The branch of science that investigates the possibility of life beyond Earth.

A large chunk of rock (or a loosely bound “rubble pile” of smaller rocks) orbiting the Sun closer than Jupiter. Smaller space rocks (under about one meter across) are known as meteoroids.

A process in which humans consciously select for or against particular features in organisms. For example, the human may allow only organisms with the desired feature to reproduce or may provide more resources to the organisms with the desired feature. This process causes evolutionary change in the organism and is analogous to natural selection, only with humans, not nature, doing the selecting.

Any member of the large animal clade, Arthropoda. Living lineages include crustaceans, arachnids, centipedes, millipedes, and insects. Fossil lineages include the extinct trilobites. All arthropods have a hard exoskeleton that is periodically shed during growth, a body that is divided into segments, and jointed legs. These traits were inherited from the common ancestor of all arthropods.

In evolutionary biology, a process in which two or more lineages coevolve such that each, in turn, evolves more and more extreme/efficient defenses and weapons in response to the other parties’ evolution. For a more detailed explanation, see our resource on arms races in Evolution 101.

A group of islands.

Any limb that extends from the body. Arms and legs, for example, are appendages. Arthropods’ mouthparts are often small, limb-derived extensions of the body, and so are considered appendages.

The derived or changed character state for a particular clade under consideration. For example, within the clade of terrestrial vertebrates (in which “has four legs” is the ancestral, or plesiomorphic, character state), birds have the apomorphic character state “has two legs and two wings.”

A scientist who studies humans. This can include studying human evolution.

Centering on humans and considering all other things in relation to humans.

A situation in which a single allele results in effects that incur both costs and benefits for the organism. Antagonistic pleiotropy has been proposed as an explanation for why organism age: perhaps alleles that result in fitness benefits early in life also cause senescence later in life, but were favored because those early benefits outweighed the later costs.

The character state present in a lineage immediately before a character state change. Ancestral character states are sometimes called primitive; however, this term is misleading because it suggests that the ancestral state is less advanced than the derived state and there is no way to measure evolutionary advancement. Use of the term primitive should be avoided.

Similar because of convergent evolution, and not because of common ancestry. Two characters are analogous if the two lineages evolved them independently. See also homologous, homoplasious.

A building block of proteins. There are about 20 amino acids and protein-coding DNA tells the cellular machinery which amino acids use to build a particular protein.

A behavior that benefits another individual, at the expense of the individual performing the behavior.

Speciation that depends on an external barrier to gene flow (such as geographic isolation) to begin or complete the process of speciation.

When some part of the organism grows at a rate different from the rest of the organism during development. For example, the neck vertebrae of fetal giraffes must grow at a faster rate than the rest of the body (in comparison to giraffe’s short-necked relatives).

One of the versions of a gene that may exist at a locus. For example, the pea color locus may have either the yellow allele or the green allele. Different alleles of the same locus are often symbolized by capital and lowercase letters (e.g., the Y and y alleles).

An event in which a lineage rapidly diversifies with the newly formed lineages evolving different adaptations. For a more detailed explanation, see our resource on adaptative radiation in Evolution 101.

A feature produced by natural selection for its current function. For a more detailed explanation, see our resource on adaptation in Evolution 101.

In terms of evolution, to undergo natural selection so that members of a population are, on average, better able to survive and reproduce. In everyday usage, to adapt may simply mean to adjust to a situation, which does not necessarily imply that evolution has occurred.

Section of the body of an animal that is furthest from the mouth and usually contains reproductive organs and part of the digestive system.