drive
Explanation
This comic features an alien (a tall green figure) addressing a group of humans. The alien reveals that it has introduced a "gene drive" into the human population -- a real genetic engineering concept where a gene modification is designed to spread rapidly through a population by ensuring it is inherited at higher-than-normal rates.
The alien explains that this gene drive causes an ever-higher percentage of incredibly attractive people who want non-stop sex with average-looking people. Someone asks if this is really happening, and the alien confirms it is already spreading through the population. The twist comes at the end: the alien reveals that "because of the thing it does to you," and a human responds, "I suppose it's driven a wedge between us" (or a similar punchline about the social consequences).
The joke plays on wish-fulfillment fantasies (attractive people wanting to have sex with ordinary people) combined with the real science of gene drives, which are genuinely controversial in biology. The humor also comes from the alien essentially weaponizing human horniness as a way to disrupt civilization. SMBC frequently uses aliens and scientists as vehicles for thought experiments about human nature, and this comic satirizes both the promises and fears around genetic engineering while sneaking in jokes about human vanity and desire.