Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift.
If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome.
Natural selection is defined as a process by which species of animals and plants that are best adapted to their environment survive and reproduce, while those that are less well adapted die out.
Natural selection ensures only the fittest survive to pass their genes on to the next generation.
In evolution, the process by which those individuals (of a species) with characters that help them to become adapted to their specific environment tend to leave more offspring and transmit their characters, while those less able to become adapted tend to leave fewer offspring or die out, so that in the course of generations there is a progressive tendency in the species to a greater degree of adaptation.