"The Galapagos Tortoise had separated due to the change in environments from the mainland tortoises.
They adapted to the new environment because they mutate, those mutations can be adaptive or non adaptive and in this case they were adaptive.
Since they were adaptive tortoises with those mutations lived longer and were able to pass those mutations to the next generation and when a whole lot of these mutations are introduced they are able to adapt to that new environment."
"One of the changes was a upward curved shell for the Galapagos tortoise, and it happened because of an mutation that later became an adaptive trait as the adaptive trait allowed the Galapagos tortoise to live long enough to reproduce and pass their traits to the next generation."