1. Heritable variation: Individuals within a population must have differences in their traits that can be passed down to their offspring. This variation can arise from mutations in DNA or other genetic material.

2. Differential reproductive success: Some individuals must be more successful at surviving and reproducing than others. This success can be influenced by factors like environmental pressures, competition for resources, or the ability to attract mates.

These two factors work together to drive evolution. Individuals with traits that make them better suited to their environment are more likely to survive and reproduce, passing those beneficial traits on to their offspring. Over generations, this process leads to changes in the genetic makeup of a population, ultimately resulting in the evolution of new species.