Subject: How do species evolve different numbers of chromosomes?

Different species have different numbers of chromosomes.  It would
seem to me that in order to get a new species with a different number
of chromosomes either (1) there would have to be two different
mutations at the same time, each producing an organism with the new
number of chromosomes, or (2) organisms having different numbers of
chromosomes could mate and produce fertile offspring. The former seems
unlikely and the latter disagrees with what I know about, for
instance, mules.  Thanks

Re: How do species evolve different numbers of chromosomes?