The diameter of the aperture determines the intensity of light admitted. 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.
The pupil of a human eye is a self-adjusting aperture. archetype: The original form or body plan from which a group of organisms develops.
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.
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.
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.
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.
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. 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. 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.
amino acid sequence: A series of amino acids, the building blocks of proteins, usually coded for by DNA.