Evolutionary Mechanisms and Natural Selection (1C) Practice Test

A rare allele r in a small isolated snail population has no measurable effect on survival or reproduction. Over 20 generations, allele r fluctuates in frequency and is eventually lost, despite no consistent environmental change. The population size remains low throughout due to limited habitat area. Based on the scenario, which outcome is most consistent with genetic drift?

In a population of freshwater snails, shell thickness varies continuously. A new fish predator is introduced that can crush only thin shells. After several generations, the mean shell thickness increases and variance decreases. Which statement best reflects the process of natural selection described?