"kstone seems to be asserting that if existing chromosomes mutate by fusing, breaking apart, and by other such means changing their number, then they CAN'T possibly assort and pair. This is false (as a universal assertion) but a different issue from polyploidy.
False. What I am asserting is that you have to have identical (or at least near-enough) fusion in both a male and female of the original species and that they have to form a mating pair in order for the novelty to be propagated and create a new species. You don't seem to be thinking this through. There are all sorts of ways to get a genetic variant. It is the small matter of PROPAGATING that novelty that is problematic.
And yet, in the example I cited, the common house mouse has managed to do it literally dozens of times. It has spawned more than 40 chromosomal races. (I believe it's close to 60 world wide, and there are likely more that have not been discovered.) In the paper I linked the chromosome numbers vary from 25 to 40.
How did Mus musculus manage this? Maybe you need to find a mouse and explain to it that this is impossible? Or maybe you are proposing that God not only creates separate species but separate races as well? This was a common view in the not too distant past, and applied to humans as well.