The sharp reduction of numbers of a population and with it the gene pool through a severe epidemic is an example of :

The Bottleneck Effect

Explanation:

A Bottleneck effect is the sharp reduction of numbers of a population through environmental effects or human activity. It causes a loss of the genetic pool and its variety; genetic variation lowers and increases with time as random mutations occur.  

multiplying and dividing the two decimals and the eqations by the same number of the overalls

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.