Why would natural selection not occur without genetic variations in species

Natural selection cannot occur without genetic variation because without genetic variation all the species would be the same and as such there would be no fitter species thus nature cant select one for survival.

When the stimulation frequency was increased to the point in which muscle relaxation did not occur the amount of force progressively increased with each stimulus. When muscle twitches overlap, no muscles relax, and stimuli occurs one after another in a short amount of time, the muscle contraction is higher than if a single stand alone contraction had occurred; also known as wave summation.