A half-baked thought occurred to me the other day. The old Catholic Encyclopedia treats of three types of evil, one of which it calls "metaphysical evil," which seems to largely correspond to the idea of survival of the fittest, or more broadly that the nature of the universe is that it seems to be in a struggle with itself.
We know that the universe and many things in it preceded us, and that, cosmically speaking, we are very late to the party. We know that this "metaphysical evil" is a fact of nature, and that it seems to have in very large part shaped the world that we came into, and that it continues to shape it. We were not brought into a peaceful utopia that was shattered as a result of original sin. The world was in turmoil before us.
Our primordial nature, while given to us with original justice, contained its right-ordered-ness by grace, not by our natural nature itself, which without God's grace shared in this metaphysical evil. Our primordial nature was, shaped by this world striving with itself, inclined also to this striving. But through that original gift of grace, we were enabled to subject that nature and order it properly, in subjection to God and to live truly freely in that state of original justice, with true freedom to choose to embrace that right ordering or to turn away from it, as Adam did, thereby destroying that state of original justice and, in the economy of salvation, calling for God's further action to restore it.
At the same time, we must acknowledge that God knew Adam's choice before he made it. He knew that he would forfeit that original gift of justice. He knew before the foundations of the cosmos this would happen. Indeed, he ordered the cosmos in such a way to allow for it to happen--precisely so that he could bless us with the greater gift of his Incarnation and Redemption through the Cross.
Thus it seems to me--and this is just my speculation on the matter, not any Catholic teaching per se that I am aware of--that this striving nature, this principle of the survival of the fittest, this mechanism of evolution was part and parcel of God's Providence that led to the need for his own Incarnation and Sacrifice as part of his plan. Thus it seems it can be said that evolution is part of the economy of salvation.