Natural Selection is the process in nature by which organisms better adapted to their environment tend to survive and reproduce more than those that are less adapted to their environment
The frogs have lots of babies. (up to 50 eggs)
Every one of the frogs have different variations from their parents , but are not identical to them
The frogs have many predators and struggle to survive
The ones that do survive reproduce and thrive.
Summary: Green tree frogs survive in a more swampy area where there is more green vegetation. Brown tree frogs survive more in dark wooded areas on tree bark. If the green tree frog were to be in an area that had lots of dark trees, the green frogs would not blend in and would not survive in the environment. Same thing for the brown tree frog. If the brown tree frog were in an area with mossy greenery, it would probably not survive and would not be able to reproduce.