How did similar, closely-related species evolve to have different numbers of chromosomes? Wouldn’t the partial, in-between stages render them sterile?

Down Syndrome people have a third copy of Chromosome 22.

That’s a birth accident that happens when something goes wrong in generating an egg or sperm and both copies of number 22 wind up in one gamete and none in the other. This kind of copying problem is known to us because the Down Syndrome effect isn’t fatal. Other copying errors also occur, for instance a child is born with an unusual number of X and Y chromosomes – 1 or 3 are commonest.

You can see that chromosome abnormalities happen. Some are simple, some involve a chromosome fracturing at some point so that there are two new ones. That’s not necessarily a good thing or a bad thing, but if it becomes dominant across a population, further evolution on either of the two new chromosomes will come to make cross-breeding with a specimen having the old number of chromosomes less and less successful.

Perfect example: horses and donkeys make mules, but mules have DNA that doesn’t work with a horse OR a donkey OR another mule, except in at least one documented case where a jack (male mule) mounted a hinny (female mule) and she gave birth to – – – a horse. Just the horse chromosomes made it into both the jack’s sperm cell and the hinny’s egg. The odds against this are way above a billion to one.

So, how many jacks have mounted how many hinnys? You tell me.

I hope this helps shed a tiny light on species drift. One thing that makes drifting into new species is dividing one successful species into two non-interacting populations, such as by earthquake, continental drift, and so on.