They have moist scaleless skin which is used to supplement the lungs in gas exchange.
The eggs are soft and vulnerable to drying, therefore reproduction commonly occurs in water.
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.
Most amphibians are found in damp environments and they occur on all continents except Antarctica.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.antibacterial: Having the ability to kill bacteria.antibiotics: Substances that destroy or inhibit the growth of microorganisms, particularly disease-causing bacteria.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).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.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.anatomy: (1) The structure of an organism or one of its parts. ancestral homology: Homology that evolved before the common ancestor of a set of species, and which is present in other species outside that set of species. anthropoid: A member of the group of primates made up of monkeys, apes, and humans.