I agree with you, but let’s be fair. This is some tough biology, because you can’t go back and do the experiment or even have all the data you want. Microevolution, from one species to another with the same number of chromosomes, that’s been observed. Evolution that starts with one population with one set of chromosomes and ends with another with another set of chromosomes…well, anybody who knows about horses, donkeys, and mules knows that isn’t a trivial problem. Evolution has lots of assumptions built in, with small odds adding up over a tremendously long time frame. If you haven’t committed yourself to studying the figurative past kicking those tires, it seems like an awful lot of assumptions.
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Remember the huge timescale involved. I don’t know how much you know about polyploidy, but that is a mechism by which new plant species have evolved. They gain complete extra sets of chromosomes. It happens due to an error in gamete formation. In the case of mules, a very small proportion of mules are fertile. It is possible that at some point they could devolpe a new specise if exactly the right mules mated. Its unlikly, but theres plenty of time.
Mutations are often fatal. Chromosome mutations, where by extra individual chromosomes develop often produce sterile offspring. But its not always the case. If two groups of one specise are isolated, chromosome numbers can change.
Sometimes in science you have to make assumptions. Blood cappilaries were once a hypothesis, someone assumed their existance. Evidence for evolution is overwhelming and I’m sure as our understanding of increases we will be able to make less and less assumptions.