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Allometric relations can be studied during the growth of a single organism, between different organisms within a species, or between organisms in different species. amino acid: The unit molecular building block of proteins, which are chains of amino acids in a certain sequence.

allopatric speciation: Speciation that occurs when two or more populations of a species are geographically isolated from one another sufficiently that they do not interbreed. There are 20 main amino acids in the proteins of living things, and the properties of a protein are determined by its particular amino acid sequence.

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

Most amphibians are found in damp environments and they occur on all continents except Antarctica.

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.

adaptation: Any heritable characteristic of an organism that improves its ability to survive and reproduce in its environment.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.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.