Consider this. There is an amoeba. It divides. Gives two exactly identical amoebas. This goes on for another 100 generations or so. In the end all you will have are identical copies of the amoeba you began your experiment with.
Now, let us consider the same scenario on a larger scale. I doubt we would have evolved from the ingenious RNAs.
When an organism suffers mutations that represent itself in a phenotypic scale, it results in variations. These variations may be harmful or beneficial. When harmful, the individual perishes, not being able to sustain the given environment. But when the mutation turns out to be beneficial, it gives the organism an edge to survival when compared with its counterparts. It successfully mates, thus passing on the trait to the next generation. Bingo, it evolves. The nature selects this organism among the other unmutated ones. Bingo, natural selection.