The Galapagos Islands contain about 15 species of finches that vary in terms of beak size and strength. The differences are believed to have occurred as a result of allopatric speciation acting on the finch populations. Which of the following best explains how the finch populations developed different beaks?

(A) As finches migrated between islands, alleles were transferred between populations through the process of gene flow.
(B) Genetic drift occurred due to a hurricane near the islands, causing a bottleneck effect.
(C) Through convergent evolution, the finches developed comparable features due to being exposed to comparable selective pressures.
(D) Geographic isolation between the different islands kept the populations from breeding with each other, and each population evolved separately.