Imagine a scenario in which part of a population of South American finches is blown by a storm onto an island 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?
Individuals from the different populations sometimes mate with each other, but all of the resulting eggs are sterile.