I need someone who has been keeping an eye on the biological literature. It is by now an established fact that the human chromosome 2 originated as a fusion of two chimpanzee chromosomes (and I'm not about to dispute that, this post is in no way an advocacy of intelligent design There are other species in nature who's number of chromosomes differ by two, suggesting that fusion (or splitting) of chromosomes is quite common event. However, it is a quite drastic change of the genome and it is well-known that species with different chromosome number have great difficulty in reproducing, and always produces infertile offspring. In light of this, the chromosome fusion is a bit bizare; It cannot happen gradually (two chromosomes are either fused or they are not) and unless the exact same fusion happens to occur in a whole number of individuals of the population at the same time, it could not spread because of the infertility of the offspring. At least in the laymans world. Which brings me to my question: Has a mechanism for 1) how/why such chromosome fusions/splittings occur and 2) how such a fusion/splitting spreads in a population, been proposed?