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Search | Glossary | Home Find definitions for the terminology used throughout the Understanding Evolution pages. Browse alphabetically: A-F | G-M | N-R | S-Z abdomen Section of the body of an animal that is furthest from the mouth and usually contains reproductive organs and part of the digestive system. adapt 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. adaptation 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 . allele 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. Understanding Evolution: Glossary http://evolution.berkeley.edu/evolibrary/glossary/glossary.php 1 of 10 11/16/2010 9:42 PM

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Search | Glossary | Home

Find definitions for the terminology used throughout the UnderstandingEvolution pages.

Browse alphabetically:A-F | G-M | N-R | S-Z

abdomenSection of the body of an animal that is furthest from the mouth and usuallycontains reproductive organs and part of the digestive system.

adaptIn terms of evolution, to undergo natural selection so that members of apopulation are, on average, better able to survive and reproduce. In everydayusage, to adapt may simply mean to adjust to a situation, which does notnecessarily imply that evolution has occurred.

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

adaptive radiationAn event in which a lineage rapidly diversifies with the newly formedlineages evolving different adaptations. For a more detailed explanation, seeour resource on adaptative radiation in Evolution 101.

alleleOne of the versions of a gene that may exist at a locus. For example, the peacolor locus may have either the yellow allele or the green allele. Differentalleles of the same locus are often symbolized by capital and lowercaseletters (e.g., the Y and y alleles).

allometric growthWhen some part of the organism grows at a rate different from the rest ofthe organism during development. For example, the neck vertebrae of fetalgiraffes must grow at a faster rate than the rest of the body (in comparisonto giraffe's short-necked relatives).

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

amino acidA 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 aparticular protein.

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analogy/analogous structureSimilar because of convergent evolution, and not because of commonancestry. Two characters are analogous if the two lineages evolved themindependently. See also homologous, homoplasious.

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

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

apomorphyThe derived or changed character state for a particular clade underconsideration. For example, within the clade of terrestrial vertebrates (inwhich "has four legs" is the ancestral, or plesiomorphic, character state),birds have the apomorphic character state "has two legs and two wings."

appendageAny limb that extends from the body. Arms and legs, for example, areappendages. Arthropods' mouthparts are often small, limb-derivedextensions of the body, and so are considered appendages.

archipelagoA group of islands.

arms racein evolutionary biology, a process in which two or more lineages coevolvesuch that each, in turn, evolves more and more extreme/efficient defensesand weapons in response to the other parties' evolution. For a more detailedexplanation, see our resource on arms races in Evolution 101.

arthropodAny member of the large animal clade, Arthropoda. Living lineages includecrustaceans, arachnids, centipedes, millipedes, and insects. Fossil lineagesinclude the extinct trilobites. All arthropods have a hard exoskeleton that isperiodically shed during growth, a body that is divided into segments, andjointed legs. These traits were inherited from the common ancestor of allarthropods.

artificial selectionA process in which humans consciously select for or against particularfeatures in organisms. For example, the human may allow only organismswith the desired feature to reproduce or may provide more resources to theorganisms with the desired feature. This process causes evolutionary changein the organism and is analogous to natural selection, only with humans, notnature, doing the selecting.

bacteriumA microscopic, single-celled organism lacking a well-defined nucleus. Neitherplants nor animals, bacteria are similar to the first life forms on Earth andare widespread today. Although some bacteria cause diseases in humans, the

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vast majority do not harm humans and are essential to the health of otherorganisms and Earth's ecosystems. (plural = bacteria)

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

bilateral symmetryA condition in which the right and left sides of an item (e.g., a shape or ananimal) are mirror images of one another. For example, since the right sideof the human body generally mirrors the left side, humans are bilaterallysymmetric.

biochemistrySet of chemical reactions that occur within or are associated with livingthings.

biodiversity hotspotA region that provides a home to an unusually high density of differentspecies.

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

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

biomassTotal mass of all living organisms in a particular area. In measures orestimates of biomass, often the mass of the water in organisms is notcounted towards their total biomass.

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

bottleneckAn 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 thepopulation are likely to change just by random chance and many genes maybe lost from the population, reducing the population's genetic variation. Fora more detailed explanation, see our resource on bottlenecks in Evolution101.

Brongniart, Alexandre(1770-1847)French geologist and student of Cuvier who, along with his mentor, was oneof the first to identify and cross-reference geologic strata using fossils, amethodological innovation credited to William Smith. Brongniart and Cuvieridentified the same fossil layers all across the Paris region and showed thatthe regional fossil fauna had alternated between marine and freshwater formsover geologic time.

Buckland, William(1784-1856)English geologist and teacher of Lyell. Buckland is known for his attempts toreconcile religion and geology and for being among the first to identifydinosaur fossils. As a natural theologist, he believed that new life forms werecontinually created. He also believed that the Earth had been shaped by aseries of catastrophes and tried to find evidence that a worldwide flood —Noah's biblical flood — was the most recent of these.

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

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

carnivoreAn organism that eats almost exclusively animals (caro = flesh, vorare = toswallow up).

character

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

chelicerateChelicerates are a group of arthropods distinguished by the followingcharacters:

a body divided into a cephalothorax and abdomen

no antennae, but two pairs of appendages on the anteriorcephalothorax (chelicerae and pedipalps), and four pairs of walkinglegs

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

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

chitinHard, tough substance that occurs widely in nature, particularly in theexoskeletons of arthropods. Chemically, chitin is a carbohydrate and is madefrom sugar molecules.

chloroplast

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

chordateAny member of the animal clade Chordata, a large group of vertebrates andsome marine invertebrates. Chordates have a notochord, a rod-likecartilaginous structure supporting the nerve cord, that they inherited fromtheir common ancestor. Modern chordates include vertebrates, tunicates,hagfish, and lancelets.

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

cladeA group of organisms that includes all the descendents of a commonancestor and that ancestor. For example, birds, dinosaurs, crocodiles andtheir extinct relatives form a clade. For a more detailed explanation, see ourresource on clades in Evolution 101.

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

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

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

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

convergent evolutionProcess in which two distinct lineages evolve a similar characteristicindependently of one another. This often occurs because both lineages facesimilar environmental challenges and selective pressures.

coproliteFossilized dung.

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crustaceanCrustaceans are a group of arthropods distinguished by the followingcharacters:

a body divided into cephalothorax and abdomen

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 rightcharacters!).

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

deleterious allelea version of a gene that, on average, decreases the fitness of the organismcarrying it.

developmentChange in an organism over the course of its lifetime; the processes throughwhich a zygote becomes an adult organism and eventually dies.

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DeVries, Hugo(1848-1935)Dutch botanist famous for his contributions to genetics. He rediscovered theresults first obtained by Mendel and described genetic changes in his plants.Based on his observations, DeVries argued that individual mutations hadwide-ranging effects and could cause speciation in a single step; however, T.H. Morgan later discovered that many mutations seemed to have rather smalleffects. DeVries, it turns out, had observed changes in chromosome number,not the minor change in base pair sequence that are typical of mutation.

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

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

dispersala process in which a species' range changes because some or all individualsmove to a new location. Dispersal is usually contrasted with vicariance as abiogeographic mechanism.

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

DNADeoxyribonucleic acid, the molecule that carries genetic information fromgeneration to generation. For a more detailed explanation, see our resourceon DNA in Evolution 101.

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

ectothermicTerm used to describe an organism that relies on the environment and itsown behavior (e.g., moving to a sunny spot) to regulate its bodytemperature (ecto = outside, therm = heat). Many lizards, for example, areectothermic.

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

endoskeleton

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Evolution, simply put, is descent with modification. This definitionencompasses small-scale evolution (changes in gene frequency in apopulation from one generation to the next) and large-scale evolution (thedescent of different species from a common ancestor over manygenerations).

endosymbiosisA relationship in which one organism lives inside another, to the mutualbenefit of both. It is generally accepted that early in the history ofeukaryotes, eukaryote cells engulfed bacteria, forming a symbioticrelationship. Over time, they became so mutually interdependent, that theybehaved as a single organism. The bacteria became what we know asmitochondria and chloroplasts.

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

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

eukaryoteAn organism with eukaryotic cells — cells with a membrane-enclosed nucleiand membrane-enclosed organelles.

evolutionEvolution (evolve - v.), simply put, is descent with modification. Thisdefinition encompasses small-scale evolution (changes in gene frequency ina population from one generation to the next) and large-scale evolution (thedescent of different species from a common ancestor over manygenerations). For a more detailed explanation, see our resource on evolutionin Evolution 101.

exaptationA feature that performs a function but that did not arise through naturalselection for its current use. For a more detailed explanation, see ourresource on exaptations in Evolution 101.

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

extantNot extinct, existing.

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

fitness

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

food chain/food webAll the feeding interactions of predator and prey, along with the exchange ofnutrients into and out of the soil. These interactions connect the variousmembers of a community, and describe how energy passes from oneorganism to another. Also referred to as the "food web."

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

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

Fourier, Joseph(1768-1830)French physicist and mathematician, most famous for creating themathematical tools to study how heat flows through solids. His studies ofheat led him to argue that Earth's history had a direction, beginning warmand cooling through time — an idea at odds with Lyell's view of Earth'shistory as one of constant, but directionless, change.

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Find definitions for the terminology used throughout the UnderstandingEvolution pages.

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gene flowThe movement of genes between populations. This may happen through themigration of organisms or the movement of gametes (such as pollen blownto a new location).

gene frequency(also called allele frequency) Proportion of genes/alleles in a population thatare of a particular type. For example, at a particular locus, pea plants mayhave either a "yellow pea" allele or a "green pea" allele — so a population ofpea plants would have some frequency of yellow pea alleles ranging fromzero to one (100%).

gene poolAll of the genes in a population. Any genes that could wind up in the sameindividual through sexual reproduction are in the same gene pool.

geneThe unit of heredity. Generally, it means a region of DNA with a particularphenotypic effect. Technically, it may mean a stretch of DNA that includes atranscribed and regulatory region.

genetic driftRandom changes in the gene frequencies of a population from generation togeneration. This happens as a result of sampling error — some genotypesjust happen to reproduce more than other genotypes, not because they are"better," but just because they got lucky. This process causes genefrequencies in a population to drift around over time. Some genes may even"drift out" of a population (i.e., just by chance, some gene may reach afrequency of zero). In general, genetic drift has the effect of decreasinggenetic variation within a population. For a more detailed explanation, seeour resource on genetic drift in Evolution 101.

genetic variationLoosely, a measure of the genetic differences there are within populations orspecies. For example, a population with many different alleles at a locus maybe said to have a lot of genetic variation at that locus. Genetic variation isessential for natural selection to operate since natural selection can onlyincrease or decrease frequency of alleles already in the population.

genome

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All the genetic information an organism carries.

genotypeThe set of genes an organism has. Sometimes, genotype refers to the entiregenome of an organism and sometimes it refers to the alleles carried at aparticular locus.

genus(genera — pl.) The rank above species in Linnaean classification. Forexample, the genus of humans is Homo. Other species in our genus includeHomo erectus and Homo neanderthalensis.

germ line mutationMutation that occurs in reproductive cells and ends up being carried bygametes (e.g., eggs and sperm).

gillAn organ used for breathing in many water-dwelling animals, including mostfish and many arthropods. Gills generally have a large surface area and arefilled with blood; gas exchange occurs by diffusion across the surface area ofthe gill as oxygen passes into the blood and carbon dioxide passes out of theanimal.

habitatPlace and conditions in which an organism normally lives.

herbivoreAn organism that eats almost entirely plants (herb = plant, vorare = toswallow up).

heterochronyAn evolutionary change in the timing of a developmental event. For example,relative to the lineage's ancestor, the early maturation of sex organs is anexample of heterochrony.

hominidHere, humans and their extinct relatives (i.e., organisms on the "human side"of the human/chimpanzee lineage split). However, some scientists use theterm hominid to refer to a larger group: humans, other great apes (gorillas,chimpanzees, bonobos, and orangutans), and their extinct relatives. Howeveryou decide to name the groups, the important thing is how all these speciesare related to one another and not exactly what we decide to call eachlineage.

homininA member of the group Hominini. This clade includes all living and extinctlineages that are more closely related to humans than they are tochimpanzees. It is, essentially, the human branch of the tree of life.

homology/homologous structureInherited from a common ancestor. Human eyes and mouse eyes arehomologous structures because we each inherited them from our common

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ancestor that also had the same sort of eyes. Contrast this withhomoplasious and analogous.

homoplasiousSimilar but not because of inheritance from a common ancestor.Homoplasious characters may be explained by convergent evolution in twodifferent organisms or character reversals.

horizontal transfera process which results in the transfer of genetic material between membersof different species. Bacteria, for example, frequently pass copies ofparticular genes to one another and pick up foreign genetic material fromtheir environment, resulting in horizontal transfer.

hostOrganism that serves as a habitat for another organism. A host may providenutrition to a parasite or simply a place in which to live.

Hox geneA gene that regulates the development and organization of the major bodyunits. For a more detailed explanation, see our resource on Hox genes inEvolution 101.

Hutton, James(1726-1797)Scottish farmer and geologist. In his travels around Britain, he madeobservations which suggested to him that the geologic processes that shapedthe ancient Earth could be seen operating all the time, an idea which wouldlater form the basis of Lyell's uniformitarianism. Hutton used hisobservations and hypothesis to argue that the Earth must be extremely old.

hybridizationThe production of offspring from different parental forms. For example, iftwo recognizably different species of plant fertilized one another andproduced viable, fertile offspring, the process would be called hybridization.

hydrostatic skeletonA fluid-filled cavity that supports the body of an animal because the fluidcannot be compressed into a smaller volume (hydro = liquid or water, statos= standing, unchanging).

hypothesisa proposed explanation for a narrow set of phenomena. A hypothesis mustbe testable with evidence from the natural world. If an explanation can't betested with experimental results, observation, or some other means, then itis not a scientific hypothesis.

inbreedingmating between relatives. Technically, this is defined as a pattern of matingin which mates are more closely related than two individuals selected atrandom from the population.

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incipient speciesA group of organisms that is about to become a separate species from other,related individuals.

insectInsects are a group of arthropods distinguished by the following characters:

a body divided into head, thorax, and abdomenone pair of antennae, three pairs of mouth appendages, three pairs oflegs on thorax, and often one or two pairs of wings

Examples of insects include flies, moths and beetles.

Photos by T. W. Davies © California Academy of Sciences

intelligent design movementThe intelligent design (ID) movement promotes the idea that many aspectsof life are too complex to have evolved without the intervention of asupernatural being — the intelligent designer. Because it relies onsupernatural explanations, ID is not science. To learn more, read our brief onthe intelligent design movement.

intermediate formA partially assembled adaptation. Complex adaptations evolve in a series ofsmaller steps and these steps along the history of an adaptation's evolutionare called intermediate forms.

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iridiumA rare element that is found in relatively high concentrations in asteroids.

junk DNADNA that doesn't code for proteins. The term "junk DNA" is a bit of amisnomer since some of this non-coding DNA performs important functionslike helping to turn genes on and off.

key innovationAn adaptation that allows an organism to exploit a new niche or resource.

life historyTraits that make up the life cycle of an organism. An organism's life historyincludes characteristics related to reproduction, development, and growth(e.g., fecundity, types of larval stages passed through, size at adulthood, andhabitats used at different points in the life cycle).

lineage splittingAn event in which a single historical lineage gives rise to two or moredescendent lineages. Every node on a phylogeny is a lineage-splitting event.

lineageA continuous line of descent; a series of organisms, populations, cells, orgenes connected by ancestor/descendent relationships.

Linnaean classificationThe standard system of classification in which every organism is assigned akingdom, phylum, class, order, family, genus, and species. This systemgroups organisms into ever smaller and smaller groups (like a series ofboxes within boxes, called a nested hierarchy).

locusThe place in the DNA where a gene is located. For example, the pea colorlocus is the place in a pea plant's DNA that determines what the color of thepeas will be. The pea color locus may contain DNA that makes the peasyellow or DNA that makes the peas green — these are called the yellow andgreen alleles.

longevityLong life; long duration of existence.

LucyThe name given to a particular female hominid (of the speciesAustralopithecus afarensis) who lived in what is now Ethiopia about threemillion years ago. "Lucy" is famous because she left behind a very completefossilized skeleton found in 1974.

macroevolutionevolution above the species level. The adaptive radiation of a lineage intomany different niches is an example of macroevolution. Since evolutionarychange above the species level, means that populations and species must beevolving, macroevolutionary change entails microevolutionary change.

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marsupial mammalA mammal, such as an opossum or kangaroo, whose young are suckled andprotected inside a maternal pouch.

mass extinctionEvent in which many different lineages go extinct around the same time.Mass extinctions involved higher rates of extinction than the usual rate ofbackground extinction that is going on all the time. For a more detailedexplanation, see our resource on mass extinctions in Evolution 101.

migrationThe movement of individuals between populations.

mitochondrionAn organelle in eukaryotic cells where cellular respiration takes place.Mitochondria contain a short loop of DNA that is distinct from the DNAcontained in the cell's nucleus.

molecularIn evolutionary biology, having to do with DNA sequences or the amino acidsequences of proteins.

moleculeGroup of two or more atoms bonded together.

moltingA process in which an animal sheds all or part of its outer covering, which isthen regenerated in some way. For example, arthropods molt theirexoskeletons in order to grow, and birds molt their feathers in order toreplace worn out feathers or to prepare for a different season or forbreeding.

morphologyThe study of the form and structure of organisms. For example, comparingthe shape of the femur in different grazing mammals is a morphologicalstudy.

mutationA change in a DNA sequence, usually occurring because of errors inreplication or repair. Mutation is the ultimate source of genetic variation.Changes in the composition of a genome due to recombination alone are notconsidered mutations since recombination alone just changes which genesare united in the same genome but does not alter the sequence of thosegenes. For a more detailed explanation, see our resource on mutation inEvolution 101.

mutualismA species interaction in which both of the interacting species profit from theinteraction.

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myotomeA segment of muscle.

myriapodMyriapods (myria = ten thousand, pod = foot) are a group of arthropodsdistinguished by the following characters:

a body built from a head and long, repeating trunkone pair of antennae (number of other appendages on head varies),many (but not 10,000!) limbs on trunk

Examples of myriapods include centipedes and millipedes.

Centipede Millipede

Centipede photo by James T. Johnson © California Academy of Sciences; Millipede photo © 2003 JohnWhite

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natural selectionDifferential survival or reproduction of different genotypes in a populationleading to changes in the gene frequencies of a population. For a moredetailed explanation, see our resource on natural selection in Evolution 101.

neutral theoryThe idea that most of the molecular variation within populations is not beingselected for or against — it is just neutral variation "drifting" around. Theneutral theory de-emphasizes the role of natural selection in explainingmolecular variation and emphasizes the importance of mutation and geneticdrift. For a more detailed explanation, see our resource on neutral theory inEvolution 101.

nicheIn ecology, the part of the environment occupied by a particular speciesalong with the resources it uses and produces. A species' niche includes suchfactors as energy consumed, time of consumption, space occupied,temperature required, mode of reproduction, and behavior.

nodeA recognizable feature of an organism. Characters may be morphological,behavioral, physiological, or molecular. They are used to reconstructphylogenies. A point on a phylogeny where a single ancestral lineage breaksinto two or more descendent lineages.

notochordA flexible rod running the length of a chordate, providing structural support.The notochord is one of the inherited characteristics shared by all chordates.

nucleotideThe building blocks of DNA. A chain of nucleotides forms DNA. Nucleotidesare made of a sugar, a phosphate, and a base. See also base.

omnivoreAn organism that eats both plants and animals (omni = all, vorare = toswallow up).

ontogenySee development

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onychophoranOnychoporans (also known as velvet worms) share certain characters witharthropods, but are lacking a hard exoskeleton or jointed legs.Onychophorans are probably closely related to arthropods and branched offthe tree just before a fully hardened exoskeleton and jointed legs evolved.

Onychophoran photo provided by Dr. Lynn Kimsey and the Bohart Museum of Entomology, University ofCalifornia Davis

organismAny living creature.

outbreedingmating between very distantly related individuals.

outgroupA lineage in a phylogenetic analysis that falls outside the clade beingstudied. All members of the clade being studied will be more closely relatedto each other than to the outgroup, so the outgroup will branch off at thebase of that phylogeny.

Owen, Richard(1804-1892)English anatomist and student of Cuvier. Owen reconstructed the skeletons ofmany extinct animals, even working on some of Darwin's specimens. He was,nonetheless, an early opponent of Darwin, arguing that God created newspecies by modifying a basic anatomical idea — an "archetype." Later hemodified his own views to accept a kind of "divine" evolution. Owen is alsoknown for overstating the differences between the human brain and those ofother apes in his struggle to place humans on a kind of pedestal, apart fromthe rest of the animal kingdom.

paedomorphosishaving some features of the ancestral juvenile stage, but being an adult (witha mature reproductive system). This word means "child form," and apaedomorphic change is any evolutionary change in the development of anorganism that generates an adult with a "child's form."

paleontologistA scientist who studies fossils (paleo = ancient, onto = being, ology = studyof; study of ancient beings).

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parasiteOrganism that lives on or within another organism, on which it feeds.

parsimonyA principle stating that the simplest explanation accounting for theobservations is the preferred explanation. When reconstructing theevolutionary relationships among lineages, the principle of parsimony impliesthat we should prefer the phylogeny that requires the fewest evolutionarychanges.

phenotypeThe physical features of an organism. Phenotype may refer to any aspect ofan organism's morphology, behavior, or physiology. An organism'sphenotype is affected by its genotype and by its environment.

phenotypic plasticityDegree to which an organism's phenotype changes depending upon theenvironment that it is currently in or its past environment. Two organismswith the same genotype (e.g., identical twins) may have different phenotypes(e.g., one may be taller or heavier) if raised in different environments; thosedifferences represent phenotypic plasticity. All organisms exhibit somedegree of phenotypic plasticity (e.g., an animal that receives more food willgenerally be heavier than a genetically identical animal that receives lessfood), but sometimes phenotypic plasticity can be extreme (e.g., some fishbecome either male or female depending upon the temperatures they wereexposed to as an egg).

phylogenetic classificationA system of classification that names groups of organisms according to theirevolutionary history. Like Linnaean classification, phylogenetic classificationproduces a nested hierarchy where an organism is assigned a series ofnames that more and more specifically locate it within the hierarchy.However, unlike Linnaean classification, phylogenetic classification onlynames clades and does not assign ranks to hierarchical levels.

phylogenyThe evolutionary relationships among organisms; the patterns of lineagebranching produced by the true evolutionary history of the organisms beingconsidered. Many of the phylogenies you encounter are the "family trees" ofgroups of closely related species, but we can also use a phylogeny to depictthe relationships between all life forms. For a more detailed explanation, seeour resource on phylogenies in Evolution 101.

pigmentsubstance that absorbs light. Pigments absorb light of particularwavelengths, which gives the pigment a characteristic color.

placentaIn placental mammals, the organ that connects a fetus to the wall of itsmother's uterus. Nutrients and oxygen pass through the placenta from themother to the developing embryo and waste products pass back through itinto the mother's bloodstream.

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placental mammalA mammal, such as a human, whose young completes its embryonicdevelopment in the uterus, joined to the mother by a placenta.

plate tectonicsa broad theory that uses movements of continental plates to explain manygeographic, geologic, seismic, and even biological observations. The idea isthat the Earth's crust and upper mantle are made up of many differently sizedand irregularly shaped plates that "slide around" on the lower mantle. Theplates may crash into one another, slide under one another, and changeshape as they are broken down and reformed.

plesiomorphyThe ancestral character state for a particular clade. This character state maychange depending on the clade under consideration. For example, "has fourlegs" is plesiomorphic for the clade of terrestrial vertebrates, but "has twolegs and two wings" is plesiomorphic for the clade of owls.

ploidyThe number of copies of each chromosome an organism carries. Forexample, humans are diploid (i.e., we have a ploidy of two) because wecarry two copies of each chromosome.

polytomyA node on a phylogeny where more than two lineages descend from a singleancestral lineage. A polytomy may indicate either that we don't know howthe descendent lineages are related or that we think that the descendentlineages speciated simultaneously. For a more detailed explanation, see ourresource on polytomies in Evolution 101.

population bottleneckSee bottleneck.

populationGenerally, a group of organisms living close to one another that interbreedwith one another and do not breed with other similar groups; a gene pool.Depending on the organism, populations may occupy greater or smallergeographic regions.

predatorAn organism that hunts and eats other organisms. Predators may eat plantsor meat.

preyOrganism killed for food by a predator.

proboscisElongated organ associated with the mouth. For example, in elephants, thetrunk is the proboscis, while in butterflies, the long, coiled feeding tube isthe proboscis.

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proteinA molecule made of a string of amino acids. Proteins are coded for by DNAand are essential molecules for life.

radial symmetryA property of an item (e.g., a shape or an animal) that can be divided intotwo matching halves by many different lines, which all intersect one anotherat a single point in the center. For example, pies, snowflakes, and starfishare radially symmetric because they have many different lines of symmetry(dividing them into matching halves) and the lines cross one another at thecenter.

radiometric datingA method of determining the date at which an igneous rock solidified basedupon the rate of decay of radioactive atoms within the rock. For a moredetailed explanation see our resource on radiometric dating.

randomUnpredictable in some way. Mutations are "random" in the sense that the sortof mutation that occurs cannot generally be predicted based upon the needsof the organism. However, this does not imply that all mutations are equallylikely to occur or that mutations happen without any physical cause. Indeed,some regions of the genome are more likely to sustain mutations thanothers, and various physical causes (e.g., radiation) are known to causeparticular types of mutations.

recombinationA process in which pairs of chromosomes swap DNA with one another. Thishappens during gamete formation. A single parent cell (containing two setsof chromosomes) will form four daughter cells (with one complete set ofchromosomes each). In the process of forming these daughter cells,recombination happens so that the chromosomes the daughter cells have are"mosaic," composed of different pieces of the parent cells' chromosomes.Recombination is important for evolution because it brings new combinationsof genes together — a source of variation for natural selection to act upon.

regulatory geneA gene that controls when protein-coding genes are turned on or off.

RNARibonucleic acid, a molecule similar to DNA involved in carrying informationand producing proteins in cells. Some viruses carry RNA as their geneticmaterial instead of DNA.

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Sedgewick, Adam(1785-1873)English geologist who studied the fossils in different geologic strata andhelped give the strata (and corresponding time periods) the names we usetoday — Cambrian, Devonian, etc. Although he accepted naturalisticexplanations for geologic events and studied them using the biostratigraphicmethods of William Smith, Sedgwick rejected Darwin's naturalisticexplanation for the origin of species and argued that God created new formsof life at the beginning each geologic period.

segregationThe process in which pairs of chromosomes separate and are shuttled off todifferent gametic daughter cells. When gametes are formed, a single parentcell (containing two sets of chromosomes) will form four daughter cells (withone complete set of chromosomes each). In the process, the pairedchromosomes of the parent cell separate into different daughter cells. Thisprocess is segregation.

sexual selectionSelection acting on an organism's ability to obtain or successfully copulatewith a mate. This process may produce traits that seem to decrease anorganism's chance of survival, while increasing its chances of mating. For amore detailed explanation, see our resource on sexual selection in Evolution101.

shocked quartzCrystals with a pattern of fracturing that can be caused by the intensepressure and heat of events such as asteroid impacts.

sickle cell anemiaA genetically caused disease that generally results in the death of the personwith it unless medical interventions are available. Sickle cell anemia is apopular topic for biology courses because it is one the few, well-worked outexamples of heterozygote advantage that we have. People carrying twocopies of the sickle cell allele have the disease, people with no copies of thesickle cell allele are normal, but people carrying just one copy of the sicklecell allele are resistant to malaria (though they may occasionally havesymptoms of sickle cell). So, if you live in a region where malaria iscommon, you are at an advantage if you are a heterozygote (i.e., if you carryone sickle cell allele and one normal allele). For a more detailed explanation,see our resource on sickle cell anemia in Evolution 101.

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single-celledRefers to an organism consisting of one cell, such as bacteria, protozoa, andsome algae, fungi, and yeasts.

sister groups(sometimes called sister taxa) Clades that are each other's closest relatives.On a phylogeny, sister groups occur anytime a single ancestral lineage givesrise to two daughter lineages: the daughter lineages are sister groups, andsince they arose from the same ancestor at the same time, sister groups arealways the same age. Sister groups may differ widely in diversity level: oneclade may be comprised of a single species, while its sister group may becomprised of 100 species.

somatic mutationMutations occurring in cells that do not form gametes, mutations that do notend up being carried by eggs or sperm. For example, mutations in your skin,muscle, or liver tissue are somatic mutations.

speciationThe process by which species form. This involves the reproductive isolationof different parts of an ancestral species so that they form distinctdescendent species. For a more detailed explanation, see our resource onspeciation in Evolution 101.

speciesMembers of populations that actually or potentially interbreed. In this sense,a species is the largest gene pool possible under natural conditions. For amore detailed explanation, see our resource on species in Evolution 101.

subspeciesA grouping of organisms less inclusive than a species. The term is usuallyapplied to groups within a species that have distinct forms and live in arestricted area.

symbiosisA relationship between two different organisms that live in close contact witheach other. The relationship may be beneficial to both organisms(mutualism), beneficial to just one (commensalism), or harmful to one(parasitism).

symplesiomorphyAn ancestral character state (i.e., a plesiomorphy) shared by two or morelineages in a particular clade. For example, within the clade of terrestrialvertebrates (in which the ancestral character state is "has four legs"), bothelephants and salamanders have four legs — and so having four legs is asymplesiomorphy for those two lineages.

synapomorphyA derived or changed character state (i.e., an apomorphy) shared by two ormore lineages in a particular clade. Synapomorphies are indicators ofcommon ancestry. For example, within the clade of terrestrial vertebrates theancestral, or plesiomorphic, character state is "has four legs." However, bothowls and parrots have the synapomorphic character state "has two legs and

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two wings," indicating that owls and parrots are closely related.

taxon(taxa — pl.) Any named group of organisms (e.g., the reptiles, Felidae,beetles, Homo sapiens), whether or not it forms a clade.

tetrapodThe animal clade containing vertebrates with sturdy legs (as opposed tofins).

theorya broad explanation for a wide range of phenomena. Theories are concise,coherent, systematic, predictive, and broadly applicable. They usuallyintegrate many individual hypotheses. A scientific theory must be testablewith evidence from the natural world. If a theory can't be tested withexperimental results, observation, or some other means, then it is not ascientific theory.

thoraxIn animals with three body regions, the middle body region, usually betweenthe head and abdomen.

tracheaAn internal tube that carries air into the body of an animal for breathing. Forexample, in humans, a trachea carries air to the lungs; in insects, a networkof tracheae carries air directly to tissues throughout the body. (plural =tracheae)

transcriptionThe process of building an RNA molecule using DNA as a template. In thisprocess, complimentary RNA bases are matched to their DNA counterparts sothat the strand of RNA that is produced carries the "imprint" of one strand ofthe DNA molecule.

transitional formsFossils or organisms that show the transformation from an ancestral form todescendant species' form. For example, there is a well-documented fossilrecord of transitional forms for the evolution of whales from theiramphibious ancestor. For a more detailed explanation, see our resource ontransitional forms.

translationPart of the process of decoding an RNA molecule composed of nucleotidebases into a protein composed of amino acids.

trilobiteTrilobites are an extinct group of arthropods, distinguished by the followingcharacters:

a body built from a cephalon, thorax, and pygidiuma body divided into three lobes, running from head to tailone pair of antennae

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The last trilobites went extinct about 245 million years ago, but they are wellrepresented by the fossil record.

vertebrateAny member of the animal clade Vertebrata. All vertebrates have a backbonethat surrounds and protects the nerve cord, a character that they all inheritedfrom their common ancestor. Vertebrates are a subgroup of the chordates.Modern vertebrates include fish, sharks, mammals, and amphibians.

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vestigial structureA feature that an organism inherited from its ancestor but that is now lesselaborate and functional than in the ancestor. Usually, vestigial structures areformed when a lineage experiences a different set of selective pressures thanits ancestors, and selection to maintain the elaboration and function of thefeature ends or is greatly reduced.

vicariancea process in which a species' range is divided even though the species hasremained in place. This might happen through tectonic action, geologicactivity (like the rise of a mountain range or shift in the course of a river), orother processes. Vicariance is usually contrasted with dispersal as abiogeographic mechanism.

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