After reading the paragraph below, answer the questions that follow.
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, through evolution, has become very different from populations of pupfish in other springs.
If in one population of pupfish all of the individuals have a blood pigment that is extraordinarily effective at carrying oxygen, but this trait is not seen in any of the other populations, what likely happened?
◦ The ancestral population probably had this type of blood pigment, but it was lost through genetic drift in the other 29 populations.
◦ This population was lucky to have an individual with a random mutation for an effective blood pigment, and the frequency of this allele was increased in subsequent generations through natural selection.
◦ Because oxygen was low where these pupfish lived, a new allele for an effective blood pigment arose.
◦ The other populations did not need this pigment, so they did not evolve it.
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