Allele: One of several alternative forms of a gene, differing from other forms in nucleotide sequence, and usually in its effect on some character.
Allele frequency: The proportion of gene copies in a population that are a specific allele. If the population has N individuals, each with 2 gene copies, the total number of genes in the population is 2N.
Biodiversity: The number of alleles or taxa in a specified geographic area (ranging from a local region to the whole earth). The number of taxa is often referred to as “taxonomic diversity” or, simply, “diversity.”
Character: A specific feature, e.g., “molar teeth.” A character state is one of several alternative conditions of a feature, e.g., the specific number of molars. A quantitative character varies continuously (e.g., weight) rather than discretely, usually because of the effects of both the environment and the action of several or many genes, hence the term “polygenic” character.
Coalescent theory: A body of population genetics theory that uses relationships among DNA sequences to infer the evolutionary processes that have affected genes and populations.
Development: The changes that an individual organism undergoes during its lifetime, from egg, seed, etc., through maturity to death.
Extinction: The death of all individuals in a local population, a species, or a higher taxon.
Fixation: The state of an allele that replaces all other alleles in a population, so that its frequency is 1 (i.e., 100%).
Fitness: The contribution to the next generation of a genotype, relative to that of other genotypes, reflecting its probability of survival and its reproductive output.
Fossil: Any recognizable trace of an ancient organism preserved in a geologic deposit.
Gamete: A cell, such as an egg or sperm, that unites with another cell to form a new organism.
Gene: A unit of heredity, usually a sequence of DNA that encodes a protein or other product that influences the development of one or more characters. Each amino acid in a protein chain is encoded by one or more specific triplets made up of four kinds of nucleotide bases.
Gene flow: The movement of genes from one population into another (usually of the same species) resulting from movement of individuals or their gametes.
Genetic drift: Random changes in the frequencies of alleles within a population due to random sampling of genes.
Genotype: A specific combination of alleles at one or more loci. Organisms such as humans carry two copies of each gene at each of most loci (one from the mother and one from the father); the genotype at a given locus is homozygous if the two copies are the same allele, and heterozygous if they are different alleles.
Locus (pl., loci): The site on a chromosome occupied by a gene; this term is often used to refer to the gene itself.
Mass extinction: A large increase in the number of extinctions (with a concomitant decline in diversity) over a geologically short interval of time (years to many thousands of years).
Mutation:Alteration of the DNA sequence of a gene; hence, the origin of a new allele.
Neutral alleles: Two or more alleles that do not differ in their effect on fitness. Such alleles are said to be “selectively neutral.”
Phenotype: An observable characteristic(s) of an organism, e.g., eye color, respiration rate, number of offspring produced. Both genetic and environmental factors often determine the phenotype.
Phylogeny: The historical pattern of branching, produced by speciation or population isolation, that has resulted in a diversity of taxa or differentiated populations.
Polymorphism: The presence in a population of two or more alleles at a given genetic locus.
Pleiotropy: The effects of a single gene on more than one character.
Population: A local group of individuals of a species; in sexually reproducing organisms, the members of a population interbreed with each other more frequently than they do with members of other populations.
Selection: Shorthand for “natural selection,” i.e., consistent differences in the rate of survival or reproduction between different genotypes or alleles due to differences in the phenotypes they produce.
Species: As used by most evolutionary biologists, a species is a population of organisms that actually or potentially exchange genes by interbreeding, and which are reproductively isolated from other such populations by biological differences that reduce or prevent gene exchange. Speciation is the origin of two or more species by the division of an ancestral species into reproductively isolated populations.
Taxon (pl., taxa): A named entity in biological classification, such as a species (e.g., Homo sapiens) or an order (e.g., Primates). A higher taxon is one above the species level (e.g., a genus or family), and ideally represents a group of species that have descended from their most recent common ancestor.