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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.
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.
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.
amniotes: The group of reptiles, birds, and mammals.allometry: The relation between the size of an organism and the size of any of its parts.For example, an allometric relation exists between brain size and body size, such that (in this case) animals with bigger bodies tend to have bigger brains.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.