How did varying chromosome numbers evolve?

Listener Mattie asked "If humans have too many or too few chromosomes it can cause them to be infertile. So how did different creatures get different numbers of them? For example, humans have 46 chromosomes, but mice have 40." Eva Higginbotham asked genome evolution expert Hugues Roest Crollius... Like this podcast? Please help us by supporting the Naked Scientists