On the Galapagos Islands, Darwin observed tortoises with domed and saddle-shaped shells. How did Darwin's observations of these tortoises help him form the theory of evolution by natural selection? A. The tortoises provided evidence that different species have adaptations suited to their particular environments. B. The tortoises helped to confirm that environmental changes cannot affect the characteristics of a species. C. The tortoises provided evidence that the Galapagos had been colonized by tortoises multiple independent times. D. The tortoises helped to confirm that two similar species do not necessarily share a common ancestor.