a-beer-gut
Explanation
The Joke
A man tells a woman he is "developing a beer gut." She tells him to relax, assuming he means the common phrase for gaining belly fat from drinking beer. But he clarifies: he means he is literally evolving a specialized intestinal structure just for processing beer. He reveals that he has an island of kidnapped human beings where the only nutritional resource is beer, and the beer stream is seeded with Cesium-137 to encourage novel mutations.
He explains that in time, a new branch of Homo sapiens will sprout, with a distinct gastrointestinal tract for the consumption of barley-derived alcoholic beverages. And then... he will have created "a pretty okay pun." The woman laughs and asks how he is paying for all of this, and he asks if she is interested in a long-term investment.
The Humor
The entire elaborate, horrifying scheme -- kidnapping humans, stranding them on an island, irradiating their beer supply to force mutations, and waiting generations for natural selection to produce a new human subspecies -- exists solely to create a literal version of the pun "beer gut." The joke is the absurd disproportion between the effort (crimes against humanity, mad science, evolutionary timescales) and the payoff (a mediocre pun). The man himself acknowledges the pun is only "pretty okay," making the enormous investment even more ridiculous.
The final panel adds another layer: the woman's reaction is not horror at the kidnapping and radiation, but curiosity about the finances. And the man's pitch for a "long-term investment" is both literally accurate (evolution takes a very long time) and a darkly funny understatement of what he is actually proposing. The comic plays on the archetype of the obsessive punster who will go to any length for a wordplay payoff.
References
Cesium-137 is a radioactive isotope that is a common byproduct of nuclear fission. It is referenced here as a mutagen -- a substance that increases the rate of genetic mutation, which is essential for driving evolution. The comic plays on the concept of directed evolution (selectively breeding organisms for specific traits), though the character's approach is far more extreme than any real scientific application. The "beer gut" being referenced is the colloquial term for abdominal obesity associated with heavy beer consumption.