A population of the Galapagos finch Geospiza fortis undergoes directional selection for larger bill size after Geospiza fuliginosa colonizes the same island. This is an example of:

character displacement

Explanation:

Character displacement is an evolutionary change which happens in a scenario whereby two identical species live in same environment. When this occurs, natural selection takes place which then favors a divergence based on the characters that the organisms possess.

Therefore, when a population of the Galapagos finch Geospiza fortis undergoes directional selection for larger bill size after Geospiza fuliginosa colonizes the same island is an example of character displacement.