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.

allometry: The relation between the size of an organism and the size of any of its parts.

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.

Then, even if circumstances change such that it no longer provides any survival or reproductive advantage, the behavior will still tend to be exhibited -- unless it becomes positively disadvantageous in the new environment.

adaptive radiation: The diversification, over evolutionary time, of a species or group of species into several different species or subspecies that are typically adapted to different ecological niches (for example, Darwin's finches).

antibacterial: Having the ability to kill bacteria.

antibiotics: Substances that destroy or inhibit the growth of microorganisms, particularly disease-causing bacteria.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 for breeding on land.In a diploid cell there are usually 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 nucleotide sequence.analogous structures: Structures in different species that look alike or perform similar functions (e.g., the wings of butterflies and the wings of birds) that have evolved convergently but do not develop from similar groups of embryological tissues, and that have not evolved from similar structures known to be shared by common ancestors. Note: The recent discovery of deep genetic homologies has brought new interest, new information, and discussion to the classical concepts of analogous and homologous structures.