An organ used for breathing in many water-dwelling animals, including most fish and many arthropods. Gills generally have a large surface area and are filled with blood; gas exchange occurs by diffusion across the surface area of the gill as oxygen passes into the blood and carbon dioxide passes out of the animal.

Mutation that occurs in reproductive cells and ends up being carried by gametes (e.g., eggs and sperm).

(genera — pl.) The rank above species in Linnaean classification. For example, the genus of humans is Homo. Other species in our genus include Homo erectus and Homo neanderthalensis.

The set of genes an organism has. Sometimes, genotype refers to the entire genome of an organism and sometimes it refers to the alleles carried at a particular locus.

All the genetic information an organism carries.

Loosely, a measure of the genetic differences there are within populations or species. For example, a population with many different alleles at a locus may be said to have a lot of genetic variation at that locus. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles already in the population.

Random changes in the gene frequencies of a population from generation to generation. This happens as a result of sampling error — some genotypes just happen to reproduce more than other genotypes, not because they are “better,” but just because they got lucky. This process causes gene frequencies in a population to drift around over time. Some genes may even “drift out” of a population (i.e., just by chance, some gene may reach a frequency of zero). In general, genetic drift has the effect of decreasing genetic variation within a population. For a more detailed explanation, see our resource on genetic drift in Evolution 101.

The unit of heredity. Generally, it means a region of DNA with a particular phenotypic effect. Technically, it may mean a stretch of DNA that includes a transcribed and regulatory region.

All of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool.

(also called allele frequency) Proportion of genes/alleles in a population that are of a particular type. For example, at a particular locus, pea plants may have either a “yellow pea” allele or a “green pea” allele — so a population of pea plants would have some frequency of yellow pea alleles ranging from zero to one (100%).

The movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location).

A mechanism of mutation associated with recombination in which a cell’s DNA repair systems “convert” the sequence on a section of one chromosome to a sequence that is found on the homologous chromosome. Gene conversion can be biased, tending to favor some alleles over others.

A collection of stars (and associated gas and dark matter) held together by its own gravity. Galaxies can contain many millions to hundreds of billions of stars. Our own Galaxy is known as the Milky Way.

A change in how a particular allele is expressed that is caused by an environmental influence. Gene-environment interactions are an example of phenotypic plasticity.