Imagine that part of a population of South American finches is blown by a storm onto an island far offshore and manages to survive and reproduce there for a period of 10,000 years. After that period, a climate change results in lower sea levels and the reconnection of the island with the mainland. Members of the formerly isolated island finch population can now interact freely with members of the original mainland population. Which of the following observations would, by itself, lead you to conclude unequivocally that the island finch population had evolved into a distinct species, according to the biological species concept?

• A. The island birds all have red feathers, but the mainland birds have green feathers.
• B. The preferred food of the island birds is quite different from the diet of mainland birds.
• C. Individuals from the different populations frequently direct courtship behavior toward members of the other population.
• D. Hybrid offspring of matings between individuals from the two populations do not look like either parent.
• E. Individuals from the different populations sometimes mate with each other, but all of the resulting eggs are sterile.