On Pharyngula, PZ Myers treats us with an incredibly accessible explanation why chromosome number can change.
The posting was in response to an email PZ received about the evolution of chromosome numbers.
How did life evolve from one (I suspect) chromosome to... 64 in horses, or whatever organism you want to pick. How is it possible for a sexually reproducing population of organisms to change chromosome numbers over time?
Firstly: there would have to be some benefit to the replication probability of the organisms which carry the chromosomes