Final answer:
The process by which beak size in a population of birds may change between generations due to environmental conditions is known as microevolution.
Step-by-step explanation:
In response to environmental conditions, the average beak size in a population of birds may change between successive generations. This process of change is referred to as microevolution. Microevolution is the process by which organisms change in small ways over time, such as the beak size of Darwin's finches changing as the availability of different-sized seeds changes.
These small, incremental changes do not create new species, but rather, they lead to changes within a species. In contrast, macroevolution refers to much larger evolutionary changes that can lead to the creation of new species over much longer periods of time or due to major environmental changes.
Understanding the concept of microevolution helps explain how a population adapts to its environment through natural selection, such as the various beak shapes found in Darwin's finches, with each suited to a specific food source based on the ecological niche the bird occupies.