| A | B |
|---|
| species | group of similar organisms that can mate with each other and produce fertile offspring |
| fossil | preserved remains or traces of an organism that lived in the past |
| adaptation | a trait that helps an organism to 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 | process by which individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the same species |
| variation | any difference between individuals of the same species |
| homologous structures | similar structures that species have inherited from a common ancestor |
| branching tree | 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 decay |
| fossil record | the millions of fossils that scientists have collected |
| extinct | the disappearance of all members of a species from Earth |
| gradualism | the theory that evolution occurs slowly but steadily |
| punctuated equilibria | the theory that species evolve during short periods of rapid change |
