Final answer:
The disruptive selection, favored by different seed sizes available to the Galapagos finches, led to evolution in beak size among the finch population, as documented in studies by Peter and Rosemary Grant.
Step-by-step explanation:
The situation described in the question regarding the Galapagos finches is an example of c. disruptive selection. This form of selection occurs when extreme values for a trait are favored over intermediate values. The ancestors of the Galapagos finches had two different types of seeds to eat: very small seeds that required small beaks and very large seeds that required large, strong beaks.
As a result, finches with medium-sized beaks were less favored because they could not handle either type of seed as effectively as finches with specialized beak sizes. Therefore, the population of finches split into two distinct groups: one with small beaks suited for handling small seeds, and another with large beaks suited for cracking large seeds. This adaptive divergence due to the selection pressures of different seed types led to the evolution of distinct beak shapes among the finches.
Research by Peter and Rosemary Grant supports this phenomenon, as they observed significant changes in beak sizes among the medium ground finches on Daphne Major, demonstrating natural selection in action. When food sources changed due to environmental conditions like drought and variations in seed availability, finch populations with beak sizes suited to the available food sources were more likely to survive and reproduce, leading to evolution in beak shape.