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.
Amphibian larvae are aquatic, and have gills for respiration; they undergo metamorphosis to the adult form.
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.
acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weightlifter).
adaptation: Any heritable characteristic of an organism that improves its ability to survive and reproduce in its environment.
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.Also used to describe the process of genetic change within a population, as influenced by natural selection.adaptive landscape: A graph of the average fitness of a population in relation to the frequencies of genotypes in it.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.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.