Evolution is the process by which populations of organisms change over generations. Genetic variations underlie these changes. Genetic variations can arise from gene variants (also called mutations) or from a normal process in which genetic material is rearranged as a cell is getting ready to divide (known as genetic recombination). Genetic variations that alter gene activity or protein function can introduce different traits in an organism. If a trait is advantageous and helps the individual survive and reproduce, the genetic variation is more likely to be passed to the next generation (a process known as natural selection). Over time, as generations of individuals with the trait continue to reproduce, the advantageous trait becomes increasingly common in a population, making the population different than an ancestral one. Sometimes the population becomes so different that it is considered a new species.

Not all variants influence evolution. Only hereditary variants, which occur in egg or sperm cells, can be passed to future generations and potentially contribute to evolution. Some variants occur during a person’s lifetime in only some of the body’s cells and are not hereditary, so natural selection cannot play a role. Also, many genetic changes have no impact on the function of a gene or protein and are not helpful or harmful. In addition, the environment in which a population of organisms lives is integral to the selection of traits. Some differences introduced by variants may help an organism survive in one setting but not in another—for example, resistance to a certain bacteria is only advantageous if that bacteria is found in a particular location and harms those who live there.

So why do some harmful traits, like genetic diseases, persist in populations instead of being removed by natural selection? There are several possible explanations, but in many cases, the answer is not clear. For some conditions, such as the neurological condition Huntington's disease, signs and symptoms occur later in life, typically after a person has children, so the gene variant can be passed on despite being harmful. For other harmful traits, a phenomenon called reduced penetrance, in which some individuals with a disease-associated variant do not show signs and symptoms of the condition, can also allow harmful genetic variations to be passed to future generations. For some conditions, having one altered copy of a gene in each cell is advantageous, while having two altered copies causes disease. The best-studied example of this phenomenon is sickle cell disease: Having two altered copies of the HBB gene in each cell results in the disease, but having only one copy provides some resistance to malaria. This disease resistance helps explain why the variants that cause sickle cell disease are still found in many populations, especially in areas where malaria is prevalent.