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 thecarbon-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 ScottWeatherbee, 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 to1962. The book looks at the effects of insecticides and pesticides on song bird 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,f ungi, 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; arodlike 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 ofmonophyletic 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 valueof a character.
clock: See molecular clock.clone: A set of genetically identical organisms asexually reproduced from oneancestral 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 judgment. 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.
conodont: A jawless fish that had tiny, tooth-like phosphate pieces that are abundant in the fossil record, these were the earliest known vertebrates.
continental drift: The process by which the continents move as part of large plates floating on Earth's mantle. See plate tectonics.
contrivance: An object or characteristic used or modified to do something different from its usual use.
convergence: The process by which a similar character evolves independently in two species. Also, a synonym for analogy; that is, an instance of a convergently evolved character, or a similar character in two species that was not present in their common ancestor. Examples include wings (convergent in birds, bats, and insects) and camera-type eyes (convergent in vertebrates and cephalopod mollusks).
convergent evolution: The evolution of species from different taxonomic groups toward a similar form; the development of similar characteristics by taxonomically different organisms.
Conway Morris, Simon: Paleobiologist and professor in the Department of Earth Sciences at Cambridge University in the U.K. His research centers around the early evolution of the metazoans, and he is a leading authority on Cambrian and Precambrian fossils. Conway Morris established a link between the Ediacaran fossils, a Burgess Shale fernlike frond Thaumaptilon, and the modern seapens, colonial animals related to the corals.
Cope's rule: The evolutionary increase in body size over geological time in a lineage of populations.
coral (also, rugose coral, tabulate coral): These tiny animals make calcium carbonate skeletons that are well known as a key part of tropical reefs. The skeletons of the extinct rugose and tabulate corals are known from fossils.
cranium: The part of the skull that protects the brain in vertebrates.
creationism: The religious doctrine that all living things on Earth were createdseparately, in more or less their present form, by a supernatural creator, as stated in the Bible; the precise beliefs of different creationist groups vary widely. See separate creation.
creation science: An assortment of many different, non-scientific attempts to disprove evolutionary theory, and efforts to prove that the complexity of living things can be explained only by the action of an "intelligent designer."
Cretaceous: The final geological period of the Mesozoic era that began 144million years ago and ended 65 million years ago. The end of this period is defined most notably by the extinction of the dinosaurs in one of the largest mass extinctions ever to strike the planet.
crinoid: A marine invertebrate animal belonging to a class (Crinoidea; about 700species) of echinoderms, including sea lilies and feather stars. They have a small cup-shaped body covered with hard plates and five radiating pairs of feathery flexible arms surrounding the mouth at the top. Sea lilies, most of which are extinct, are fixed to the sea bottom or some other surface such as a reef by a stalk. Feather stars are free-swimming and are usually found on rocky bottoms. Crinoids occur mainly in deep waters and feed on microscopic plankton and detritus caught by the arms and conveyed to the mouth. The larvae are sedentary. They arose in the Lower Ordovician (between 500 and 460 million years ago), and fossil crinoids are an important constituent of Palaeozoic limestones.
crossing over: The process during meiosis in which the chromosome of a diploid pair exchange genetic material, visible in the light microscope. At a genetic level, it produces recombination.
crustacean: A group of marine invertebrates with exoskeletons and several pairs of legs. They include shrimp, lobsters, crabs, amphipods (commonly known as "sand fleas"), and many more.
Currie, Cameron: A Canadian ecologist and recipient of the 2001 Natural Sciences and Engineering Research Council Doctoral Prize for his research on the complex symbiotic relationship of fungus-growing ants, the fungi they cultivate, mutualistic bacteria that the ants carry on their bodies, and pathogens that attack the fungi.
cytoplasm: The region of a eukaryotic cell outside the nucleus.