The idea is that as the climate in northern Europe gets warmer, redheads will gradually die out as their paler skin – adapted to cloudy conditions – proves unable to cope with the massive amounts of sun that Scotland will suddenly have.
But even if redheads were fatally allergic to sunlight, they still wouldn't go extinct. Why? Because of how genetics works.
Being recessive means that you need to get a redhead copy of the gene from both your mother and your father to have red hair.
And if two non-redhead parents (who both carry a single redhead version of the gene) have kids, there's only a 1 in 4 chance that any given child will have red hair.
If a gene's effects mean you're less likely to survive and pass on your genes to your children, it will naturally become less common in the gene pool.
But with a recessive gene, even if the people who have two copies of it die off and get replaced in the population, there's still plenty of people who carry a single copy of the gene but don't suffer any ill effects.
This means that even if a recessive gene is actually deadly (which red hair definitely isn't), it's still almost impossible to completely breed it out of a population.
And despite its rareness, you'll still get occasions when two carriers meet and produce a kid with both copies of the gene.
These magical ginger children will then go on to reproduce a huge amount, thanks to the fact that redheads will be rare, almost mythical creatures of astonishing beauty and jaw-dropping hotness.