6. 2. The species of tortoise that lived on an island with tall cacti had with lots of short grass had necks; the species of tortoise that lived on an island necks. 3. Darwin thought all the Galápagos tortoises shared a common that came to one of the islands millions of years ago. 4. Darwin knew that in any species, members of the same species each have slight differences, called 5. Darwin didn't know about but he realized that in populations could help explain how the different species of Galápagos tortoises and other organisms evolved. a. Darwin knew that food is a(n) of a species that live in the same area compete for food. b. If a variation resource, so members a tortoise, allowing it to compete for food better than other tortoises, the tortoise lived longer, reproduced more, and passed on its variations to its is the process by which populations of organisms with variations that help them survive in their environments live longer, compete better, and reproduce more than populations that do not have the​