Evolution by natural selection requires three ingredients:
If any of these three factors are absent, no evolution can occur.
They work *together*.
The reason that variation is important is because there is competition among members of the same species. Namely, more individuals are born than can possibly survive to reproduce. So that means that some will produce more offspring than others.
Variation refers to the fact that some combinations of traits will produce some slight advantage over others in this competition for survival and reproduction. So *ON AVERAGE* they produce more offspring, and so those traits are the ones that spread into a higher percentage of the population.
If there is no variation ... nothing that gives any one individual an advantage over any other in this competition for survival and reproduction ... then it is pure randomness which ones get to reproduce more. So the next generation will be exactly the same as this one ... and so on. So there is no evolution.