Final answer:
The correct choice explaining the evolution of beak size on Daphne Major is D, which states that beak size must be heritable and affected by alleles and that there must be environmental changes affecting food availability. These conditions enable natural selection to influence beak size in finches.
Step-by-step explanation:
The evolution of beak size on Daphne Major involves various principles of natural selection. The correct answers are A and B, making option D the correct choice. Beak sizes of the finches on Daphne Major must be at least partially determined by the additive effect of alleles for there to be variation in the trait that selection can act upon. Moreover, the characteristics of the average seed must change from year to year, creating a changing environment that selects for different beak sizes. Due to a drought on the island, the number of smaller seeds diminished, leading to a situation where birds with larger, stronger beaks had a survival advantage, as they could also crack larger seeds. This resulted in an increase in the average beak size in subsequent generations, as birds with larger beaks were more likely to survive and reproduce.
It is not necessary for individuals with smaller beaks to grow larger beaks during drought years; instead, the population's average beak size changes due to the differential survival and reproduction of individuals with genetic traits (larger beaks) that are advantageous under the current environmental conditions. This effect, clearly demonstrated by the work of the Grants, showcases natural selection in action influencing the evolution of beak size on Daphne Major.