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.

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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.

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.

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.

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.

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