| 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 | Individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the species |
| variation | Any difference between individuals of the same species |
| homologous structures | Body parts that are structurally similar in related species; evidence of common ancestry |
| branching tree | A diagram that shows how scientists think different groups of organisms are related |
| petrified fossil | A fossil formed when minerals replace all or part of an organism |
| mold | A type of fossil formed when a shell or other hard part of an organism dissolves, leaving an empty space in the shape of the part |
| cast | A type of fossil that forms when a mold becomes filled in with minerals that then harden |
| relative dating | A technique used to determine which of two fossils is older |
| radioactive dating | A technique used to determine the actual age of a fossil on the basis of the amount of a radioactive element it contains |
| radioactive element | An unstable element that breaks down into a different element |
| half-life | The time it takes for half of the atoms in a radioactive element to break down |
| fossil record | The millions of fossils that scientists have collected |
| extinct | Term used to indicate a species that does not have any living members |
| gradualism | The theory that evolution occurs slowly but steadily |
| punctuated equilibria | The theory that species evolve during short periods of rapid change |
