Over thousands or millions of years, living things can change, through the process of evolution. One kind of evolution is when a species changes over time, such as giraffes growing longer necks. Most of the time, the species because better suited to its environment, a process called adaptation. Evolution can also cause one group of living things to split into two groups. This is called speciation if it makes a new species. An example is mockingbirds on the Galapagos Islands — one species of mockingbird lives on each island, but all the species split from a shared ancestor species. Groups that are bigger than species can also split from a shared ancestor — for example, reptiles and mammals. A group of living things and their shared ancestor is called a clade. 