A preserve contains the last population of the very rare desert pupfish which is only found in one pool. There are two color forms of this pupfish, one is silver and one is black and the color is...

A preserve contains the last population of the very rare desert pupfish which is only found in one pool.  There are two color forms of this pupfish, one is silver and one is black and the color is inherited.  The color forms are  equally successful in surviving and having offspring over the long term but the frequency of the two forms varies greatly from generation to generation.  Which evolutionary force is causing the fluctuation in frequency?