[CharlesWayneCT]

If you define those "forces" as agents which limit viability, you just said what I said. If you suggest forces that DON'T limit viability, but somehow otherwise conveniently limit mutations that should occur and would be viable simply so we don't have a huge variability, I'm not sure what the point OR the mechanism would be.

Put another way, I see neither the point nor the operability of selection forces that don't effect viability or opportunity to succeed but still manage to prevent mutations that have occured from spreading in the population. I guess that seems like the definition of viability and opportunity -- at least that's what I was trying to cover with those terms, which I admit may not be the scientific technical terms for whatever processes you envision.

[b_sharp]

"If you define those "forces" as agents which limit viability, you just said what I said. If you suggest forces that DON'T limit viability, but somehow otherwise conveniently limit mutations that should occur and would be viable simply so we don't have a huge variability, I'm not sure what the point OR the mechanism would be.

Although it is silly to call the many forms of selection 'forces', calling them that does simply the language when discussing them.

Organisms tend to cluster around the point of optimum fitness given normal morphological and environmental constraints simply because it is more likely for a specific mutation to be neutral and not contribute immediately to fitness, or deleterious and quickly removed, than they are to be immediately beneficial. But yes, relative viability is the mechanism that limits variability. However, the contribution a mutation makes to viability is dependent on the environment and a change in environment can change which mutation (which is the source for new alleles) fixes in a population.

"Put another way, I see neither the point nor the operability of selection forces that don't effect viability or opportunity to succeed but still manage to prevent mutations that have occured from spreading in the population. I guess that seems like the definition of viability and opportunity -- at least that's what I was trying to cover with those terms, which I admit may not be the scientific technical terms for whatever processes you envision.

Random drift can control the frequency of an allele without viability having anything to do with it but deleterious mutations don't usually last long enough to be affected.

[b_sharp]

The problem is in your choice of words. 'Anything can happen' is simply too broad to be accurate. It implies that evolution can get anywhere from anywhere which is simply not true. Because successful mutations generally affect a simple modification of existing genes and need to retain viability (cost is lower than or equal to benefit) some paths are not available.