Because similar species are thought to share a relatively recent common ancestor, they are assumed to have not had much time to evolve differences between them. That explains why they are similar, and it also predicts that such species do not have significant differences. Their genome differences should be minor. This is because evolution is limited by the rate at which genetic variations can appear and subsequently spread throughout the respective lineages. But we now know of significant numbers of unique genes between allied species and even between different variants within the same species. Read more