Answer:
Evolution theory
Step-by-step explanation:
The evolution theory was developed by Darwin and it is a generally accepted theory that deals with gradual changes in the characteristics of organisms due to their quests to adapt to continuous changes in the environment.
As the environment of organisms keeps changing, the population of the organism keeps adjusting. Individuals that are able to adjust to the changes survive and are able to reproduce while the weak ones gradually die off from the population.
Hence, the entire characteristics of the population is changed with time as a result. The continuous changes sometimes lead to some species within the population giving rise to a new species entirely.