The Head Nurse at the Swedish embassy wore a uniform so white the Assistant had to avert his eyes to avoid being blinded. She coolly noted that the proportion of blue-haired people seemed to be increasing over time.

"That's very strange," mumbled the Assistant apprehensively. "I did the algebra, it has to come out that way! The Hardy-Weinberg law clearly states that the proportions of blue-hair and brown-hair alleles should stay constant over time!"

"Unless the Hardy-Weinberg Law doesn't apply here..."

"How is that possible?" sputtered the Assistant. "I thought Hardy-Weinberg was a universal law. Like an apple a day keeps the doctor away."

"A common error. In fact you could call it the Hardly-Ever Law, because the assumptions of the Law are hardly ever satisfied. Don't they teach you anything in school these days?"

"Well, I knew about the apples..."

"OK, you want me to lay it out for you? Mutation. Migration. Natural Selection. Even pure chance, if your population is small enough. Any of them cause changes in allelic frequencies -- the proportions of brown vs. blue alleles. All of them invalidate the Hardy-Weinberg Law. Null and void. No warranty. No shirt, no shoes, no service!" finished the Nurse with a flourish.