First that populations and species change over time.
Second, those heritable traits help organisms to survive.
Explanation:
Charles Darwin proposed these two Ideas to explain Evolution. The first one is that populations and species change over time because the conditions of the environments let them change to adapt in the best way to the environment. Also, that certain organisms pass heritable traits to their offspring because they manage to survive and reproduce. Therefore their offspring will inherit those traits to guarantee survival and some of them even develop a new change that will improve the survival of its offspring. So, these two ideas define how a species develop to adapt.
