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?

competition for food and survival on the mainland provided stabilizing selection.

Explanation:

Based on the information provided within the question it seems that the most plausible biological explanation is that the amount of competition for food and survival on the mainland provided stabilizing selection. This was most likely not seen in the islands from which they came and therefore provided the ideal conditions in order for the Finches to diversify rapidly into a multitude of new forms,

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.