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

According to the biological concept of species, a set of individuals are from the same species if they are able to reproduce and leave fertile offspring. This is key so one species can be successful and perpetuate through time. If they cannot leave descendants, then the species will be extinct. However, it’s possible that two individuals from different species mate and have descendants, but they cannot leave fertile offspring. When they are from different species, the offspring will be sterile and it is called a hybrid.

In the case of the South American finch, the island's finch population and mainland's finch population belong to different species because their offspring is sterile. To have different features, as different feather's color, or to have other dietary preference is not enough to say that they are different species. This is because within the same species, individuals can have variations. For example, we can different eye color and belong to the same species.