Final answer:
If finches with large beaks and finches with small beaks increase in a population, while those with average-sized beaks decrease, the type of natural selection is disruptive selection, which favors extreme variations of a trait.
Step-by-step explanation:
If finches with large beaks and finches with small beaks increase in a population while finches with average-sized beaks decrease, the pattern of natural selection that has occurred is disruptive selection. This type of selection occurs when the environment favors both extremes of a trait over the intermediate forms, leading to an increase in the diversity of the trait within the population.
Disruptive selection is evident in this situation as the finches with larger and smaller beaks are favored, likely due to different environmental pressures or available resources that suit these extreme variants better than the average beak size. The finches with large beaks might be better equipped to crack large, hard seeds, while those with small beaks might specialize in feeding on small, soft seeds.
The famous studies on Galápagos finches by Peter and Rosemary Grant provide rich examples of natural selection, including directional selection, where a single extreme phenotype is favored, and stabilizing selection, where the average phenotype is favored, ensuring less variation within a population. Conversely, the scenario described with finches with large and small beaks increasing in frequency aligns with patterns of disruptive selection, where intermediate phenotypes are selected against.