1. Variation: Individuals within a population must exhibit variations in their traits. These variations can be in physical characteristics, behaviors, or even biochemical processes.

2. Heritability: The variations must be heritable, meaning they can be passed down from parents to offspring. This allows advantageous traits to become more common in future generations.

3. Differential Reproduction: Individuals with certain traits must have a higher chance of surviving and reproducing than others. This means that those with advantageous traits will leave more offspring.

4. Selective Pressure: There must be some environmental pressure that favors certain traits over others. This pressure could be a change in climate, the introduction of a new predator, or competition for resources.

These four requirements work together to drive natural selection. Individuals with advantageous traits are more likely to survive and reproduce, passing those traits on to their offspring. Over time, this leads to the evolution of populations, as advantageous traits become more common.