In order for natural selection to occur, populations must have

Populations must have:
1. Reproduction (ongoing births that may result in mutated or beneficial traits)
2. Heredity (the ability to pass down mutated or beneficial traits to offspring)
3. Variation in fitness or organisms (differences that may serve beneficial)
4. Variation in individual characteristics among members of the population (diversity in traits to allow for the most beneficial to remain)

To summarize, genetic variation or environmental pressures are the most generalized answers.