I think Darwin's theories were just a piece of the way scientific thought was being transformed in the latter part of the nineteenth century. After all, preceding him by decades was Mary Shelley's Frankenstein, which itself reflects a transformational look at how humans interact with scientific knowledge. DNA was first identified by a Swiss chemist named Friedrich Miescher around the same time that On the Origin of Species was published. Hermann Ludwig Ferdinand von Helmholtz was also a contemporary, forever changing scientific thought with his principle of the law of conservation of energy.
So I would argue that Darwin found himself in the midst of a shifting perspective of scientific influences, and this shift can also be found in the literature of that century. Early in the 1800s, Jane Austen was scripting novels like Pride and Prejudice, which is a work of Romantic literature (popular during this period) and focused on an emotional response and the innate goodness of nature.
As Darwin's theories became more well-known, this sentiment began to shift toward the latter part of the century. An emotional connection to literature was replaced by novels reflecting reasoning and judgement. The Strange Case of Dr. Jekyll and Mr. Hyde, the story of a scientist's desire to eliminate human evil, was published in the late 1800s; the fact that Stevenson was contemplating the origin of human evil and that there could be a scientific means of eliminating it reflects this new scientific thought. Another scientist appears in The Invisible Man; this time, the quest is to make oneself invisible. Again, Wells was testing the limits of what could be possible through new understandings of science.
I would argue that ideas such as the capacity of the strongest individuals to survive shows up in literature such as The Adventures of Huckleberry Finn and even in the poetry of Emily Dickinson. Darwin's theories reshaped human thought by allowing people to see the ever-changing nature of human capacities, and these types of characters are also reflected in works such as A Doll's House and A Christmas Carol—works where the protagonists completely transform their character by the plot's conclusion.