Desert pupfish live in springs of the American Southwest. Today there are about 30 species of pupfish, but they all evolved from a common Pleistocene ancestor. The southwestern United States was once much wetter than it is now, and the Pleistocene pupfish flourished over a wide geographic area. Over thousands of years, however, the Sierra Nevada Mountain range was pushed upward by geological forces, blocking rainfall from the Pacific Ocean. As the large lakes dried up, small groups of pupfish remained in springs and pools fed by groundwater seepage. Now, although many of these small springs still have pupfish, each population has evolved to become very different from pupfish in other springs.
1. The variation in gene pools between the 30 pup-fish populations occurred through an evolutionary mechanism called .
A) Bottleneck Effect
B) Random Mating
C) Interaction of alleles between populations
D) Directional Selection
E) Hardy-Weinberg Equilibrium