Natural selection is the process by which helpful variations are disseminated throughout populations at the expense of neutral or harmful variations. The process improves the chance for beneficial mutations to survive succeeding generations, and it tends to weed out the less desirable traits.
Natural selection is the key mechanism of the theory of evolution. Populations tend to produce more offspring than could possibly survive on the resources available to them. Some members of each generation succumb to disease, predators or starvation. Others live to adulthood but fail to reproduce for a variety of reasons. Still others are unusually successful and leave behind a disproportionate number of progeny. The relative success or failure of an organism may be partly due to luck, but the driving force that determines whether certain traits become more or less common is the sorting effect of natural selection.
Any individual variation is unlikely, in itself, to be positive or negative. The inclusive fitness of a genetically determined trait is either helpful for the organism in a given environment or an impediment. Impeding traits are harmful and tend to inhibit reproduction. Genes that cause impeding traits become less common in each succeeding generation before disappearing altogether. Genes that aid survival and reproduction, however, are favored by selection with gradually increasing representation, until every member of the population has the successful gene.