Final answer:
The finches showing an increase in both large and small beaks while average-sized beaks decrease is an example of disruptive selection, where extreme phenotypes are favored and intermediate phenotypes are selected against due to environmental pressures.
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. Disruptive selection, also known as diversifying selection, occurs when extreme phenotypes are favored over intermediate phenotypes, leading to the increase in the population of organisms with those extreme traits. This pattern can result in a bimodal distribution of traits, where two extreme forms within a population are more common than intermediate forms.
An example of this can be seen in the Galápagos finches studied by Peter and Rosemary Grant. Different beak sizes were favored depending on food availability caused by environmental changes like drought or periods of high rainfall. During a drought, larger-beaked finches could access larger seeds, while smaller-beaked finches could access smaller seeds more efficiently in periods of abundant small seeds, demonstrating the natural selection process leading to disruptive selection.