Final answer:
Natural selection affected Galápagos finches' beak sizes, with larger beaks favored during droughts and smaller beaks favored during floods, as documented by the Grants' research. These changes are clear examples of evolution responding to environmental pressures.
Step-by-step explanation:
Natural selection played a significant role in shaping the beak size and influencing the sex ratio of finches during periods of drought and flooding in the Galápagos. During the drought, the Galápagos finches with larger beaks had a survival advantage because they could eat the fewer, larger seeds that were still available, while finches with smaller beaks died off.
This led to the increase in the average beak size within the finch population, demonstrating evolution by natural selection. Conversely, during the flood caused by El Niño, small seeds were abundant, giving an advantage to small-beaked finches, which resulted in a decrease in the average beak size. These changes, documented by the Grants through extensive research on finch populations, are excellent examples of natural selection driving evolution based on environmental conditions.