This is a good question. If we define Darwinism as the belief that the strong will see their wealth and power increase while the weak should see the opposite, then the there is Darwinism in the novella, The Pearl.
Kino, Juana, and Coyotito are a poor family. When Coyotito is bit by a scorpion, they rush him to the doctor. However, they realize that the doctor won't treat him, because they have no money. So, they have to treat him at home. This shows that there is a huge divide between the wealthy and the poor—or the colonizers and the natives. We can say the rich get richer and the poor get poorer.
Later that day when Kino finds a pearl of great price, he has dreams of all that he can do for his family. So he sets out to find a buyer. As the story unfolds, people are not kind and try to take advantage of him. The pearl only brings destruction and eventually the death of Coyotito.
In the end, they are still poor—in fact, poorer—and the rich are still rich.