Darwin's theory of natural selection explains how species evolve over time. It suggests that individuals with traits better suited to their environment are more likely to survive and reproduce. This process leads to the gradual change of species as advantageous traits become more common in the population. Natural selection operates on the principle of variation within a species. Some individuals may have characteristics that give them an edge, such as better camouflage or faster speed. Over generations, these traits can become prevalent, shaping the species and allowing it to adapt to changing environments.