Natural Selection is the process whereby organisms better adapted to their environment tend to survive for longer. For example, if you were to put an Elephant in the middle of Antarctica, it most likely would not survive because it is adapted to heat not cold.
Overpopulation can occur when a species' population exceeds the carrying capacity of the environment. Overpopulation can lead to several outcomes such as starvation and an increase in predators
Variation is when one species has more than one form or color. A variation of the color could have been an adaptation formed because of predication etc.
Selection occurs when environmental or genetic influences determine which species thrive best in certain environments.
Adaptation is when a organism better adapts to its environment over time. This can be physical and mental changes.