Final answer:
The study conducted by Andrew Hendry and colleagues focused on the process of speciation in threespine sticklebacks by studying the variation in gill raker length.
Step-by-step explanation:
The subject of the question is Biology. The question is asking about a study conducted by Andrew Hendry and colleagues on the process of speciation, specifically the variation in gill raker length in a certain group of organisms.
The correct answer to the question is option 4) threespine sticklebacks. The study conducted by Hendry and colleagues focused on the gill raker length in threespine sticklebacks as a way to understand the gradual process of speciation.