Andrew hendry and colleagues demonstrated that the process of speciation is gradual by studying the distribution and variation in gill raker length in
a) brook trout
b) aquatic copepods
c) salmonids
d) threespine sticklebacks
e) razorfish

Threespine sticklebacks

Explanation: Three spine sticklebacks is a species of fish that is found majorly in inland coastal waters at 30 degree north.

Andrew Hendry is an American- Canadian professor of evolution biology and ecology. He is a big name in world of biology due to his researches and observations on various species of aquatic and land animals.

One of his biggest achievement is the observation he passed on the threespine sticklebacks in a research he made with his colleagues.

Hence from the above we can conclude that the correct option is D.

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