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.
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.
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 certain viruses, such as HIV.
amphibians: The class of vertebrates that contains the frogs, toads, newts, and salamanders.
Species adapt when succeeding generations emphasize beneficial characteristics.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).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.