For natural selection to occur, several premises must be satisfied. Firstly, there must be variation in a population. If all individuals are the same, there is no variation on which selection can occur. Secondly, these traits must be heritable, meaning they are passed from parent to offspring. Natural selection cannot operate on traits that have no genetic basis. There must also be differential reproduction, which means that some individuals must reproduce more than others. When all of these factors occur, natural selection results in evolution as advantageous traits propagate throughout the population.
Natural selection is quite variable, as changes in the environment impact what traits are advantageous in a particular time and place. A good of example of this comes from Galapagos finches. These birds have varying beak sizes. Wide beaks are best for eating hard seeds, while long narrow beaks are more adept at exploiting the fleshy fruit from trees. Typically narrow beaks are more prevalent, but during drought conditions when trees cannot produce fruit, seeds become a more abundant food item. This results in those finches with wide beaks having more food available and reproducing more than those with narrow beaks. When droughts persist over many years, wide beaks replace narrow beaks as the most common, but this trend reverses when droughts end and fruits once again become available.