The goal of the evolution is to produce better and better creatures with time. But new better creatures can only arise from existing best creatures. The preservation force prevents the best creature by some mechanisms which prevent mutation or the effect of mutation. The example for such mechanisms are proof reading machinery, introns, and diploid genes, etc.
On the other hand, there is an evolutionary pressure on the creature to diversify (in order to explore better life forms). This diversification force is a natural result of “survival of the fittest”, fitter the creature is better is its survival chances. Therefore, the search for fittest creature is always one.
So it is the balance of these two forces which drives evolution. This can be seen as a paradox that: Evolution wants the fittest creature, but the fitter the creature, more the difficulty in searching fittest creature. But it isn’t a paradox, it is a balance of these two forces, for if it were a paradox we(humans) wouldn’t be here to discuss this.