Severe flooding in an area caused a population of minnows to become split into two separate ponds. Prior to the separation, the male minnows all had a moderate number of spots. Following the separation, the first pond was filled with larger fish that fed on minnows, so male minnows with fewer spots were better able to avoid detection by predators. The minnows in the second pond, however, had very few predators, and female minnows in this pond preferred to mate with males that had the most spots. After many generations, the two minnow populations evolved into different species and could no longer interbreed.