The basis of natural selection is that individuals with some traits are more successful than others and thus have more offspring. The frequency of those traits within the population changes because of this.
For natural selection to occur, four criteria must be met: a variation in traits, differential reproduction, inheritance of traits and a struggle for resources. A variation in traits is important because if there is only one set of traits, no change can occur from generation to generation. Differential reproduction is when some individuals in a population reproduce more than others. If traits can't be passed from parents to offspring, natural selection cannot work on them, because it works through survival and reproduction. Areas with no struggle for resources do not experience natural selection, because survival is due to chance rather than specific traits.