On his trip aboard the HMS Beagle, Darwin visited the Galapagos Islands. One of the groups of animals that he studied there was giant tortoises. He noticed that there was variation in the shapes of their shells. The two extremes of this variation can be seen below. One Galapagos tortoise has a dome-shaped shell and the other has a saddle back shell

What best explains why there was variation in the types of shells Darwin saw?
A.
Each tortoise selected its shell shape when it was born.
B.
The different shell shapes were completely random to each tortoise.
C.
Each tortoise had a different mutation in the gene that affects shell shape.
D.
The different shells were advantageous in different environments.