While many mutations in a genome may decrease an organism’s ability to survive and reproduce, also known as fitness, these mutations are selected against and not passed on to future generations. The most commonly observed mutations detectable as variation in the genetic makeup of organisms and populations appear to have no visible effect on the fitness of individuals and are therefore neutral. The identification and study of neutral mutations has led to the development of the neutral theory of molecular evolution. The neutral theory of molecular evolution is an important and often controversial theory proposing that most molecular variation within and among species is essentially neutral and not acted on by selection. Neutral mutations are also the basis for using molecular clocks to identify such evolutionary events as speciation and adaptive or evolutionary radiations.