acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on tothe next generation (for example, the large muscles of a weightlifter).
adaptation: Any heritable characteristic of an organism that improves itsability to survive and reproduce in its environment. Also used to describe theprocess of genetic change within a population, as influenced by naturalselection.
adaptive landscape: A graph of the average fitness of a population in relationto the frequencies of genotypes in it. Peaks on the landscape correspond togenotypic frequencies at which the average fitness is high, valleys to genotypicfrequencies 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 numberof offspring that an individual contributes to the next and following generations. If such a behavior is even partly genetically determined, it willtend to become widespread in the population. Then, even if circumstances changesuch that it no longer provides any survival or reproductive advantage, thebehavior will still tend to be exhibited -- unless it becomes positivelydisadvantageous in the new environment.
adaptive radiation: The diversification, over evolutionary time, of a species orgroup of species into several different species or subspecies that are typicallyadapted to different ecological niches (for example, Darwin's finches). The term can also be applied to larger groups of organisms, as in "the adaptive radiationof mammals."
adaptive strategies: A mode of coping with competition or environmentalconditions on an evolutionary time scale. Species adapt when succeedinggenerations emphasize beneficial characteristics.
agnostic: A person who believes that the existence of a god or creator and thenature 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 inmoist situations on land. The term has no direct taxonomic significance. Algae range from macroscopic seaweeds such as giant kelp, which frequently exceeds 30m in length, to microscopic filamentous and single-celled forms such asSpirogyra and Chlorella.
allele: One of the alternative forms of a gene. For example, if a genedetermines the seed color of peas, one allele of that gene may produce greenseeds and another allele produce yellow seeds. In a diploid cell there areusually 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 nucleotidesequence.
allometry: The relation between the size of an organism and the size of any ofits parts. For example, an allometric relation exists b etween brain size andbody size, such that (in this case) animals with bigger bodies tend to havebigger brains. Allometric relations can be studied during the growth of a single organism, between different organisms within a species, or between organisms indifferent species.
allopatric speciation: Speciation that occurs when two or more populations of aspecies are geographically isolated from one another sufficiently that they donot 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 certainviruses, such as HIV.
ammonoid: Extinct relatives of cephalopods (squid, octopi, and chamberednautiluses), these mollusks had coiled shells and are found in the fossil recordof 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 forbreeding on land.
amphibians: The class of vertebrates that contains the frogs, toads, newts, and salamanders. The amphibians evolved in the Devonian period (about 370 millionyears 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 inwater. 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 performsimilar functions (e.g., the wings of butterflies and the wings of birds) that have evolved convergently but do not develop from similar groups ofembryological tissues, and that have not evolved from similar structures known to be shared by common ancestors. Contrast with homologous structures. Note: Therecent discovery of deep genetic homologies has brought new interest, new information, and discussion to the classical concepts of analogous andhomologous 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, andhumans.
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 tosurvive 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, andother 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 tolearn 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 plantor 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 thatususally 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 tomate with other individuals of genotype AA) or phenotype (e.g., tall individualsmate with other tall individuals).
asteroid: A small rocky or metallic body orbitting the Sun. About 20,000 havebeen observed, ranging in size from several hundred kilometers across down todust 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).