Answer: A. They were all descended from a common ancestor.
Explanation: Darwin theorized that the finches he found on the Galapagos Islands were all descended from a common ancestor. He observed that the finches had different characteristics such as beak shape and size, which varied depending on the food sources available on each island. This led him to propose that the finches had adapted to their respective environments through natural selection.
By studying the finches, Darwin realized that their traits were advantageous for survival in their specific habitats. For example, finches with larger, stronger beaks were able to crack open hard seeds, while those with smaller beaks were better suited for feeding on insects or nectar. Over time, these advantageous traits became more common in the population as the finches with the best-suited beak shapes were more likely to survive and reproduce.
This observation of finch adaptation provided evidence for Darwin's theory of evolution by natural selection. It suggested that over generations, a species could gradually change as individuals with advantageous traits were more successful in their environment and passed those traits on to their offspring.