Final answer:
Selective pressures during a drought on Daphne Major, exemplified by the presence of Geospiza magnirostris and scarcity of small seeds, led to natural selection for larger beak sizes in Geospiza fortis. The Grants' research provided evidence for rapid evolutionary change in finch populations, supporting Darwin's theory of natural selection.
Step-by-step explanation:
The analysis of the Geospiza fortis beak size graph during the colonization of Daphne Major by Geospiza magnirostris suggests that selective pressures, such as competition and availability of food resources, led to natural selection favoring finches with larger beak sizes. After a severe drought which made small seeds scarce, larger-beaked G. fortis were more successful at cracking open and consuming the more abundant, larger seeds. Consequently, birds with smaller beaks, which struggled with the harder seeds, died in greater numbers. This natural selection against small beaks increased the average beak size of the surviving population. Over several generations, such selective pressures could potentially lead to evolutionary changes in the population, including an increase in beak size.
Observations and studies of this phenomenon support Darwin's theory of evolution through natural selection, which posits that traits that enhance survival and reproduction become more common in a population over time. The Grants' research on the finches of the Galápagos Islands highlighted how rapidly such evolutionary changes can occur, especially in response to environmental changes like the introduction of a competitor and scarcity of food resources.