Most natural selection is purifying selection, that is it weeds out harmful random mutations. For the most part, natural selection actually keeps things simple and just maintains what is currently in place. If there are alleles in a population and 1 does not have any fitness advantage over another, then 1 eventually taking over the population can simply be explained by a random evolutionary mechanism such as genetic drift.
Natural selection on the other hand is the only mechanism for adaptive evolutionary change. The complex adaptations we see in nature such as the vertebrate eye for example, represent the accumulation of small changes such that each were more fit than another. There is no mechanism for natural selection to see ahead, each small step had to be beneficial enough on its own to be selected for, there is no perfect archetype being strived for.
Since natural selection can only work with existing variation which must necessarily arise from random variation, it is not possible for any adaptation to simply come out of nowhere. Natural selection tinkers with existing structures to give rise to new ones.
Is there a way to reconcile the fact that in general natural selection tends to be conservative rather than creative with the fact that natural selection is the only known mechanism to generate adaptive structures? I think so. Sean Carroll explained in his book Endless Forms Most Beautiful that over evolutionary time the force of natural selection tends to reduce the number of structures while at the same time increasing their specialization. For example, if a gene is duplicated several times, the resulting redundancy will loosen the constraints of natural selection on these gene sequences. As the conservative force of natural selection pares down this redundancy, new functions will be inevitably carved out from this variation.
In other words, natural selection is not a builder or an inventor. Natural selection is a sculptor.
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