Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution.
The struggle for existence is individuals competing for living space, food,water, and other essentials for life.
Variation and adaptation is where individual variation plays a vital role. Darwin hypothesized that some traits are better suited than others in a species.
Survival of the fittest is the most fit of a species surviving and passing on their traits so the species is better suited for the environment they inhabit.
Natural selection is the best of a species surviving because their traits help them to survive.