Why is it more common to have colorful eyes (like blue or green) in northern countries?


I’m asking for North hemisphere of the world and I don’t have any idea for the south hemisphere.

In: Biology

If you have dark brown eyes, it protects your eyes from the sun. It’s like how dark brown skin protects your skin from the sun. But your body has to continually burn extra calories to make the brown pigment. Food is scarce so it’s only worth burning the calories if the risk of damage from the sun is high. Since it’s extremely sunny at the equator, it’s better to have dark skin and eyes.

If you leave the equator and go to the North or South poles, it’s less sunny. This means your body doesn’t need to protect itself from the sun as much. Instead of making the brown pigment, it saves the energy for something else. This means that eyes can be blue or green because they don’t need dark brown to protect them.

As a result people who live near the poles tend to have light skin and eyes. They tend to be a lot more sensitive to the sun (e.g., they burn easier and their eyes are sensitive to sunlight.) People who live near the equator have darker skin and darker eyes to protect themselves. They tend to burn slightly more calories to make the brown pigment.

This is more common in the northern hemisphere because the land in the northern hemisphere is really far north. Even Australia is still relatively close to the equator compared to Canada, Russia, Norway, etc.

Light eyes are caused by specific mutations in the genes that produce melanin ([see link for details](https://www.news-medical.net/health/Genetics-of-Eye-Color.aspx)).

The mutation for blue eyes happened spontaneously 6000-10000 years ago around the Black sea and spread into Europe as humans migrated there. They’ve traced blue-eyed people from all over the world back to that one mutation.

So it’s probably the timing and geography – low chances of having the same mutation happen again in a relatively short time and the fact that people were migrating North.