evolutionist
Explanation
The Joke
A child is in bed, and tells a parent (presumably the mother) that there's an evolutionist under the bed. The mother reassures the child: "Oh son, that's just a stereotype. All he wants to do is rant about Richard Dawkins and natural selection. He won't even stay awake through a whole sentence."
In the final panel, we see under the bed, where an exhausted-looking man is sprawled out, muttering "Mmmm... out... line... 6..." — apparently having fallen asleep mid-lecture, confirming the mother's description.
The Humor
The comic inverts the classic "monster under the bed" childhood fear. Instead of a scary creature, the "evolutionist" is just a boring academic who lectures about evolution and falls asleep mid-sentence. The mother's reassurance mirrors the standard parental response to monsters under the bed, but instead of "there's no monster," she says the equivalent of "yes, he's there, but he's harmless because he's incredibly dull."
The joke works on two levels: it parodies anti-evolution sentiment (where "evolutionist" is treated as a scary boogeyman), and it pokes fun at academics who can't help but lecture about their subject, even to the point of boring themselves to sleep.
Broader Context
SMBC frequently plays with the cultural war between science and religion. Here, Weinersmith playfully mocks both sides — the anti-science crowd for treating evolutionary biologists as monsters, and the academics themselves for being so tediously absorbed in their subject that they're more soporific than scary.