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
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
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
Gamete: A cell, such as an egg or sperm, that unites with another cell to form a
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
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
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
Polymorphism: The presence in a population of two or more alleles at a given
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.