>If a member of a species was born with an extra chromosome, or two chromosomes fused, their offspring have a high change of being sterile. How could the increase of decrease of a chromosome become wide spread in a species if that happens?

I think I understood the question and answered it. 1. The sterility of an offspring with an additional / fused chromosome isn't that high as shown by examples, it can be neutral; and 2. neutral mutations can become fixed.

The argument is quite similar to mutations in general. There is the general opinion that mutations are bad and overwhelmingly deleterious. They aren't. Most are neutral; the result is most people have mutations, often quite a few. Those mutations can become fixed simply because there are so many of them and they are not selected out. There are certainly bad mutations, which cause developmental or functional problems. They are sometimes really bad and really obvious, and people remember those. Sometimes they are good and increase selection.

Similarly, sometimes chromosomes fuse or split, and it doesn't make a difference. Sure, sometimes, in fact more often than not, they are bad and get selected out. But sometimes they are neutral, and sometimes the different number gets fixed. This is not unexpected.