This is current orthodoxy. But it is widely admitted, nevertheless, that the concept is of doubtful validity.
Nature can do nothing of the kind.
Nature may eliminate what is not immediately useful but it cannot by itself foresee what might be useful in the future unless one attributes to it some kind of purposeful planning, and this is precisely what the evolutionist is most anxious to avoid. Yet by capitalizing the word Nature and personifying it by referring to it as her or she, evolutionists do really deify Nature while dethroning Nature's Creator. Man is in the position of being able to see ahead and can therefore take advantage of chance mutations that are recognized to be of possible use sometime in the future. As a rule, Nature would merely eliminate them, for Nature's way is to discourage the exceptional and favor only the normal. Populations of animals do not encourage the persistence of the extremes of their range because they are constantly crossed with the normals which therefore overwhelm them and thus place natural limits on variability, once the form is established for any particular habitat.