An ancestor species is simply an older species that serves as an ancestor to a more recently living species. An ancestral species is a given species descended from, but which otherwise is not a past population that is part of the same species.
According to the theory of evolution, overtime, the traits that enable species to survive and reproduce will become more frequent in the population and the population will change, or evolve. Through natural selection, genetically diverse species could arise from a common ancestor.
