1. the phrase darwin used to describe his broad theory of evolution is ''descent with .'' 2. all of life is related through common ancestry, accounting for the of life. 3. the of life arises from the adaptation of species to different habitats over long spans of time. 4. darwin proposed a mechanism for how evolution occurs, which he called . 5. two key observations underlying natural selection are that members of a population vary in their and that all species can produce more offspring than the environment can support. 6. darwin inferred that those organisms with traits best suited to the environment tend to leave more than other members of a population. 7. this unequal reproduction in a population leads to the gradual accumulation of to the environment.