# Organismic and Evolutionary Biology Terms

Abdomen - Section of the body of an animal that is furthest from the mouth & usually contains reproductive organs & 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 & 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.
Adaptive radiation - Periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles, or niches, in their communities
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 & lowercase letters (e.g. the Y & 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 & 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 & not because of common ancestry. Two characters are analogous if the two lineages evolved them independently. See also homologous homoplasious.
Anthropocentric Centering on humans & considering all other things in relation to humans.
Anthropologist - A scientist who studies humans. This can include studying human evolution.
Apomorphy - 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 & two wings."
Appendage - Any limb that extends from the body. Arms & legs for example are appendages. Arthropods' mouthparts are often small limb-derived extensions of the body & so are considered appendages.
Archipelago - 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 & more extreme/efficient defenses & weapons in response to the other parties' evolution.
Arthropod - Any member of the large animal clade Arthropoda. Living lineages include crustaceans arachnids centipedes, millipedes, & 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 & jointed legs.
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 & is analogous to natural selection only with humans not nature doing the selecting.
Bacterium - A microscopic single-celled organism lacking a well-defined nucleus. Neither plants nor animals bacteria are similar to the first life forms on Earth & are widespread today. Although some bacteria cause diseases in humans the vast majority do not harm humans & are essential to the health of other organisms & Earth's ecosystems. (plural = bacteria)
Base - 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 & 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 str& of DNA bases are paired & are lined up across from one another: A pairs with T & G pairs with C.
Bilateral symmetry - A condition in which the right & left sides of an item (a shape or 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.
Biochemistry - Set of chemical reactions that occur within or associated with living things.
Biodiversity - The variety & variability among organisms inhabiting a particular region. However the term may be more specifically defined & measured in different ways. For example sometimes biodiversity refers to the number of species in a particular area sometimes to the number of different ecological niches occupied by organisms in a particular area & sometimes to the amount of genetic divergence that organisms in a particular area experienced.
Biodiversity hotspot - Region that provides a home to a unusually high density of different species
Biogeography - The study of where organisms live & how they came to live where they do.
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 l&-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 & the outside is exposed to air allowing oxygen & 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 & many genes may be lost from the population reducing the population's genetic variation.
Brongniart Alexandre(1770-1847) - French geologist & student of Cuvier who along with his mentor was one of the first to identify & cross-reference geologic strata using fossils a methodological innovation credited to William Smith. Brongniart & Cuvier identified the same fossil layers all across the Paris region & showed that the regional fossil fauna had alternated between marine & freshwater forms over geologic time.
Buckland William(1784-1856) - English geologist & teacher of Lyell. Buckl& is known for his attempts to reconcile religion & geology & 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 & 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 & plants lived in the Earth's oceans. Many organisms that we recognize as members of modern animal groups (including the arthropods sponges chordates & 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: a body divided into a cephalothorax & abdomen * no antennae but two pairs of appendages on the anterior cephalothorax (chelicerae & pedipalps) & four pairs of walking legs chitinHard tough substance that occurs widely in nature particularly in the exoskeletons of arthropods. Chemically chitin is a carbohydrate & is made from sugar molecules.
Chloroplast - In plants & photosynthetic protists a cellular body that uses energy from the sun (sunlight) to create organic compounds from carbon dioxide & water.
Chordate - Any member of the animal clade Chordata a large group of vertebrates & 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 & 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.
Clade - A group of organisms that includes all the descendents of a common ancestor & that ancestor. For example birds dinosaurs crocodiles & their extinct relatives form a clade.
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
Common ancestor - An ancestor that they have in common. For example the common ancestors of two biological siblings include their parents & grandparents; the common ancestors of a coyote & a wolf include the first canine & 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 & selective pressures.
Coprolite - Fossilized dung.
Crustacean - Crustaceans are a group of arthropods distinguished by the following characters: a body divided into cephalothorax & abdomen * two pairs of antennae & three pairs of mouth appendages.
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 & eventually dies.
DeVries Hugo(1848-1935) - Dutch botanist famous for his contributions to genetics. He rediscovered the results first obtained by Mendel & described genetic changes in his plants. Based on his observations DeVries argued that individual mutations had wide-ranging effects & could cause speciation in a single step; however T. H. Morgan later discovered that many mutations seemed to have rather small effects. DeVries had observed changes in chromosome number not minor change in base pair sequence that are typical of mutation.
Diffusion - Process in which the r&om movement of molecules causes different types of molecules to mix moving from regions of higher concentration to regions of lower concentration & 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.
Dispersal - 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.
Diversity - In biology a measure of the variety of the Earth's animal plant & 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 is the molecule that carries genetic information from generation to generation.
Ectoderm - Layer of tissue present in developing animals that will eventually form organs such as the skin & brain. Other tissue layers (the mesoderm & endoderm) will form other parts of the body.
Ectothermic - Term used to describe an organism that relies on the environment & 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 & found only in that place.
Endoskeleton - Evolution is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) & 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 & chloroplasts.
Endothermic - Term used to describe an organism that regulates its body temperature by generating its own heat internally. 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 & membrane-enclosed organelles.
Evolution - Evolution is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) & large-scale evolution (the descent of different species from a common ancestor over many generations).
Exaptation - A feature that performs a function but that did not arise through natural selection for its current use.
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 & observations can allow researchers to estimate a genotype's fitness assigning it a numerical value.
Food chain/food web - All the feeding interactions of predator & prey along with the exchange of nutrients into & out of the soil. These interactions connect the various members of a community & describe how energy passes from one organism to another. Referred to as the "food web."
Fossil - Trace of a living creature (body part of body burrow footprint) preserved over time.
Founder effect - Changes in gene frequencies that accompany starting a new population from a small number of individuals. The newly founded population is likely to have different gene frequencies than the source population because of sampling error (genetic drift). The newly founded population is to have a less genetic variation than the source population
Fourier Joseph (1768-1830) - French physicist & 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 & cooling through time — an idea at odds with Lyell's view of Earth's history as one of constant but directionless change.
Gene - The unit of heredity. Generally it means a region of DNA with a particular phenotypic effect. A stretch of DNA that includes a transcribed & regulatory region.
Gene flow - The movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location).
Gene frequency - (also called allele frequency) Proportion of genes/alleles in a population that are of a particular type. For example at a particular locus pea plants may have either a "yellow pea" allele or a "green pea" allele — so a population of pea plants would have some frequency of yellow pea alleles ranging from zero to one (100%).
Gene pool - All of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool.
Genetic drift - Random changes in the gene frequencies of a population from generation to generation. This happens as a result of sampling error — some genotypes just happen to reproduce more than other genotypes not because they are "better but just because they got lucky. This process causes gene frequencies in a population to drift around over time. Some genes may even drift out" of a population (just by chance some gene may reach frequency of 0). Genetic drift has the effect of decreasing genetic variation within a population.
Genetic variation - Loosely a measure of the genetic differences there are within populations or species. For example a population with many different alleles at a locus may be said to have a lot of Genetic variation at that locus. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles already in the population.
Genome - All the genetic information an organism carries.genotype. The set of genes an organism has. Sometimes genotype refers to the entire genome of an organism & sometimes it refers to the alleles carried at a particular locus.genus (genera — pl.) The rank above species in Linnaean classification.
Germ line mutation - Mutation that occurs in reproductive cells & ends up being carried by gametes (egg/sperm).
Gill - Organ used for breathing in many water-dwelling animals including most fish & many arthropods. Gills generally have a large surface area & are filled with blood; gas exchange occurs by diffusion across the surface area of the gill as oxygen passes into the blood & carbon dioxide passes out of them.
Habitat - Place & conditions in which an organism normally lives.
Herbivore - An organism that eats almost entirely plants.
Heterochrony - An evolutionary change in the timing of a developmental event. For example relative to the lineage's ancestor the early maturation of sex organs is an example of heterochrony.
Hominid - Humans & their extinct relatives (i.e. organisms on the "human side" of the human/chimpanzee lineage split). However some scientists use the term hominid to refer to a larger group: humans other great apes (chimpanzees, gorillas & orangutans) & their extinct relatives. However you decide to name the groups the important thing is how all these species are related to one another & not what we decide to call each lineage.
Hominin - This clade includes all living & extinct lineages that are more closely related to humans than they are to chimpanzees. It is essentially the human branch of the tree of life.
Homology/homologous structure - Inherited from a common ancestor. Human eyes & mouse eyes are homologous structures because we each inherited them from our common ancestor that also had the same sort of eyes. Contrast this with homoplasious & analogous.
Homoplasious - Similar but not because of inheritance from a common ancestor. Homoplasious characters may be explained by convergent evolution in 2 different organisms or character reversals.
Horizontal transfer - A process which results in the transfer of genetic material between members of different species. Bacteria for example frequently pass copies of particular genes to one another & pick up foreign genetic material from their environment resulting in horizontal transfer.
Host - Organism that serves as a habitat for another organism. A host may provide nutrition to a parasite or simply a place in which to live.
Hox gene - Gene that regulates development & organization of the major body units.
Hutton James (1726-1797) - Scottish farmer & geologist. In his travels around Britain he made observations which suggested to him that the geologic processes that shaped the ancient Earth could be seen operating all the time an idea which would later form the basis of Lyell's uniformitarianism. Hutton used his observations & hypothesis to argue that the Earth must be extremely old.
Hybridization - The production of offspring from different parental forms. For example if two recognizably different species of plant fertilized one another & produced viable fertile offspring the process would be called hybridization.
Hydrostatic skeleton - A fluid-filled cavity that supports the body of an animal because the fluid cannot be compressed into a smaller volume (hydro = liquid statos = standing unchanging).
Hypothesis - A proposed explanation for a narrow set of phenomena. A hypothesis must be testable with evidence from the natural world. If an explanation can't be tested with experimental results observation or some other means then it is not a scientific hypothesis.
Inbreeding - Mating between relatives. Technically this is defined as a pattern of mating in which mates are more closely related than two individuals selected at random from the population.
Incipient species - Group of organisms that is to become separate species from other related individuals.
Insect - Insects are a group of arthropods distinguished by the following characters: a body divided into head thorax & abdomen one pair of antennae three pairs of mouth appendages three pairs of legs on thorax & often one or two pairs of wings
Intelligent design movement - The intelligent design (ID) movement promotes the idea that many aspects of life are too complex to have evolved without the intervention of a supernatural being — the intelligent designer. Because it relies on supernatural explanations ID is not science. To learn more read our brief on the intelligent design movement.
Intermediate form - A partially assembled adaptation. Complex adaptations evolve in a series of smaller steps & these steps along the history of an adaptation's evolution are called intermediate forms.
Iridium - A rare element that is found in relatively high concentrations in asteroids.
Junk DNA - DNA that doesn't code for proteins. The term "junk DNA" is a bit of a misnomer since some of this non-coding DNA performs important functions like helping to turn genes on & off.
Key innovation - An adaptation that allows an organism to exploit a new niche or resource.
Life history - Traits that make up the life cycle of an organism. An organism's life history includes characteristics related to reproduction development & growth (e.g. fecundity types of larval stages passed through size at adulthood & habitats at different points in life cycle).
Lineage - A continuous line of descent; a series of organisms populations cells or genes connected by ancestor/descendent relationships.
Lineage splitting - An event in which a single historical lineage gives rise to two or more descendent lineages. Every node on a phylogeny is a lineage-splitting event.
Linnaean classification - The standard system of classification in which every organism is assigned a kingdom phylum class order family genus & species. This system groups organisms into ever smaller & smaller groups (like a series of boxes within boxes called a nested hierarchy).
Locus - The place in the DNA where a gene is located. For example the pea color locus is the place in a pea plant's DNA that determines what the color of the peas will be. The pea color locus may contain DNA that makes the peas yellow or DNA that makes the peas green — these are called the yellow & green alleles.
Longevity - Long life; long duration of existence.
Lucy - The name given to a particular female hominid (of the species Australopithecus afarensis) who lived in what is now Ethiopia about three million years ago. "Lucy" is famous because she left behind a very complete fossilized skeleton found in 1974.
Macroevolution - Evolution above the species level. The adaptive radiation of a lineage into many different niches is an example of macroevolution. Since evolutionary change above the species level means that populations & species must be evolving macroevolutionary change entails microevolutionary change.
Marsupial mammal - A mammal such as an opossum or kangaroo whose young are suckled & protected inside a maternal pouch.
Mass extinction - Event in which many different lineages go extinct around the same time. Mass extinctions involved higher rates of extinction than the usual rate of background extinction that is going on all the time.
Mitochondrion - An organelle in eukaryotic cells where cellular respiration takes place. Mitochondria contain a short loop of DNA that is distinct from the DNA contained in the cell's nucleus.
Molecular - In evolutionary biology having to do with DNA sequences or the amino acid sequences of proteins.
Molecule - Group of two or more atoms bonded together.
Molting - A process in which an animal sheds all or part of its outer covering which is then regenerated in some way. For example arthropods molt their exoskeletons in order to grow & birds molt their feathers in order to replace worn out feathers or to prepare for a different season or for breeding.
Morphology - The study of the form & structure of organisms. For example comparing the shape of the femur in different grazing mammals is a morphological study.
Mutation - A change in a DNA sequence usually occurring because of errors in replication or repair. Mutation is the ultimate source of genetic variation. Changes in the composition of a genome due to recombination alone are not considered mutations since recombination alone just changes which genes are united in the same genome but does not alter the sequence of those genes.
Mutualism - A species interaction in which both of the interacting species profit from the interaction.
Myotome - A segment of muscle.
Myriapod - Myriapods (myria = ten thousand pod = foot) are a group of arthropods distinguished by the following characters: a body built from a head & long repeating trunk one pair of antennae (number of other appendages on head varies) many (but not 10000!) limbs on trunk
Natural selection - Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population.
Neutral theory - The idea that most of the molecular variation within populations is not being selected for or against — it is just neutral variation "drifting" around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation & emphasizes the importance of mutation & genetic drift.
Niche - In ecology the part of the environment occupied by a particular species along with the resources it uses & produces. A species' niche includes factors as energy consumed time of consumption space occupied temperature required mode of reproduction & behavior.
Node - A recognizable feature of an organism. Characters may be morphological behavioral physiological or molecular. They are used to reconstruct phylogenies. A point on a phylogeny where a single ancestral lineage breaks into two or more descendent lineages.
Notochord - A flexible rod running the length of a chordate providing structural support. The notochord is one of the inherited characteristics shared by all chordates.
Nucleotide - The building blocks of DNA. A chain of nucleotides forms DNA. Nucleotides are made of a sugar a phosphate & a base. See also base.
Omnivore - An organism that eats both plants & animals (omni = all vorare = to swallow up).
Onychoporans (also known as velvet worms) - Share certain characters with arthropods but are lacking a hard exoskeleton or jointed legs. Onychophorans are probably closely related to arthropods & branched off the tree just before a fully hardened exoskeleton & jointed legs evolved.
Organism - Any living creature.
Outbreeding - Mating between very distantly related individuals.
Outgroup - A lineage in a phylogenetic analysis that falls outside the clade being studied. All members of the clade being studied will be more closely related to each other than to the outgroup so the outgroup will branch off at the base of that phylogeny.
Owen Richard (1804-1892) - English anatomist & student of Cuvier. Owen reconstructed the skeletons of many extinct animals even working on some of Darwin's specimens. He was nonetheless an early opponent of Darwin arguing that God created new species by modifying a basic anatomical idea of an "archetype." Later he modified his own views to accept a kind of "divine" evolution. Owen is known for overstating the differences between the human brain & those of other apes in his struggle to place humans on a kind of pedestal apart from the rest of the animal kingdom.
Paedomorphosis - Having some features of the ancestral juvenile stage but being an adult (with a mature reproductive system). This word means "child form & a paedomorphic change is any evolutionary change in the development of an organism that generates an adult with a child's form."
Paleontologist - A scientist who studies fossils (paleo = ancient onto = being ology = study of; study of ancient beings).
Parasite - Organism that lives on or within another organism on which it feeds.
Parsimony - A principle stating that the simplest explanation accounting for the observations is the preferred explanation. When reconstructing the evolutionary relationships among lineages the principle of parsimony implies that we should prefer the phylogeny that requires the fewest evolutionary changes.
Phenotype - The physical features of an organism. Phenotype may refer to any aspect of an organism's morphology behavior or physiology. An organism's phenotype is affected by its genotype & by its environment.
Phenotypic Plasticity - Degree to which an organism's phenotype changes depending upon the environment that it is currently in or its past environment. Two organisms with the same genotype (e.g. identical twins) may have different phenotypes (e.g. one may be taller or heavier) if raised in different environments; those differences represent phenotypic plasticity. All organisms exhibit some degree of phenotypic plasticity (e.g. an animal that receives more food will generally be heavier than a genetically identical animal that receives less food) but sometimes phenotypic plasticity can be extreme (e.g. some fish become either male or female depending upon the temperatures they were exposed to as an egg).
Phylogenetic classification - A system of classification that names groups of organisms according to their evolutionary history. Like Linnaean classification phylogenetic classification produces a nested hierarchy where an organism is assigned a series of names that more & more specifically locate it within the hierarchy. However unlike Linnaean classification phylogenetic classification only names clades & does not assign ranks to hierarchical levels.
Phylogeny - The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered. Many of the phylogenies you encounter are the "family trees" of groups of closely related species but we can also use a phylogeny to depict the relationships between all life forms.
Pigment - substance that absorbs light. Pigments absorb light of particular wavelengths which gives the pigment a characteristic color.
Placenta - In placental mammals the organ that connects a fetus to the wall of its mother's uterus. Nutrients & oxygen pass through the placenta from the mother to the developing embryo & waste products pass back through it into the mother's bloodstream.
Placental mammal - A mammal such as a human whose young completes its embryonic development in the uterus joined to the mother by a placenta.
Plate tectonics - a broad theory that uses movements of continental plates to explain many geographic geologic seismic & even biological observations. The idea is that the Earth's crust & upper mantle are made up of many differently sized & irregularly shaped plates that "slide around" on the lower mantle. The plates may crash into one another slide under one another & change shape as they are broken down & reformed.
Plesiomorphy - The ancestral character state for a particular clade. This character state may change depending on the clade under consideration. For example "has four legs" is plesiomorphic for the clade of terrestrial vertebrates but "has two legs & two wings" is plesiomorphic for the clade of owls.
Ploidy - The number of copies of each chromosome an organism carries. For example humans are diploid (i.e. we have a ploidy of two) because we carry two copies of each chromosome.
Polytomy A node on a phylogeny where more than two lineages descend from a single ancestral lineage. A polytomy may indicate either that we don't know how the descendent lineages are related or that we think that the descendent lineages speciated simultaneously.
Population - Generally a group of organisms living close to one another that interbreed with one another & do not breed with other similar groups; a gene pool. Depending on the organism populations may occupy greater or smaller geographic regions.
Predator - An organism that hunts & eats other organisms. Predators may eat plants or meat. prey Organism killed for food by a predator.
Proboscis - Elongated organ associated with the mouth. For example in elephants the trunk is the proboscis while in butterflies the long coiled feeding tube is the proboscis.
Protein A molecule made of a string of amino acids. Proteins are coded for by DNA & are essential molecules for life.
Radial symmetry - A property of an item (e.g. a shape or an animal) that can be divided into two matching halves by many different lines which all intersect one another at a single point in the center. For example pies snowflakes & starfish are radially symmetric because they have many different lines of symmetry (dividing them into matching halves) & the lines cross one another at the center.
Radiometric dating - A method of determining the date at which an igneous rock solidified based upon the rate of decay of radioactive atoms within the rock.
Random - Unpredictable in some way. Mutations are "r&om" in the sense that the sort of mutation that occurs cannot generally be predicted based upon the needs of the organism. However this does not imply that all mutations are equally likely to occur or that mutations happen without any physical cause. Indeed some regions of the genome are more likely to sustain mutations than others & various physical causes (e.g. radiation) are known to cause particular types of mutations.
Recombination - A process in which pairs of chromosomes swap DNA with one another. This happens during gamete formation. A single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes 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 combinations of genes together — a source of variation for natural selection to act upon.
Regulatory gene - A gene that controls when protein-coding genes are turned on or off.
RNA - Ribonucleic acid a molecule similar to DNA involved in carrying information & producing proteins in cells. Some viruses carry RNA as their genetic material instead of DNA.
Sedgewick Adam(1785-1873) - English geologist who studied the fossils in different geologic strata & helped give the strata (& corresponding time periods) the names we use today — Cambrian Devonian etc. Although he accepted naturalistic explanations for geologic events & studied them using the biostratigraphic methods of William Smith Sedgwick rejected Darwin's naturalistic explanation for the origin of species & argued that God created new forms of life at the beginning each geologic period.
Segregation - The process in which pairs of chromosomes separate & are shuttled off to different gametic daughter cells. When gametes are formed a single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process the paired chromosomes of the parent cell separate into different daughter cells. This process is segregation.
Sexual selection - Selection acting on an organism's ability to obtain or successfully copulate with a mate. This process may produce traits that seem to decrease an organism's chance of survival while increasing its chances of mating.
Shocked quartz Crystals with a pattern of fracturing that can be caused by the intense pressure & heat of events such as asteroid impacts.
Sickle cell anemia - A genetically caused disease that generally results in the death of the person with it unless medical interventions are available. Sickle cell anemia is a popular topic for biology courses because it is one the few well-worked out examples of heterozygote advantage that we have. People carrying two copies of the sickle cell allele have the disease people with no copies of the sickle cell allele are normal but people carrying just one copy of the sickle cell allele are resistant to malaria (though they may occasionally have symptoms of sickle cell). So if you live in a region where malaria is common you are at an advantage if you are a heterozygote (i.e. if you carry one sickle cell allele & one normal allele).
Single-celled - Refers to an organism consisting of one cell such as bacteria protozoa & some algae fungi & 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 gives rise to two daughter lineages: the daughter lineages are sister groups & since they arose from the same ancestor at the same time sister groups are always the same age. Sister groups may differ widely in diversity level: one clade may be comprised of a single species while its sister group may be comprised of 100 species.
Somatic mutation - Mutations occurring in cells that do not form gametes mutations that do not end up being carried by eggs or sperm. For example mutations in your skin muscle or liver tissue are somatic mutations.
Speciation - The process by which species form. This involves reproductive isolation of different parts of an ancestral species so that they form distinct descendent species.
Species- Members of populations that actually or potentially interbreed. In this sense a species is the largest gene pool possible under natural conditions.
Subspecies - A grouping of organisms less inclusive than a species. The term is usually applied to groups within a species that have distinct forms & live in a restricted area.
Symbiosis - A relationship between two different organisms that live in close contact with each other. The relationship may be beneficial to both organisms (mutualism) beneficial to just one (commensalism) or harmful to one (parasitism).
Symplesiomorphy - An ancestral character state (i.e. a plesiomorphy) shared by two or more lineages in a particular clade. For example within the clade of terrestrial vertebrates (in which the ancestral character state is "has four legs") both elephants & salam&ers have four legs — & so having four legs is a symplesiomorphy for those 2 lineages.
Synapomorphy - A derived or changed character state (i.e. an apomorphy) shared by two or more lineages in a particular clade. Synapomorphies are indicators of common ancestry. For example within the clade of terrestrial vertebrates the ancestral or plesiomorphic character state is "has four legs." However both owls & parrots have the synapomorphic character state "has two legs & two wings " indicating that owls & 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.
Tetrapod - The animal clade containing vertebrates with sturdy legs (as opposed to fins).
Theory - A broad explanation for a wide range of phenomena. Theories are concise coherent systematic predictive & broadly applicable. They usually integrate many individual hypotheses. A scientific theory must be testable with evidence from the natural world. If a theory can't be tested with experimental results observation or some other means then it is not a scientific theory.
Thorax - In animals with 3 body regions the middle body region between the head & abdomen.
Trachea - An internal tube that carries air into the body of an animal for breathing. For example in humans a trachea carries air to the lungs in insects a network of tracheae carries air directly to tissues throughout the body.
Transcription - The process of building an RNA molecule using DNA as a template. In this process complimentary RNA bases are matched to their DNA counterparts so that the str& of RNA that is produced carries the "imprint" of one str& of the DNA molecule.
Transitional forms - Fossils or organisms that show the transformation from an ancestral form to descendant species' form. For example there is a well-documented fossil record of transitional forms for the evolution of whales from their amphibious ancestor.
Translation Part of the process of decoding an RNA molecule composed of nucleotide bases into a protein composed of amino acids.
Trilobite - Trilobites are an extinct group of arthropods distinguished by the following characters: a body built from a cephalon thorax & pygidium a body divided into three lobes running from head to tail one pair of antennae
Vertebrate - Any member of the animal clade Vertebrata. All vertebrates have a backbone that surrounds & protects the nerve cord a character that they all inherited from their common ancestor. Vertebrates are a subgroup of the chordates. Modern vertebrates include fish sharks mammals & amphibians.
Vestigial structure - A feature that an organism inherited from its ancestor but that is now less elaborate & functional than in the ancestor. Usually vestigial structures are formed when a lineage experiences a different set of selective pressures than its ancestors & selection to maintain the elaboration & function of the feature ends or is greatly reduced.
Vicariance - Vicariance is a process in which a species' range is divided even though the species has remained in place. This might happen through tectonic action geologic activity (like the rise of a mountain range or shift in the course of a river) or other processes. Vicariance is usually contrasted with dispersal as a biogeographic mechanism.

### References

* ↑ 'Reece and Campbell. Biology, 7th ed. New York, 2005. ISBN 0-8053-7171-0 '