Two groups of related living things are no longer considered to be the same species when a. They can no longer interbreed.
b. They do not eat the same food.
c. They do not look the same.
d. They become physically separated from each other.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products

this would be called the responsiveness