lamarckianism
Explanation
The Joke
A couple is lying in bed. The woman says, "Ever since we got pregnant, when we lie in bed together I get this strange tingling feeling at the small of my back. Like someone's drawing on it with a marker." In the next panel, the man nervously says, "Hey, unrelated, are you familiar with Lamarck's theory of evolution, where changes to the parents' bodies are inherited by their children?" She asks, "Sure, why?" He responds, "No reason." The final panel reveals that their baby has "DAD WUZ HERE" and a heart drawn on its back -- confirming that the father had been drawing on her back and, through a Lamarckian mechanism, the markings were inherited by the baby.
The Humor
The joke works by taking the discredited scientific theory of Lamarckian inheritance -- the idea that organisms can pass on traits acquired during their lifetime to their offspring -- and applying it in the most absurd possible way. Instead of the classic example of giraffes stretching their necks and producing long-necked offspring, the father has been doodling graffiti on the mother's back ("DAD WUZ HERE"), and the baby is born with these doodles. The humor is amplified by the father's transparent attempt to seem casual ("Hey, unrelated...") when he clearly knows exactly what happened.
References
Jean-Baptiste Lamarck (1744-1829) proposed a theory of evolution in which organisms could pass on characteristics acquired during their lifetime to their offspring. This theory, known as Lamarckism or Lamarckian inheritance, was largely supplanted by Darwinian natural selection and modern genetics, which established that acquired traits are generally not heritable. The theory is a common example used in biology education to contrast with Darwinian evolution.