| A | B |
|---|
| evolution | the process by which a species gradually changes over time |
| fossil | the remains of an organism that lived long ago |
| natural selection | the survival of offspring that have favorable traits |
| variation | a difference in a trait among individuals in its environment |
| adaptation | a trait that helps an organism to survive in its environment |
| gene pool | all the genes in a population |
| genetic drift | a random change in a gene pool caused by chance events |
| relative dating | a way to estimate the ago of a fossil by comparing its position in arock layer |
| radioactive dating | a way to determine the age of a fossil by measuring the amount of radioactive elements in it |
| geologic time scale | the record of Earth's history based on the types of organisms that lived at different times |
