Commonly used evolution assessments often ask about the evolution of blindness in cavefish or salamanders, running speed in cheetahs, and/or the long necks of giraffes. Explaining the loss of function in cave animals, however, is more difficult than explaining evolution involving gains of function resulting from natural selection. In fact, the evolution of cavefish blindness is not yet well understood by scientists. This article presents the three current hypotheses for explaining the evolution of blindness in Mexican tetras (Astyanax mexicanus), related to the Next Generation Science Standards and the Advanced Placement curriculum.