the common sequences of can be used to show that two organisms come from to common ancestor
Sequences of DNA, RNA and protein can be used to show that the two organisms are related. However, different regions or types of these molecules can be compared for different groups of organisms. In animals, mitochondrial DNA is usually compared while in plants chloroplast DNA is used. In bacteria, 16S rRNA is used for comparison between species. In aerobic organisms cytochrome oxidase gene or protein sequences are compared. In photosynthetic organisms RuBisCo gene and protein sequences are used for comparison and so on.