Answer: character displacement
Step-by-step 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.