Darwin's theory of natural selection is based on the concepts of variation, inheritance, and survival of the fittest. This theory posits that individuals within a species exhibit variations in traits, some of which are inherited by their offspring. Those individuals with advantageous traits are more likely to survive and reproduce in a given environment, thereby passing on these favorable traits to future generations. Over time, this leads to the evolution of species, as adaptations accumulate and populations become better suited to their environments, illustrating the dynamic nature of biological life.