The explanation that accounts for the characteristics of Darwin's finches, particularly their similar features but differing beak sizes and shapes, is that all finch species evolved from a common ancestor.
This common ancestry explains the similarities among the species, while the differences in beak size and shape reflect adaptations to different environments and dietary needs on the Galapagos Islands. Over time, natural selection would have favored variations in beak morphology that allowed different species to exploit various food sources effectively.