“Now if we observe a 0.001% change in a population over 20 years, what is going to stop it from becoming a 1% change in 20,000 years?”
What will stop it is any event or events within the process which render it nonlinear. It’s far more likely to be nonlinear than linear.
“Why would this 2% difference not be sufficient to call them two different species?”
Again, the answer is very simple. The 2% change is more likely than not to be empty code—meaning it doesn’t have meaningful product.
For the benefit of anyone that might be reading this, let’s clarify the fact that the theory of speciation does not exist as a mathematical statement. It’s theorized to be a real world process. This tells us that throwing numbers out there—especially elementary math—will do nothing to illustrate it.
But what if the change we are talking about IS in genetic DNA, as humans and chimps are only 2% different in genetic DNA (but some 6-10% different over the entire genome)? Would you STILL not quality a 2% accumulated difference to be a different species? After that much it is most likely that the two cannot interbreed with fertile offspring - that is the most “hard” definition of species there is.
What is going to stop the accumulation of change between two separated populations? Even if it is nonlinear, it NEVER goes down to zero, are you suggesting it goes exponential?
That might explain the supposed mechanism behind the supposed mass extinctions that should take place due to rampant mutation - if mutation is really the bad thing you said it was several posts ago - the cause of extinction. But of course that is all just Creationist delusion with no basis in observation or scientific theory. Mutations don't cause populations to go extinct.
Changes accumulate, because DNA polymerase (of even the highest fidelity) ALWAYS will introduce errors when replicating a chromosome.
Even if the slope of the accumulating change changes, and it does, it will never go to zero.
So if a 0.001% change is observed in a population over 20 years, it may not take EXACTLY 20,000 years to reach a 1% difference - but what is going to STOP the accumulation of change?