Natural selection refers to the phenomenon of genotype-dependent reproduction rates. Selection therefore constitutes an evolutionary process by which the diversity of functional genomic regions is shaped via interactions between single organisms and their environment. The rate at which genetic variants are passed on to the next generation can be modified via natural selection in a positive (positive selection), negative (negative selection), or, depending on whether a mutation exists in the heterozygote or homozygote state, positive-or-negative (balancing selection) way. Both positive and negative selection decrease the diversity of functional and function-associated regions of the genome, whereas balancing selection increases such diversity. Studying natural selection is not only important for improving our evolutionary understanding but can also yield information relevant for human health.