Faith,
The reduction process cancels out any increase in variability conferred by the addition of a mutation. You get a mutation, it changes something, it's selected and that reduces the variability by eliminating other genetic possibilities in the new subspecies. Not a formula for evolution.
What stops a new allele in one locus increasing the possible variability at another? In other words, one novelty allows the potential for more elsewhere. Meaning that variability, as you define it, never really gets near to zero.
Mark
There are 10 kinds of people in this world; those that understand binary, & those that don't
™ Version 4.2