This page describes the Dobzhansky-Muller model and I can follow the illustration of the split leading to incompatible hybrids.
But at the start of the page the author states "[Dobzhansky and Muller] both realized that hybrid inviability or sterility was unlikely to arise from a single change at one locus." I don't really understand the explanation that follows i.e. what exactly is the issue with having a single change at one locus?
I wonder if someone here could explain the issue more clearly.