How natural selection occurs

Overproduction of offspring: Organisms produce large numbers of offspring, far more than the environment can support.
Variation: The offspring exhibit variation in their phenotypes due to having different alleles.
Struggle for existence: Due to limited resources, not all individuals survive and reproduce.
Selection pressure: Environmental factors (such as weather) act as selection pressures which affect an organism's chance of survival. Individuals with certain alleles are more likely to survive and reproduce than others.
Differential survival and reproduction: Individuals with advantageous alleles are more likely to survive and reproduce, passing on their genes to the next generation.
Change in allele frequencies: Over time, advantageous alleles become more common, while disadvantageous alleles become less common.
Adaptation: After many generations, the organisms become better adapted to their environment.

Types of natural selection

Stabilizing selection: Maintains allele frequencies relatively constant. It favors the average phenotype and selects against extreme variations.
Directional selection: Causes a gradual change in allele frequency over generations. This occurs when environmental conditions change (favouring one extreme) or a new allele arises.
Disruptive selection: Favors both extremes of a population, maintaining different phenotypes (polymorphism). Intermediate phenotypes are selected against.