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.