* Variation is the raw material for natural selection: If all individuals in a population were identical, there would be no differences for natural selection to act upon. Some individuals might be better suited to the environment, but there would be no advantage over their identical counterparts.
* Selection acts on existing traits: Natural selection doesn't create new traits; it favors existing traits that make individuals more likely to survive and reproduce in a given environment.
* Differential reproduction: Individuals with advantageous traits are more likely to survive, reproduce, and pass those traits to their offspring. This leads to a gradual shift in the population's traits over time, as the advantageous traits become more common.
In summary, natural selection requires variation within a population so that individuals with certain traits have a survival and reproductive advantage over others.
