You are studying three populations of an endangered species of toad that used to be common in the California central valley. Global warming has caused this species to retreat to higher elevations in recent decades where it stays cooler; the toad used to be found throughout the valley, but is now restricted to the tops of three mountains. Historical records indicate that there used to be hundreds of thousands of toads in the central valley, but sampling the mountaintops reveals that the toad population on each is now limited to a few hundred individuals. You survey allozyme diversity at one locus on each of the three mountaintops and find that each population is fixed for a different allele of the ribbit gene: mountain #1 is fixed for the A1 allele, mountain #2 for the A2 allele, and mountain #3 for the A3 allele. The average number of baby toads born in each population is the same, and the environment on each mountaintop is very similar, as far as your data show. Explain why each population is fixed for a different allele.
Sentence 1: Describe the variability that was initially present in each of the populations. Sentence 2: State which individuals had the highest fitness in a given population, or why fitness is not an issue if that is your conclusion. Sentence 3: Explain what evolutionary force acted on the population(s), and why and how it acted. Sentence 4: Describe how the populations evolved in response to their changing environment. Be as specific as you can (e. g., say "increased" or "decreased", rather than just "changed".)