selected trait to increase, the others have to decrease, it's not a "perhaps" it's a necessity. What do you think "lower number" means anyway? Yes, they may not disappear right away, that would happen only when the new trait has completely dominated the population, but decrease they will when there is selection pressure for that one trait.

Try to think of it like capitalism. Just because Bill Gates began to dominate the silicon valley market doesn't mean all other software companies went belly up. In fact, despite Microsoft's attempts to stamp out their competitions, we still have giant software corporations all over the place. Even in cases of monopolies in the past, no one single commercial entity of a particular market has ever dominated the entire market.

You are determined to put me in the wrong even though what I've said has taken all this into account.

In other words, despite selective pressure favoring one or two or a few traits doesn't mean the overall variation of the gene pool will necessarily decrease.

IF those favored traits come to dominate in the population then the overall genetic variability will tend in the direction of decrease as the other alleles decrease. In many cirdcumstances the other alleles may remain and the original trait distribution could even be regained. But then you don't have evolution, do you? I'm talking about what happens with evolution, with the establishment of new traits or phenotypes, with speciation etc.

Now, if we're talking about the bottleneck effect... that's a different story.

Not really, it's just an extreme and it ought to be taken as an illustration of the formula -- new traits, less genetic variability.