Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a populationover generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which is intentional, whereas natural selection is not.Variation exists within all populations of organisms. This occurs partly because random mutations arise in the genome of an individual organism, and offspring can inherit such mutations. Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits. The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations, species, as well as the abiotic environment. Because individuals with certain variants of the trait tend to survive and reproduce more than individuals with other, less successful, variants, the population evolves. Other factors affecting reproductive success include sexual selection(now often included in natural selection) and fecundity selection.

Charles Darwin wrote his famous book ‘Origin of Species’. He threw new insights on evolution of species. Some salient points of Darwin’s theory are as follows:Organisms have unlimited capacity to reproduce:Organisms can reproduce offspring at a fast pace. This is necessary for survival, because a higher number of offspring ensures that at least some of them could survive. Each organism has to struggle for its day to day survival. For example; a frog lays thousands of eggs at one go. The spawn is released in water and it is left to fend for itself. Most of the eggs are either washed away or are eaten by predators. However, some portion of eggs from the spawn develops into tadpoles. Once again, many tadpoles are eaten up by predators; leaving a few which develop into adults. It is evident, that a large number of eggs is needed to ensure that at least some of them develop into adults.Natural Selection:Different individuals of a particular species have different traits. Those with more suitable traits are selected by the nature. Each organism needs a particular trait for finding food and finding a mate. Those with better traits are finally able to pass on their traits to the next generation.Survival of the Fittest:Those organisms which are the fittest are able to survive, while others perish. That is how many species become extinct and some species continue to evolve over a period of time.