If the stimulus reaches the threshold an action potential will be generated. If the stimulus doesn't reach it no action potential will be generated. If the stimulus will be stronger the action potential will have the same amplitude. The refractory period makes it harder to generate an action potential so they decrease the frequency of action potential production.
DNA and the genetic code reflect the shared ancestry of life. DNA comparisons can show how related species are. Biogeography. The global distribution of organisms and the unique features of island species reflect the evolution and geological change.
