Evolution is the process by which this occurs and it is spread out over many generations. Thus, the short minimal definition of biological evolution is, Evolution is a process that results in heritable changes in a population spread over many generations. This is a good working scientific definition of evolution; one that can be used to distinguish between evolution and similar changes that are not evolution.
Bone, cartilage, or dentine is used in the Ostracoderm fish and turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters (see arthropod exoskeleton), and in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs (see Mollusc shell), brachiopods, and some tube-building polychaete worms.
Cetaceans (whales, dolphins, and porpoises) do not have hooves but are actually the closest relatives of even-toed hoofed mammals. A group of animals called subungulates are sometimes grouped with hoofed mammals. This group includes elephants, aardvarks, hyraxes, manatees, and dugongs.Elephants and aardvarks both have tough nails, but none of the subungulates have full hooves.ven-toed hoofed mammals include pigs and their relatives, hippopotamuses, camels, llamas, alpacas, vicunas, guanacos, deer, giraffes, okapis, cows and their relatives, goats and their relatives, and antelope. Odd-toed hoofed mammals include horses, donkeys, zebras, tapirs, and rhinoceroses.