The evolution of beak size in Darwin's finches is an example of microevolution, involving changes within a species rather than the formation of a new species. Option a is the correct option.
The example of microevolution provided is the evolution of beak size in Darwin's finches. Microevolution refers to small evolutionary changes within a species or population over time, leading to a change in allele frequencies. Research by Peter and Rosemary Grant showcased this with the finches on the Galápagos islands. The Grants observed that during a period of high rainfall due to an El Niño event, the food source changed in favor of small seeds, which benefitted small-billed birds. Consequently, these birds had a better rate of survival and reproduction, leading to a decrease in average beak size in the finch population over time.
This example illustrates that microevolution can occur over a relatively short period and does not result in the creation of a new species but rather in adaptative changes within a species. It contrasts with macroevolution, which involves larger evolutionary changes that can give rise to new species over much longer timeframes. Option a is the correct option.