Understanding Evolution: your one-stop source for information on evolution


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.

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.

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 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 base pairs code for a single amino acid. For example, the base pair 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.

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.

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.

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.

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.

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.

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.

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.

convergent evolution
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).

Fossilized dung.

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

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

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.

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.

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.

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.

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.

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

Evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations).

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.

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

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 frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). For a more detailed explanation, see our resource on evolution in Evolution 101.

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.

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

Not extinct, existing.

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

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 creature (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.

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