acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weightlifter).

adaptation: Any heritable characteristic of an organism that improves its ability to survive and reproduce in its environment. Also used to describe the process of genetic change within a population, as influenced by natural selection.

adaptive landscape: A graph of the average fitness of a population in relation to the frequencies of genotypes in it. Peaks on the landscape correspond to genotypic frequencies at which the average fitness is high, valleys to genotypic frequencies at which the average fitness is low. Also called a fitness surface.

adaptive logic: A behavior has adaptive logic if it tends to increase the number of offspring that an individual contributes to the next and following generations. If such a behavior is even partly genetically determined, it will tend to become widespread in the population. Then, even if circumstances change such that it no longer provides any survival or reproductive advantage, the behavior will still tend to be exhibited -- unless it becomes positively disadvantageous in the new environment.

adaptive radiation: The diversification, over evolutionary time, of a species or group of species into several different species or subspecies that are typically adapted to different ecological niches (for example, Darwin's finches). The term can also be applied to larger groups of organisms, as in "the adaptive radiation of mammals."

adaptive strategies: A mode of coping with competition or environmental conditions on an evolutionary time scale. Species adapt when succeeding generations emphasize beneficial characteristics.

agnostic: A person who believes that the existence of a god or creator and the nature of the universe is unknowable.

algae: An umbrella term for various simple organisms that contain chlorophyll (and can therefore carry out photosynthesis) and live in aquatic habitats and in moist situations on land. The term has no direct taxonomic significance. Algae range from macroscopic seaweeds such as giant kelp, which frequently exceeds 30 m in length, to microscopic filamentous and single-celled forms such as Spirogyra and Chlorella.

allele: One of the alternative forms of a gene. For example, if a gene determines the seed color of peas, one allele of that gene may produce green seeds and another allele produce yellow seeds. In a diploid cell there are usually two alleles of any one gene (one from each parent). Within a population there may be many different alleles of a gene; each has a unique nucleotide sequence.

allometry: The relation between the size of an organism and the size of any of its parts. For example, an allometric relation exists between brain size and body size, such that (in this case) animals with bigger bodies tend to have bigger brains. Allometric relations can be studied during the growth of a single organism, between different organisms within a species, or between organisms in different species.

allopatric speciation: Speciation that occurs when two or more populations of a species are geographically isolated from one another sufficiently that they do not interbreed.

allopatry: Living in separate places. Compare with sympatry.

amino acid: The unit molecular building block of proteins, which are chains of amino acids in a certain sequence. There are 20 main amino acids in the proteins of living things, and the properties of a protein are determined by its particular amino acid sequence.

amino acid sequence: A series of amino acids, the building blocks of proteins, usually coded for by DNA. Exceptions are those coded for by the RNA of certain viruses, such as HIV.

ammonoid: Extinct relatives of cephalopods (squid, octopi, and chambered nautiluses), these mollusks had coiled shells and are found in the fossil record of the Cretaceous period.

amniotes: The group of reptiles, birds, and mammals. These all develop through an embryo that is enclosed within a membrane called an amnion. The amnion surrounds the embryo with a watery substance, and is probably an adaptation for breeding on land.

amphibians: The class of vertebrates that contains the frogs, toads, newts, and salamanders. The amphibians evolved in the Devonian period (about 370 million years ago) as the first vertebrates to occupy the land. They have moist scaleless skin which is used to supplement the lungs in gas exchange. The eggs are soft and vulnerable to drying, therefore reproduction commonly occurs in water. Amphibian larvae are aquatic, and have gills for respiration; they undergo metamorphosis to the adult form. Most amphibians are found in damp environments and they occur on all continents except Antarctica.

analogous structures: Structures in different species that look alike or perform similar functions (e.g., the wings of butterflies and the wings of birds) that have evolved convergently but do not develop from similar groups of embryological tissues, and that have not evolved from similar structures known to be shared by common ancestors. Contrast with homologous structures. Note: The recent discovery of deep genetic homologies has brought new interest, new information, and discussion to the classical concepts of analogous and homologous structures.

anatomy: (1) The structure of an organism or one of its parts. (2) The science that studies those structures.

ancestral homology: Homology that evolved before the common ancestor of a set of species, and which is present in other species outside that set of species. Compare with derived homology.

anthropoid: A member of the group of primates made up of monkeys, apes, and humans.

antibacterial: Having the ability to kill bacteria.

antibiotics: Substances that destroy or inhibit the growth of microorganisms, particularly disease-causing bacteria.

antibiotic resistance: A heritable trait in microorganisms that enables them to survive in the presence of an antibiotic.

aperture: Of a camera, the adjustable opening through which light passes to reach the film. The diameter of the aperture determines the intensity of light admitted. The pupil of a human eye is a self-adjusting aperture.

aquatic: Living underwater.

arboreal: Living in trees.

archeology: The study of human history and prehistory through the excavation of sites and the analysis of physical remains, such as graves, tools, pottery, and other artifacts.

archetype: The original form or body plan from which a group of organisms develops.

artifact: An object made by humans that has been preserved and can be studied to learn about a particular time period.

artificial selection: The process by which humans breed animals and cultivate crops to ensure that future generations have specific desirable characteristics. In artificial selection, breeders select the most desirable variants in a plant or animal population and selectively breed them with other desirable individuals. The forms of most domesticated and agricultural species have been produced by artificial selection; it is also an important experimental technique for studying evolution.

asexual reproduction: A type of reproduction involving only one parent that ususally produces genetically identical offspring. Asexual reproduction occurs without fertilization or genetic recombination, and may occur by budding, by division of a single cell, or by the breakup of a whole organism into two or more new individuals.

assortative mating: The tendency of like to mate with like. Mating can be assortative for a certain genotype (e.g., individuals with genotype AA tend to mate with other individuals of genotype AA) or phenotype (e.g., tall individuals mate with other tall individuals).

asteroid: A small rocky or metallic body orbitting the Sun. About 20,000 have been observed, ranging in size from several hundred kilometers across down to dust particles.

atheism: The doctrine or belief that there is no god.

atomistic: (as applied to theory of inheritance) Inheritance in which the entities controlling heredity are relatively distinct, permanent, and capable of independent action. Mendelian inheritance is an atomistic theory because in it, inheritance is controlled by distinct genes.

australopithecine: A group of bipedal hominid species belonging to the genus Australopithecus that lived between 4.2 and 1.4 mya.

Australopithecus afarensis: An early australopithecine species that was bipedal; known fossils date between 3.6 and 2.9 mya (for example, Lucy).

autosome: Any chromosome other than a sex chromosome.

avian: Of, relating to, or characteristic of birds (members of the class Aves).

bacteria: Tiny, single-celled, prokaryotic organisms that can survive in a wide variety of environments. Some cause serious infectious diseases in humans, other animals, and plants.

base: The DNA molecule is a chain of nucleotide units; each unit consists of a backbone made of a sugar and a phosphate group, with a nitrogenous base attached. The base in a unit is one of adenine (A), guanine (G), cytosine (C), or thymine (T). In RNA, uracil (U) is used instead of thymine. A and G belong to the chemical class called purines; C, T, and U are pyrimidines.

Batesian mimicry: A kind of mimicry in which one non-poisonous species (the Batesian mimic) mimics another poisonous species.

belemnite: An extinct marine invertebrate that was related to squid, octopi, and chambered nautiluses. We know from the fossil record that belemnites were common in the Jurassic period and had bullet-shaped internal skeletons.

big bang theory: The theory that states that the universe began in a state of compression to infinite density, and that in one instant all matter and energy began expanding and have continued expanding ever since.

biodiversity (or biological diversity): A measure of the variety of life, biodiversity is often described on three levels. Ecosystem diversity describes the variety of habitats present; species diversity is a measure of the number of species and the number of individuals of each species present; genetic diversity refers to the total amount of genetic variability present.

bioengineered food: Food that has been produced through genetic modification using techniques of genetic engineering.

biogenetic law: Name given by Haeckel to recapitulation.

biogeography: The study of patterns of geographical distribution of plants and animals across Earth, and the changes in those distributions over time.

biological species concept: The concept of species, according to which a species is a set of organisms that can interbreed among each other. Compare with cladistic species concept, ecological species concept, phenetic species concept, and recognition species concept.

biometrics: The quantitative study of characters of organisms.

biosphere: The part of Earth and its atmosphere capable of sustaining life.

bipedalism: Of hominids, walking upright on two hind legs; more generally, using two legs for locomotion.

bivalve: A mollusk that has a two-part hinged shell. Bivalves include clams, oysters, scallops, mussels, and other shellfish.

Blackmore, Susan: A psychologist interested in memes and the theory of memetics, evolutionary theory, consciousness, the effects of meditation, and why people believe in the paranormal. A recent book, The Meme Machine, offers an introduction to the subject of memes.

blending inheritance: The historically influential but factually erroneous theory that organisms contain a blend of their parents' hereditary factors and pass that blend on to their offspring. Compare with Mendelian inheritance.

botanist: A scientist who studies plants.

brachiopod: Commonly known as "lamp shells," these marine invertebrates resemble bivalve mollusks because of their hinged shells. Brachiopods were at their greatest abundance during the Paleozoic and Mesozoic eras.

Brodie, Edmund D., III: A biologist who studies the causes and evolutionary implications of interactions among traits in predators and their prey. Much of his work concentrates on the coevolutionary arms race between newts that posess tetrodotoxin, one of the most potent known toxins, and the resistant garter snakes who prey on them.

Brodie, Edmund D., Jr.: A biologist recognized internationally for his work on the evolution of mechanisms in amphibians that allow them to avoid predators. These mechanisms include toxins carried in skin secretions, coloration, and behavior.

Bruner, Jerome: A psychologist and professor at Harvard and Oxford Universities, and a prolific author whose book, The Process of Education, encouraged curriculum innovation based on theories of cognitive development.

bryozoan: A tiny marine invertebrate that forms a crust-like colony; colonies of bryozoans may look like scaly sheets on seaweed.

Burney, David: A biologist whose research has focused on endangered species, paleoenvironmental studies, and causes of extinction in North America, Africa, Madagascar, Hawaii, and the West Indies.

carbon isotope ratio: A measure of the proportion of the carbon-14 isotope to the carbon-12 isotope. Living material contains carbon-14 and carbon-12 in the same proportions as exists in the atmosphere. When an organism dies, however, it no longer takes up carbon from the atmosphere, and the carbon-14 it contains decays to nitrogen-14 at a constant rate. By measuring the carbon-14-to-carbon-12 ratio in a fossil or organic artifact, its age can be determined, a method called radiocarbon dating. Because most carbon-14 will have decayed after 50,000 years, the carbon isotope ratio is mainly useful for dating fossils and artifacts younger than this. It cannot be used to determine the age of Earth, for example.

carnivorous: Feeding largely or exclusively on meat or other animal tissue.

Carroll, Sean: Developmental geneticist with the Howard Hughes Medical Institute and professor at the University of Wisconsin-Madison. From the large-scale changes that distinguish major animal groups to the finely detailed color patterns on butterfly wings, Dr. Carroll's research has centered on those genes that create the "molecular blueprint" for body pattern and play major roles in the origin of new features. Coauthor, with Jennifer Grenier and Scott Weatherbee, of From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design.

Carson, Rachel: A scientist and writer fascinated with the workings of nature. Her best-known publication, Silent Spring, was written over the years 1958 to 1962. The book looks at the effects of insecticides and pesticides on songbird populations throughout the United States. The publication helped set off a wave of environmental legislation and galvanized the emerging ecological movement.

Castle, W.E.: An early experimental geneticist, his 1901 paper was the first on Mendelism in America. His Genetics of Domestic Rabbits, published in 1930 by Harvard University Press, covers such topics as the genes involved in determining the coat colors of rabbits and associated mutations.

cell: The basic structural and functional unit of most living organisms. Cell size varies, but most cells are microscopic. Cells may exist as independent units of life, as in bacteria and protozoans, or they may form colonies or tissues, as in all plants and animals. Each cell consists of a mass of protein material that is differentiated into cytoplasm and nucleoplasm, which contains DNA. The cell is enclosed by a cell membrane, which in the cells of plants, fungi, algae, and bacteria is surrounded by a cell wall. There are two main types of cell, prokaryotic and eukaryotic.

Cenozoic: The era of geologic time from 65 mya to the present, a time when the modern continents formed and modern animals and plants evolved.

centromere: A point on a chromosome that is involved in separating the copies of the chromosome produced during cell division. During this division, paired chromosomes look somewhat like an X, and the centromere is the constriction in the center.

cephalopod: Cephalopods include squid, octopi, cuttlefish, and chambered nautiluses. They are mollusks with tentacles and move by forcing water through their bodies like a jet.

character: Any recognizable trait, feature, or property of an organism. In phylogenetic studies, a character is a feature that is thought to vary independantly of other features, and to be derived from a corresponding feature in a common ancestor of the organisms being studied. A "character state" is one of the possible alternative conditions of the character. For example, "present" and "absent" are two states of the character "hair" in mammals. Similarly, a particular position in a DNA sequence is a character, and A, T, C, and G are its possible states (see bases.)

character displacement: The increased difference between two closely related species where they live in the same geographic region (sympatry) as compared with where they live in different geographic regions (allopatry). Explained by the relative influences of intra- and inter-specific competition in sympatry and allopatry.

chloroplast: A structure (or organelle) found in some cells of plants; its function is photosynthesis.

cholera: An acute infectious disease of the small intestine, caused by the bacterium Vibrio cholerae which is transmitted in drinking water contaminated by feces of a patient. After an incubation period of 1-5 days, cholera causes severe vomiting and diarrhea, which, if untreated, leads to dehydration that can be fatal.

chordate: A member of the phylum Chordata, which includes the tunicates, lancelets, and vertebrates. They are animals with a hollow dorsal nerve cord; a rodlike notochord that forms the basis of the internal skeleton; and paired gill slits in the wall of the pharynx behind the head, although in some chordates these are apparent only in early embryonic stages. All vertebrates are chordates, but the phylum also contains simpler types, such as sea-squirts, in which only the free-swimming larva has a notochord.

chromosomal inversion: See inversion.

chromosome: A structure in the cell nucleus that carries DNA. At certain times in the cell cycle, chromosomes are visible as string-like entities. Chromosomes consist of the DNA with various proteins, particularly histones, bound to it.

chronology: The order of events according to time.

Clack, Jenny: A paleontologist at Cambridge University in the U.K., Dr. Clack studies the origin, phylogeny, and radiation of early tetrapods and their relatives among the lobe-finned fish. She is interested in the timing and sequence of skeletal and other changes which occurred during the transition, and the origin and relationships of the diverse tetrapods of the late Paleozoic.

clade: A set of species descended from a common ancestral species. Synonym of monophyletic group.

cladism: Phylogenetic classification. The members of a group in a cladistic classification share a more recent common ancestor with one another than with the members of any other group. A group at any level in the classificatory hierarchy, such as a family, is formed by combining a subgroup at the next lowest level (the genus, in this case) with the subgroup or subgroups with which it shares its most recent common ancestor. Compare with evolutionary classification and phenetic classification.

cladistic species concept: The concept of species, according to which a species is a lineage of populations between two phylogenetic branch points (or speciation events). Compare with biological species concept, ecological species concept, phenetic species concept, and recognition species concept.

cladists: Evolutionary biologists who seek to classify Earth's life forms according to their evolutionary relationships, not just overall similarity.

cladogram: A branching diagram that illustrates hypotheses about the evolutionary relationships among groups of organisms. Cladograms can be considered as a special type of phylogenetic tree that concentrates on the order in which different groups branched off from their common ancestors. A cladogram branches like a family tree, with the most closely related species on adjacent branches.

class: A category of taxonomic classification between order and phylum, a class comprises members of similar orders. See taxon.

classification: The arrangement of organisms into hierarchical groups. Modern biological classifications are Linnaean and classify organisms into species, genus, family, order, class, phylum, kingdom, and certain intermediate categoric levels. Cladism, evolutionary classification, and phenetic classification are three methods of classification.

cline: A geographic gradient in the frequency of a gene, or in the average value of a character.

clock: See molecular clock.

clone: A set of genetically identical organisms asexually reproduced from one ancestral organism.

coadaptation: Beneficial interaction between (1) a number of genes at different loci within an organism, (2) different parts of an organism, or (3) organisms belonging to different species.

codon: A triplet of bases (or nucleotides) in the DNA coding for one amino acid. The relation between codons and amino acids is given by the genetic code. The triplet of bases that is complementary to a condon is called an anticodon; conventionally, the triplet in the mRNA is called the codon and the triplet in the tRNA is called the anticodon.

coelacanth: Although long thought to have gone extinct about 65 million years ago, one of these deep-water, lungless fish was caught in the 1930s. Others have since been caught and filmed in their natural habitat.

coevolution: Evolution in two or more species, such as predator and its prey or a parasite and its host, in which evolutionary changes in one species influence the evolution of the other species.

cognitive: Relating to cognition, the mental processes involved in the gathering, organization, and use of knowledge, including such aspects as awareness, perception, reasoning, and judgement. The term refers to any mental "behaviors" where the underlying characteristics are abstract in nature and involve insight, expectancy, complex rule use, imagery, use of symbols, belief, intentionality, problem-solving, and so forth.

common ancestor: The most recent ancestral form or species from which two different species evolved.

comparative biology: The study of patterns among more than one species.

comparative method: The study of adaptation by comparing many species.

concerted evolution: The tendency of the different genes in a gene family to evolve in concert; that is, each gene locus in the family comes to have the same genetic variant.

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