My argument is very simple:
Change or variation (or "evolution") in living things is always accompanied by a reduction in genetic capacity to change further, or reduced genetic variability or reduced genetic diversity or however that should be put. On out to the ultimate extreme of complete genetic depletion or fixation at a particular characteristic beyond which change is absolutely impossible. But at any point along the line the trend is in the direction of genetic reducttion. The more "new species" you get the less GENETIC ability to change further you also get. This is a trend that may occur slowly over generations, but it may sometimes occur rapidly through events that drastically reduce a population. The point is that change is ALWAYS in the direction of genetic decrease, NEVER in the direction of genetic increase, and that means there is an ultimate end point even if it is only rarely reached. And if there is an ultimate end point to the processes of change, or variation, or evolution, this spells the end of all hope for the Theory of Evolution.
I pointed out to PB at one point that it's interesting he never brought up mutation in answer to my argument, since that's the usual recourse of evolutionists faced with the idea of inevitable genetic depletion caused by evolution itself. This was his answer:
Although it cannot be dispensed with, mutation is n ot as great a factor in evolution as natural selection and genetic drift (which was unknown to Darwin). I did not include mutation in my presentation because, in my computer simulations of evolution, mutation must be set at a very low percentage, yet it cannot be set at zero. Of course, the majority of mutations in nature are lethal or neutral and will be eliminated by natural selection.[Sept 10 4:12 AM]