Consider a population of bacteria that came from a single ancestor. because bacteria reproduce asexually, there should be no variation in this population, but that violates the first postulate of natural selection. how is it then that this population of bacteria can still evolve?

Bacteria undergo evolution through mutation. Mutation is the random (though not entirely) distribution of the DNA sequence which later then gives rise to other phenotypical features that aids the bacteria to adapt to the environment they are in. Natural selection does not necessarily apply to this evolutionary feature rather mutation does!