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Find definitions for the terminology used throughout the Understanding Evolution pages.

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

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.

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

adaptive radiation
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.

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).

allometric growth
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).

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

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

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

analogy/analogous structure
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.

ancestral character state
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.

antagonistic pleiotropy
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.

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

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

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."

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.

A group of islands.

arms race
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.

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.

artificial selection
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.

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.

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

A microscopic, single-celled organism lacking a well-defined nucleus. Neither plants nor animals, bacteria are similar to the first life forms on Earth and are widespread today. Although some bacteria cause diseases in humans, the vast majority do not harm humans and are essential to the health of other organisms and Earth's ecosystems. (plural = bacteria)

The information coding part of DNA, the letters of the genetic code. The sequence of bases on a stretch of DNA (i.e., the sequence of As, Ts, Gs, and Cs) determines what the DNA does — if it codes for a protein, turns on a gene, or whatever. In protein-coding regions, three bases code for a single amino acid. For example, the base sequence ATG codes for the amino acid methionine. In a strand of DNA, bases are paired and are lined up across from one another: A pairs with T and G pairs with C.

bilateral symmetry
A condition in which the right and left sides of an item (e.g., a shape or an animal) are mirror images of one another. For example, since the right side of the human body generally mirrors the left side, humans are bilaterally symmetric.

bilateral symmetry

Set of chemical reactions that occur within or are associated with living things.

biodiversity hotspot
A region that provides a home to an unusually high density of different species.

The variety and variability among organisms inhabiting a particular region. However, the term may be more specifically defined and measured in different ways. For example, sometimes biodiversity is used to refer to the number of species in a particular area, sometimes to the number of different ecological niches occupied by organisms in a particular area, and sometimes to the amount of genetic divergence that the organisms in a particular area have experienced.

biogenic elements
The elements that are necessary for basic life processes. Biogenic elements include, but are not limited to, carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorous.

The study of where organisms live and how they came to live where they do.

Total mass of all living organisms in a particular area. In measures or estimates of biomass, often the mass of the water in organisms is not counted towards their total biomass.

book lung
An organ used by many land-dwelling arachnids for breathing. It consists of a cavity in the abdomen containing a set of thin overlapping flaps (like the pages of a book). The inside of each flap is filled with blood, and the outside is exposed to air, allowing oxygen and carbon dioxide to be exchanged through diffusion.

An event in which a population's size is greatly reduced. When this happens, genetic drift may have a substantial effect on the population. In other words, when the population size is radically reduced, gene frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population's genetic variation. For a more detailed explanation, see our resource on bottlenecks in Evolution 101.

branch rotation
A technique for combating many misconceptions about phylogenetics by noting that branches can be rotated around any node on a tree and yield an equivalent phylogeny.

Brongniart, Alexandre
French geologist and student of Cuvier who, along with his mentor, was one of the first to identify and cross-reference geologic strata using fossils, a methodological innovation credited to William Smith. Brongniart and Cuvier identified the same fossil layers all across the Paris region and showed that the regional fossil fauna had alternated between marine and freshwater forms over geologic time.

Buckland, William
English geologist and teacher of Lyell. Buckland is known for his attempts to reconcile religion and geology and for being among the first to identify dinosaur fossils. As a natural theologist, he believed that new life forms were continually created. He also believed that the Earth had been shaped by a series of catastrophes and tried to find evidence that a worldwide flood — Noah's biblical flood — was the most recent of these.

Burgess Shale
Rich deposit of fossils from the Cambrian Period located in western Canada. This fossil bed is particularly valuable because the rarely fossilized soft parts of many ocean-dwelling organisms were preserved in these rocks along with their hard parts (e.g., the exoskeleton).

Cambrian Period
Geologic time period 543-490 million years ago. The Cambrian is the first period of the Paleozoic era, during which all animals and plants lived in the Earth's oceans. Many organisms that we recognize as members of modern animal groups (including the arthropods, sponges, chordates, and molluscs) made their first unmistakable appearance in the fossil record during the Cambrian.

carbon chauvinism
A suggestion by astronomer Carl Sagan that it is narrow-minded to view carbon as the only possible basis for life. He proposed that life in other places might be based on alternative biochemistries not found on Earth.

An organism that eats almost exclusively animals (caro = flesh, vorare = to swallow up).

A recognizable feature of an organism. Characters may be morphological, behavioral, physiological, or molecular. They are used to reconstruct phylogenies.

Chelicerates are a group of arthropods distinguished by the following characters:
  • a body divided into a cephalothorax and abdomen

    a body divided into a cephalothorax and abdomen

  • no antennae, but two pairs of appendages on the anterior cephalothorax (chelicerae and pedipalps), and four pairs of walking legs

    no antennae, but two pairs of appendages on head (chelicerae and pedipalps)

Examples of chelicerates include spiders, scorpions, and horseshoe crabs.

Spiders, scorpions, and horseshoe crabs are all chelicerates

Black Widow Spider photo by George W. Robinson © California Academy of Sciences; Scorpion photo by Dr. Antonio J. Ferreira © California Academy of Sciences; Horseshoe Crab photo © 2000 John White

Hard, tough substance that occurs widely in nature, particularly in the exoskeletons of arthropods. Chemically, chitin is a carbohydrate and is made from sugar molecules.

In plants and photosynthetic protists, a cellular body that uses energy from the sun (sunlight) to create organic compounds from carbon dioxide and water.

Any member of the animal clade Chordata, a large group of vertebrates and some marine invertebrates. Chordates have a notochord, a rod-like cartilaginous structure supporting the nerve cord, that they inherited from their common ancestor. Modern chordates include vertebrates, tunicates, hagfish, and lancelets.

chromosomal inversion
A mutation in which a section of chromosome is reversed 180 degrees. Because inversions in certain chromosomes can be observed with a light microscope, they were particularly important in early genetic studies.

A cellular structure made of DNA and proteins that contains all or part of an organism's heritable, genetic information. Humans have 23 pairs of chromosomes.

A group of organisms that includes all the descendents of a common ancestor and that ancestor. For example, birds, dinosaurs, crocodiles and their extinct relatives form a clade. For a more detailed explanation, see our resource on clades in Evolution 101.

A three base unit of DNA that specifies an amino acid or the end of a protein

A process in which two or more different species reciprocally effect each other's evolution. For example, species A evolves, which causes species B to evolve, which causes species A to evolve, which causes species B to evolve, etc. For a more detailed explanation, see our resource on coevolution in Evolution 101.

An asteroid-like object containing ice and other compounds such as methane and ammonia. When heated by the Sun, a comet develops a surrounding cloud of gas called a "coma" and often a visible tail.

common ancestor
Ancestral organism shared by two or more descendent lineages — in other words, an ancestor that they have in common. For example, the common ancestors of two biological siblings include their parents and grandparents; the common ancestors of a coyote and a wolf include the first canine and the first mammal.

In terms of evolution, an aspect of a lineage's genetic makeup that prevents the lineage from reaching a particular, potentially advantageous evolutionary outcome (e.g., an organism's developmental process prevents the evolution of a trait that would allow a lineage to invade a new habitat).

convergent evolution
Process in which two distinct lineages evolve a similar characteristic independently of one another. This often occurs because both lineages face similar environmental challenges and selective pressures.

Fossilized dung.

cosmic dust
Small particles or grains of solid material in space. Cosmic dust forms in outflows and winds from stars or supernovae and is rich in the heavy elements and organic compounds necessary for life. Dust from early generations of stars mixed with gas in nebulae and ultimately formed subsequent stars and planetary systems.

Crustaceans are a group of arthropods distinguished by the following characters:
  • a body divided into cephalothorax and abdomen

    body divided into cephalothorax and abdomen

  • two pairs of antennae and three pairs of mouth appendages

    Two pairs of antennae and three pairs of mouth appendages

Examples of crustaceans include crabs, pillbugs, and barnacles (It's true! Under that lumpy exterior, barnacles are crustaceans with all of the right characters!).

Crabs, barnacles, and pillbugs are all crustaceans

Sally Lightfoot Crab photo by Gerald and Buff Corsi © California Academy of Sciences; Acorn Barnacles photo by Sherry Ballard © California Academy of Sciences; Pillbugs photo © 2002 William Leonard

data matrix
In phylogenetics, a data set that consists of a list of taxa and a set of those taxa's attributes (called characters and character states). Data matrices are used to build evolutionary trees. For example, a data matrix for a beetle phylogeny might consist of a list of beetle species and would specify many traits of those species—how many antennal segments each has, which have spotted wings and which have solid wings, etc. Data matrices can also include the genetic sequence for a particular gene or part of the genome.

deleterious allele
A version of a gene that, on average, decreases the fitness of the organism carrying it.

derived character state
The character state present in a lineage immediately after a character state change.

derived trait
Form of a trait that evolved from an ancestral form. For example, if a light-colored species of pocket mouse begins living on the dark-colored rocks of an ancient lava flow and evolves a darker coat color, we would consider dark fur to be the derived form of the trait and light fur to be the ancestral form.

descent with modification
See evolution.

design compromise
A term borrowed from engineering indicating failure to realize an "ideal" design because improving performance in one area entails decreasing performance in another area or because of another constraint on the design. Sometimes this term is applied to biological features even though these traits evolved and were not designed. In terms of evolution, a "design" compromise indicates a constraint or trade-off during a feature's evolution. For more details, read about why evolved traits may not be perfectly "engineered" in Evolution 101.

Change in an organism over the course of its lifetime; the processes through which a zygote becomes an adult organism and eventually dies.

DeVries, Hugo
Dutch botanist famous for his contributions to genetics. He rediscovered the results first obtained by Mendel and described genetic changes in his plants. Based on his observations, DeVries argued that individual mutations had wide-ranging effects and could cause speciation in a single step; however, T. H. Morgan later discovered that many mutations seemed to have rather small effects. DeVries, it turns out, had observed changes in chromosome number, not the minor change in base pair sequence that are typical of mutation.

differential reproductive success
A situation in which some individuals leave more offspring in the next generation than do others, often due to traits that confer advantages in survival and/or reproduction.

Process in which the random movement of molecules causes different types of molecules to mix, moving from regions of higher concentration to regions of lower concentration and eventually becoming evenly distributed.

Individual or cell that carries two sets of its chromosomes. Humans are diploid: we carry two copies of each of our 22 regular chromosomes, plus two sex chromosomes (either two Xs or an X and a Y).

directed mutation
The hypothesis that mutations that are useful under particular circumstances are more likely to happen if the organism is actually in those circumstances. In other words, the idea that mutation is directed by what the organism needs. There is little evidence to support this hypothesis.

A process in which a species' range changes because some or all individuals move to a new location. Dispersal is usually contrasted with vicariance as a biogeographic mechanism.

The process of lineage splitting in which one ancestral lineage becomes two or more descendent lineages. In this process, unique traits are likely to evolve in each of the descendent lineages. These traits help us identify the descendent lineages as distinct.

In biology, a measure of the variety of the Earth's animal, plant, and microbial lineages. Different measures of biological diversity (biodiversity) include number of species, number of lineages, variation in morphology, or variation in genetic characteristics.

Deoxyribonucleic acid, the molecule that carries genetic information from generation to generation. For a more detailed explanation, see our resource on DNA in Evolution 101.

dominant gene version
Gene version with an effect that is observed even when paired with a non-identical gene version in the same individual.

Drake Equation
An equation proposed by Dr. Frank Drake to estimate the number of intelligent civilizations in our Galaxy. The equation multiplies together factors such as the rate of star formation in the Galaxy, the fraction of stars with planets, and the fraction of planets that are habitable. The values of many of the parameters are currently highly uncertain, but some (such as the fraction of stars with planets) are increasingly well known.

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

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.

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.

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

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

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.

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.

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.

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.

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

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.

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

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.

evolutionarily stable strategy
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.

evolutionary legacy
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.

evolutionary medicine
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.

evolutionary rate
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.

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

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

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

Not extinct, currently living.

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.

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.

facultative trait
A trait in which the phenotypic expression of the genotype has been shaped by natural selection such that environmental variation triggers the production of different adaptive phenotypes well-suited to that environment — i.e., a trait with adaptive phenotypic plasticity. For example, melanin level is a facultative trait. Humans that are exposed to higher levels of solar radiation produce more melanin, which provides protection from the sun. This response likely evolved through many generations of natural selection. For more details, see our resource on genotype vs. phenotype in Evolution 101.

A genotype's success at reproducing (the more offspring the genotype leaves, the higher its fitness). Fitness describes how good a particular genotype is at leaving offspring in the next generation relative to other genotypes. Experiments and observations can allow researchers to estimate a genotype's fitness, assigning it a numerical value. For a more detailed explanation, see our resource on fitness in Evolution 101.

food chain/food web
All the feeding interactions of predator and prey, along with the exchange of nutrients into and out of the soil. These interactions connect the various members of a community, and describe how energy passes from one organism to another. Also referred to as the "food web."

Any trace of a living organism (body, part of body, burrow, footprint, etc.) preserved over geologic time.

founder effect
Changes in gene frequencies that usually accompany starting a new population from a small number of individuals. The newly founded population is likely to have quite different gene frequencies than the source population because of sampling error (i.e., genetic drift). The newly founded population is also likely to have a less genetic variation than the source population. For a more detailed explanation, see our resource on adaptation in Evolution 101.

Fourier, Joseph
French physicist and mathematician, most famous for creating the mathematical tools to study how heat flows through solids. His studies of heat led him to argue that Earth's history had a direction, beginning warm and cooling through time — an idea at odds with Lyell's view of Earth's history as one of constant, but directionless, change.

frequency dependent selection
A form of natural selection in which the selective advantage of a heritable trait depends on the frequency of that trait in the population.

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