An organism that lives in an environment of extreme temperature, pressure, acidity, alkalinity, or other physical conditions normally not tolerated by other forms of life.

An event in which the last members of a lineage or species die. A single species goes extinct when all members of that species die. An entire lineage goes extinct when all the species that make it up go extinct.

Not extinct, currently living.

A planet orbiting a star other than our Sun. Exoplanets are also known as extrasolar planets.

A region of DNA that is transcribed into RNA. Exons include all the protein-coding regions of a genome, plus the regions that code for rRNA and tRNA.

A feature that performs a function but that did not arise through natural selection for its current use. For a more detailed explanation, see our resource on exaptations in Evolution 101.

A representation of the hypothesized evolutionary relationships among a group of organisms.

The speed at which some evolutionary process occurs. Evolutionary rates may refer to processes that occur at very different scales — for example, the per-generation or per-year rate at which allele frequencies change in a population or the per-million-year rate at which morphological changes are observed in the fossil record. For more details, see our resource on the pace of evolution.

An approach to human health and disease that recognizes that the human body, as well as the pathogens that attack it and symbionts that aid it, are the products of evolutionary history and are susceptible to evolutionary processes. Taking evolution into account when considering medical problems can suggest new avenues of research and new treatment options, and can help physicians make sense of a wide variety of medical phenomena.

The idea that an organism’s future evolutionary trajectories may be constrained or biased by traits or features that it has already evolved. For more details, read about why evolved traits may not be perfectly “engineered” in Evolution 101.

A characteristic of a population, species, or lineage that is likely to be stable or returned to over evolutionary timescales because mutations that would cause an alternate characteristic are acted against by natural selection.

Within biology, the idea that a species or a type of organism a contains an “essence” that makes them what they are (e.g., that dogs are all dogs because they carry the essence of “dogness”). Since essences don’t change over time, essentialist perspectives don’t cohere with evolutionary history, in which the traits of lineages change over time and single-celled ancestors evolved into multi-celled microrganisms, and these in turn diversified into the wide variety of species that have inhabited Earth over life’s history.

A layer of tissue covering an organism’s internal or external surfaces.

Term used to describe an organism that regulates its body temperature by generating its own heat internally (endo = inside, therm = heat). Mammals, for example, are largely endothermic.

A relationship in which one organism lives inside another, to the mutual benefit of both. It is generally accepted that early in the history of eukaryotes, eukaryote cells engulfed bacteria, forming a symbiotic relationship. Over time, they became so mutually interdependent, that they behaved as a single organism. The bacteria became what we know as mitochondria and chloroplasts.

An organism with eukaryotic cells — cells with a membrane-enclosed nuclei and membrane-enclosed organelles.

Support structure located on the inside of the body (endo = inside). For example, human bodies are supported by an endoskeleton made of bone and cartilage.

Organism native to a particular, restricted area and found only in that place.

Material blasted out from a crater by the force of an impact.

Term used to describe an organism that relies on the environment and its own behavior (e.g., moving to a sunny spot) to regulate its body temperature (ecto = outside, therm = heat). Many lizards, for example, are ectothermic.

Layer of tissue present in developing animals that will eventually form organs such as the skin and brain. Other tissue layers (the mesoderm and endoderm) will form other parts of the body.

In evolutionary medicine, environmental influences experienced during gestation or juvenile phases of life which influence adult evolutionary fitness (including disease states).

The study of heritable traits that are not passed on through the genetic sequence of DNA. For example, certain chemical changes in the DNA molecule that do not alter its sequence of bases (e.g., methylation patterns) can be passed down from parent to offspring and can affect which genes are turned on or off. Note that the term epigenetics is also sometimes used to refer to the study of gene expression and gene-environment interactions regardless of the heritability of these changes.

Support structure located on the outside of the body (exo = outside). Arthropod bodies, for example, are supported by an armor-like exoskeleton.

Evolution (evolve – v.), simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene — or more precisely and technically, allele — frequency in a population from one generation to the next — microevolution) and large-scale evolution (the descent of different species from a common ancestor over many generations — macroevolution). For a more detailed explanation, see our resource on evolution in Evolution 101.