An alteration of genetic material (DNA) such that a new variation in a gene is produced. For instance, a trait that has only one allele (A) can mutate to a new form (a). This is the only mechanism of evolution that can produce new alleles of a gene.

mutation

A mutation that occurs as an error in a single codon of a DNA molecule.

A general term for an agent in the environment that can cause a mutation to occur. Various kinds of chemicals, viruses, and radiation have been identified as having this capability.

A general term for a mutagen that can cause a mutation in a sex cell. Such mutations can be inherited.

Selection against extreme forms of a trait.  In the case of polygenic traits that are expressed as a continuum of phenotypes, such as human stature, it would be selection for people who are in the middle of the range and against those who are very tall or very short.  The result would be fewer people who are at the extremes in height.

stabilizing selection

Selection for or against one extreme of a trait.  In the case of polygenic traits that are expressed as a continuum of phenotypes, such as human stature, it is selection for people who are either very tall or very short.  The result is a progressive increase in the form of the trait that is being selected for and a reduction in the form that is being selected against.

directional selection

A genetically inherited recessive condition in which red blood cells are distorted resulting in severe anemia and related symptoms that are often fatal in childhood. People who are homozygous for this disease are immune to malaria but die of the disease. Those who are heterozygous for it have a high degree of immunity to malaria and have only minimal symptoms of the disease.

sickle-cell trait or sickle-cell anemia

The mutual, interactive effect on human evolution of biology and culture.  This term was developed to describe the effect our culture has on natural selection.

biocultural evolution

An inherited metabolic abnormality that is fatal in early childhood. Eastern European Jews have an unusually high frequency of this harmful recessive allele in their population.   However, it can occur in any human group.

Tay-Sachs disease

The maintenance of two or more alleles for a trait in a population at a more or less constant frequency ratio due to the selective advantage of heterozygotes.

balanced polymorphism

A life threatening disease found mostly in tropical and subtropical regions of the world.  It is caused by single-celled microorganisms that are transmitted from person to person by mosquitoes as they extract blood.  Symptoms include chills, high fever, and sometimes fatal irregularities of the brain, liver, kidney, and/or blood.

malaria