Individuals in a population are not genetically identical. They can carry different alleles and therefore differ in traits that have a genetic component. This heritable variation is the variation that can be transmitted from parents to offspring.

Heritable variation is generated and reshuffled by several mechanisms:

• mutation creates new DNA sequence variants;
• meiotic recombination produces new combinations of existing alleles;
• independent assortment places different combinations of homologous chromosomes into gametes;
• fertilization combines genetic material from two gametes.

The crucial distinction is between heritable and merely phenotypic variation. Individuals can differ because of nutrition, temperature, injury or other environmental effects without those differences being encoded in transmissible genetic information.

For example, suppose two genetically similar plants grow to different heights because one receives more nitrogen. Their height difference is phenotypic variation, but that difference alone does not provide a heritable difference on which evolution can act across generations.

By contrast, if plants carry different alleles affecting stem growth and those alleles are transmitted to offspring, the resulting variation can change in frequency over generations.

Natural selection does not create useful variants when they are needed. It acts on variation that already exists or that arises independently through mutation and recombination. Heritable variation is therefore the raw material on which evolutionary processes operate.