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Glossary Find definitions for the terminology used throughout the Understanding Evolution pages.
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abdomen
biochemistry Set of chemical reactions that occur within or are associated with living things. biodiversity 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. biomass 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. bottleneck 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 (1770-1847) 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 (1784-1856) 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. carnivore An organism that eats almost exclusively animals (caro = flesh, vorare = to swallow up). character A recognizable feature of an organism. Characters may be morphological, behavioral, physiological, or molecular. They are used to reconstruct phylogenies. chelicerate Chelicerates are a group of arthropods distinguished by the following characters:
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 chitin 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. chloroplast In plants and photosynthetic protists, a cellular body that uses energy from the sun (sunlight) to create organic compounds from carbon dioxide and water. chordate 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. clade 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. codon a three base unit of DNA that specifies an amino acid or the end of a protein coevolution 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. constraint 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. coprolite Fossilized dung. crustacean Crustaceans are a group of arthropods distinguished by the following characters:
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. development 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 (1848-1935) 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. diffusion 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. diversity 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. DNA 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. ectoderm 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. ectothermic 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. endemic Organism native to a particular, restricted area and found only in that place. endoskeleton 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). endosymbiosis 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. endothermic 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. epithelium A layer of tissue covering an organism's internal or external surfaces. eukaryote An organism with eukaryotic cells — cells with a membrane-enclosed nuclei and membrane-enclosed organelles. evolution 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. exaptation 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. exoskeleton Support structure located on the outside of the body (exo = outside). Arthropod bodies, for example, are supported by an armor-like exoskeleton. extant Not extinct, existing. extinction 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. fitness 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." fossil 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 (1768-1830) 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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