Evolutionary theories are the flexible equivalent of species variability. To evaluate and support these theories, certain molecular clocks are used to know the relationship of species. These are done by considering the variations of every species' sequences of DNA. It is relevant to know the evidence that all living things contribute identical biochemical building blocks. Common ancestry and biological evolution is thereby supported by evidence regarding differences of structures. Homologous structures explains the same ancestry. Analogous structures, on the other hand, explains that the presence of identical features is the outcome of identical selective pressures.