The sequence of amino acids in a protein is determined by DNA. In a protein that is present in all species, the amino acid sequence will be different. Once a protein has been chosen, the amino acid sequence of that protein in different species are compared. More similarities, or fewer differences, indicate that two species are closely related. (sorry if im wrong!)
A species that is not native to an ecosystem but is introduced artificially. Usually cause harm to the native species in the area.
They all have a true nucleus and complex cellular organelles and are large and eukaroyote