| A | B |
|---|
| species | a group of similar organisms that can mate with each other and produce fertile offspring |
| fossil | the preserved remains or traces of an organism that lived in the past |
| adaptation | a trait that helps an organism survive and reproduce |
| evolution | the gradual change in a species over time |
| scientific theory | a well-tested concept that explains a wide range of observations |
| natural selection | the process by which individuals that are better adapted to their environment are more likely to survive and reproduce |
| variation | any difference between individuals of the same species |
| homologous structures | similar structures that related species have inherited from a common ancestor |
| cladogram | a diagram that shows how scientists think different groups of organisms are related |
| petrified fossils | minerals replace remains, changing them into rock |
| mold | a hollow space in sediment in the shape of an organism or part of an organism |
| cast | a copy of the shape of the organism that made a mold |
| relative dating | a method used to determine which of two fossils is older |
| radioactive dating | a method used to determine the actual age of a fossil |
| radioactive elements | unstable elements that decay, or break down, into different elements |
| half-life | the time it takes for half of the atoms in a sample to decay |
| fossil record | the millions of fossils scientists have collected |
| extinct | no members of that species are still alive |
| gradualism | theory that proposes that evolution occurs slowly, but steadily |
| punctuated equilibrium | theory that proposes that species evolve quickly during relatively short periods. |
