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.

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.

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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.

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.

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.

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.

antibacterial: Having the ability to kill bacteria.

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

Amphibian larvae are aquatic, and have gills for respiration; they undergo metamorphosis to the adult form.