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

by Wesley R. Elsberry
Last updated: 19980413
SciCre Population Dynamics: An Exercise in Selective and Misleading Use of Data
 

Certain proponents of "scientific creationism" (SciCre) have put forward an argument that humans could not have evolved, simply because human population size shows that humans have only been around a few thousand years.  Those putting forward the argument tie the original population size to either two (sometimes Adam and Eve, sometimes Noah and his wife) or eight (Noah's immediate family), note a current population figure, and derive a rate of increase by use of some Biblical chronology to either creation, Noah's birth, or The Flood.  It should be noted that biblically, what should be argued is either descent from two (Adam and Eve) or from six (Noah's sons and their wives).  While some admit up front that the calculation of rate of increase yields an average value and that the actual rate of increase varies, many do not. The crux of the argument comes when they use the derived rate of increase for comparison to the deep time that evolutionary timetables give.  The numbers of humans that would be present, they say, were evolution true, would be far greater than what we observe today, and thus evolution of humans must be false. Some are precise enough to restrict their conclusion to only humans, others leave how much is disproved unspecified.  Some utilize the numbers to infer intermediate population sizes.