| A | B |
|---|
| species | a group of similar organisms that can mate and produce fertile offspring |
| adaptation | a trait that helps an organism survive |
| 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 are better able to adapted to their environment |
| variation | any differences between individuals of the same species |
| fossil | preserved remains or traces of organisms that lived in the past |
| sedimentary rock | harden layers of sediment |
| petrified fossil | the gradual replacement of minerals into remains of organisms |
| mold | a hollow space of an organism or part of an organism |
| cast | when the mold becomes filled in with harden minerals |
| radioactive element | an unstable particle that breaks down into different elements |
| 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 sciencts have collected |
| extinct | a species that does not have any living members |
| homologous structures | similar structures |
| branching tree | a diagram that shows how scientists think that organisms are related |
