In essence, natural selection is the differential reproduction of some genetic variants with respect to others. We can more rigorously define it as the process that occurs in a population of biological entities when the following three conditions are met: first, a phenotypic variation among individuals in a population, i.e., the different individuals in a population differ in their observable characters - their phenotype - with differences in their morphology, physiology or behaviour; second a differential biological efficacy associated with variation; that is, certain phenotypes or variants are associated with greater offspring and/or greater survival; and finally the inheritance of the variation, which requires that the phenotypic variation is due, at least in part, to an underlying genetic variation that allows the transmission of the selected phenotypes to the next generation. If these three conditions are present in a population of organisms, then a change in the genetic composition of the population by natural selection is necessarily followed. Selection is, therefore, the process that results from the three premises cited. And this is logically true in this as well as any other imaginable world.
Explanation:
With Darwin's Origin, this revolution in Biology was introduced. What was truly revolutionary in Darwin was to propose a natural mechanism to explain the genesis, diversity and adaptation of organisms. Darwin's great challenge was to explain the complex adaptations of living organisms, such as the functional design of an eye, by natural mechanisms. Darwin's solution was to propose the mechanism of natural selection. To impose his theory of evolution and natural selection, Darwin had to introduce a new way of understanding variation in nature, population thinking. At Darwin's time species were considered immutable fixed entities; they represented a platonic type, the perfect idea of the mind of their creator. The differences in the form, in the behavior, or in the physiology of the organisms of a species were nothing more than imperfections, errors in the materialization of the idea of the species. In contrast to this dominant essentialist view, individual variation, far from being trivial, was for Darwin the cornerstone of evolution. Variation within the populations of species is the only real thing, it is the raw material of evolution, from which all biological diversity is to be created.it is the existing differences between the organisms of a species that, when amplified in space and time, will produce new populations, new species, and by extension, all biological diversity.
Sources: 1. Curtis. Biology 8th edition.
2.LA SELECCIÓN NATURAL: "ME REPLICO, LUEGO EXISTO" Juguete de la Selección Natural. Antonio Barbadilla. Universidad Autonoma de Barcelona. Available at: http://bioinformatica.uab.es/divulgacio/lasn/
