For natural selection to occur, there must be a selective pressure.
Sources of selective pressure include
1. Limited sources of food
2. Competition for suitable mating partners
3. Outbreak of diseases.
4. Predation
Variation within individuals of the same species is increased with spontaneous mutations resulting in varieties in the selected traits.
Natural selection occurs when nature selects individuals with suitable traits best suited to cope with the selection pressure (ie "Survival of the Fittest"). These individuals that are selected for, are able to survive till reproductive maturity, producing a larger number of offsprings than individuals that are selected against.