system
Given the following post, respond as if you're commenting on Reddit:
human
How does a species evolve to have a different number of chromosomes? Wouldn't the first individual with that mutation be unable to mate with any other individuals, and so the mutation would instantly die off?
gpt
The first thing is that producing offspring is not the problem. Instead the problem is that offspring could be infertile. Evolutionarily, that amounts to the same thing, but the mechanism is different. Nothing prevents fertilization, it's the production of sperm and eggs in mixed offspring that might be problematic.

Second, though, it's a common misconception that parents with a different number of chromosomes will always produce infertile offspring. The mule is the example pretty much everyone has in mind when they think about this. In fact, though, chromosome count has pretty much nothing to do with it.  Mules are infertile because their chromosomes are not able to line up and separate correctly during the formation of sperm or eggs. That failure during gamete production is due to extensive point and structural mutations within the chromosomes so that they no longer "match" each other and can't pair properly.

If the only difference was that a single large chromosome split into two small ones or that two small ones fused into one large one (these are both called Robertsonian translocation, which is the most common way that chromosome number evolves), without any significant number of additional mutations within the chromosomes, then pairing during the formation of sperm and eggs would still work out fine. The two small chromosomes from one parent would just pair with the matching halves of the one large chromosome from the other parent. They would still match each other at the nucleotide level necessary for the process to function.

We see examples of this in many animals, like the domestic horse and the Przewalski's horse, which are considered different species, have different chromosomes counts, but are able to produce fertile hybrid offspring. Or black rats, which are all considered one species (Rattus rattus), but have different numbers of chromosomes in different populations or sometimes even in the same population, which doesn't seem to affect fertility at all.