- Abiotic Components (Aibiotic Factors) - are non-living chemicaland physical factors in the environment, which affect ecosystems. Abiotic phenomena underlie all of biology.
- Albinism - is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of tyrosinase, a copper-containing enzyme involved in the production of melanin.
- Alkaptonuria - is a rare condition in which a person's urine turns a dark brownish-black color when exposed to air.
- Allele - An allele is a viable DNA (deoxyribonucleic acid) coding that occupies a given locus (position) on a chromosome.
- Allopatric Speciation - the process by which, in theory, a new species originates when a population that is geographically separated from the rest of the species becomes unable to reproduce with the original population.
- Analgous Structure - Body parts that serve the same function in different organisms, but differ in structure and embryological development; e. g., the wings of insects and birds.
- Artificial Selection - The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
- Asthenosphere - A zone of the earth's mantle that lies beneath the lithosphere and consists of several hundred kilometers of deformable rock.
- Australopithecus - group of extinct creatures closely related to, if not actually ancestors of, modern human beings and known from a series of fossils found at numerous sites in eastern, central, and southern Africa The various species of Australopithecus lived during 5.3 to 2.6 million years ago. As characterized by the fossil evidence, they bore a combination of human- and apelike traits.