Random changes in the gene pool of a small population

gene flow or genetic drift can cause changes in a population

It's called genetic drift: when changes in the alleles of a population are altered due to chance. For example:

There's a population of red beetles. A couple of the beetles are blue due to a genetic mutation. Suddenly, an elephant steps on the beetles, which, coincidentally, most of which were red. This resulted in more blue beetles than red, and after a few generations, mist other beetles are blue, and only a few are red.