Final answer:
Darwin speculated that species on an island descended from mainland species and evolved separately on the island through natural selection.
Step-by-step explanation:
In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. Darwin observed species of organisms on different islands that were similar yet had distinct differences. He speculated that these species on the islands could have descended from mainland species and underwent separate evolution on the island.
For example, Darwin observed the ground finches inhabiting the Galápagos Islands, which comprised several species with unique beak shapes. He noticed that the finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be modified versions of the original mainland species.
Further study revealed that each finch's varied beak shape helped them acquire a specific type of food. This observation supported Darwin's theory of natural selection, where the finches with beak shapes that were most adapted to the available food sources were more likely to survive and reproduce, leading to the evolution of different species on the islands.