According to the professor, how was Darwin’s On the Origin of Species related to American realism?

[Professor (male)] Q6 Class, during the late nineteenth century, there was a movement going on
   in American literature. I’m talking about the period roughly between 1860 and
   the turn of the century. Can anyone name the movement i’m talking about?
[Student A (female)] Yeah...that was the period when realism dominated American literature.
[Professor] Exactly. And can you...um, describe any characteristics of American realism?
[Student A] Sure. During that time, writers were focusing on, um, representing reality as
   accurately as they could.That’s why it’s called realism, I guess.
[Professor] Right. In one sense, realism is a technique writers used to convey details about
   the world around them. Q7(A) But in another sense, it’s also a certain kind of
   subject matter--the life of middle-class Americans. Q7(C) Um, it was a response
   to--or a reaction against--the style of writing that was popular in American
   literature during the early nineteenth century. That was romanticism--and it
   was all about nature, art, inspiration, and spirituality. So realism was, in a way,
   resisting the ideas of romanticism. Q7(B) Instead, realism prized the scientific
   method, systematic investigation, and rationality above all else.
   Q8 I think it’s important to place realist literature in a historical context, so
   let’s spend a minute talking about what was going on in American society
   during the mid-to-late nineteenth century. Would anyone be willing to venture a
   suggestion?
[Student B (male)] Well, one major event was the Civil War. 1861 to 1865, right?
[Professor] Great. You’re absolutely right. The Civil War had a huge effect on American
   society and identity. It changed everything. I mean, look at the South--the
   war really destroyed a lot of the South. Another result of the war was a rise in
   technology, which had been developed for use in the war but ended up leading
   into a period of industrialization. Oh, and let’s not forget the way the war directly
   influenced literature--it created a new market for books and articles that featured
   everyday people as subjects, like soldiers, the working class, and the poor.
[Student B] Q10 Yeah, I studied that in my American history class last year. Some
   of the men who, uh, served in the war became really famous because, you
   know, they’d, uh...
[Professor] [interrupting] Uh...I’ll stop you there, Martin. We’ve got a lot to get
   through today and only limited time. So, uh, tell me--what else was happening
   in American society at the time?
[Student A] I know there were also lots of important scientific developments and, uh,
   breakthrough theories being developed back then. Maybe they had an effect.
[Professor] That’s a great point! Q9 Things like the publication of Darwin’s book, On
   the Origin of Species, which, uh, you know, challenged people’s fundamental
   understanding of the world...these sorts of things made a big impact on
   American culture. And this shift interested writers, who explored related topics
   in their work. Anyway, what I’m getting at here is the idea that there was an
   awful lot of change occurring during this period of American history, and it was
   all happening rather quickly. Some have argued that realism was, in essence,
   a way of, um, analyzing and digesting those changes...changes that could’ve
   seemed a little bit frightening or even threatening at the time.
   OK. And here’s how all of that translates into realist literature--these are some
   of the...characteristics of realist literature. First of all, it was very true to life--
   to the details of reality--to the point where, um, authors were making sacrifices
   in terms of plot development just so they can keep their writing as "real" as
   possible. Another feature of realist literature is the way it rejects formal or overly
   sophisticated prose in favor of more authentic, spoken styles of language. It was
   mainly written in vernacular English. Q7(A) Oh, another thing that was common
   among realist writings was a focus on class--mainly the middle class, which
   had recently begun to grow in size.
   Now, can anyone name an author who wrote during this period?
[Student B] [uncertainly] Um...Emerson wasn’t a realist writer...was he?
[Professor] Ralph Waldo Emerson? No...he actually falls into the period right before
   realism--romanticism.
[Student B] OK. How about Mark Twain?
[Professor] Yes! Q11 Mark Twain was indeed a realist author. His contribution
   to American literature was quite important, too. Some people credit Twain with
   helping American writers find their own voice.
   See, around Twain’s time--or immediately before it, rather--American authors
   were struggling to prove themselves. They were, um, intimidated...by the
   legacy of England’s literary figures, and they basically felt that, as American
   authors, they were working in the shadow of British writers. Wanting to prove
   themselves, they went overboard with, um, exaggerated styles of writing. But
   Twain--he pioneered a new style, one that American writers could claim as their
   own. Twain’s two greatest novels--Adventures of Huckleberry Finn, and The
   Adventures of Tom Sawyer--they represented this new style, or, uh, voice. And,
   uh, their most important feature was their use of authentic, colloquial American
   speech. Twain’s writing--because it was so concerned with capturing what was
   real...true in his culture--it helped Americans come to a better understanding of
   who they were.