In the case of Darwin's finches, an ancestral finch species from the mainland arrived on the Galápagos Islands and soon developed into many new species via adaptive radiation. The finches did not undergo adaptive radiation back on the mainland. What is the most plausible biological explanation?

A) Directional selection works better on island.

B) Competition from many other birds species on the mainland provided stabilizing selection that was absent on the island.

C) The environment on the mainland was completely uniform.

D) The ancestral mainland finch was reproductively isolated.

B) Competition from many other birds species on the mainland provided stabilizing selection that was absent on the island.

Explanation:

Galapagos Islands provided many vacant niches with no competitor species to the ancestral finches to facilitate their adaptive radiation. These finches diversified to become adapted to the different conditions present in different vacant niches. This process led to the development of new species.

The mainland was already inhabited with other species of birds as well. The species of the finches present on the mainland experienced stabilizing selection which in turn favored the average phenotypes that were better adapted to compete with other species of the birds. This did not allow their adaptive radiation.