The finches had a common ancestor that presided in a set place. As time progressed, and factors such as isolation and migration occurred, these finches were able to move to different locations on the Galapagos Island that required them to possess adaptive characteristics that woud have allowed them to survive and reproduce. So adaptations such as different beaks, colours and sizes in response to new food sources, water sources, predators, climatic conditions etc etc. Over time, these characteristics were passed down over generations and they became more common within their specific population. Hence, these finches became completely new species.