Question

how does DNA replication cause variation?

Answer 1

By variations I’m assuming you mean mutations that occur during DNA replication. Usually they are point mutations and have little to no effect on the protein produced. (And even more often they are caught and corrected)

Sometimes they will cause a protein to be non-functional, in humans if its fatal, then the cell dies. No harm no foul. Its usually if the mutation is non-fatal but otherwise detrimental effects.

If the mutation is in an oncology gene thats when you get cancer, but you aren’t really going to get a mutation that affects the body design. Like you wont get a mutation that causes a patch of blond hair to grow.