A process in nature in which organisms possessing certain genotypic characteristics that make them better adjusted to an environment tend to survive, reproduce, increase in number or frequency, and therefore, are able to transmit and perpetuate their essential genotypic qualities to succeeding generations
Evolution pertains to the processes and events that take place over time illustrating the gradual progression of changes in the genetic composition of a biological population over successive generations. Two major mechanisms that drive evolution are natural selection and genetic drift. Natural selection is the process by which heritable traits increase an organism’s chances of survival and reproduction. These traits are favoured than less beneficial traits. Originally proposed by Charles Darwin, natural selection is the process that results in the evolution of organism. Evolution that arises from natural selection is called adaptive evolution. In contrast, genetic drift produces random changes in the frequency of traits in a population. Evolution that arises from genetic drift is called neutral evolution.
Additional Reference A more detailed discussion about the theory and of natural selection and examples of it in practice can be found in the genetics and evolution tutorial.