How and Why
Researchers from Copenhagen University discovered that everyone with blue eyes are descended from the same ancestor, who lived near the northwest coast of the Black Sea, 6,000 to 10,000 years ago. This individual had a genetic mutation occur to the gene OCA2, which controls eye color. This mutation turned off the mechanism which produces melanin in the iris of the eye. Without melanin to give eyes their brown color, they became blue. This is the reason 300 million people worldwide have blue eyes. In most countries in Northern Europe, blue is the dominant color.
Percentage of people with blue eyes in some countries:
99% in Estonia.
95% in some Scandinavian countries.
89% in Denmark (this was higher before immigration from third world countries).
75% in Germany.
But making blue eyes a permanent trait was a near impossibility. The difficulty stems from the gene for blue eyes being a recessive gene. When brought together with the brown eye gene, the brown is the dominant one.
Brown eye gene = B
Blue eye gene = b
Each person has two copies of the eye color gene in their genome, one inherited from each parent. Now if both parents only carry the gene for brown eyes, BB and BB, then their child will receive one B from each parent, ending up as BB. The same works for blue eyes, if that's the only gene both parents carry, bb and bb. Each parents gives one b to the child, who ends up as bb.
If you have one parent who only has the gene for brown eyes, BB, and one parent who only has the gene for blue eyes, bb, then all the children will have brown eyes. Example: One parent gives a B, the other gives a b. Bb = brown eyes. Here's why: When you have two alleles (coding sequences) from genes that are at odds with each other, one version will override the the other. When dealing with eye color, B always dominates b. But these children now carry the b gene in them, and could pass it down to their own children. Some of them, depending on the other parent, could end up with blue eyes.
If one parent is Bb, and the other is bb, then each time they have a child, there is a 50% chance it will have blue eyes. Example: First parent is Bb, second parent is bb, then their children will end up as either Bb, bb, Bb, bb. If both parents carry the genes for brown eyes and blue eyes, then each time they have a child, there is a 25% chance it will have blue eyes. Example: First parent is Bb, second parent is Bb, then their children will end up as either BB, Bb, Bb, bb.
So how did the gene for blue eyes overcome near impossible odds and become a permanent trait ?
The first individual who had this genetic mutation did not have blue eyes. But he passed this gene onto his children (I'll explain why it's a male further down). His children didn't have blue eyes either, since the other parent would have had brown eyes. Not until you reach the grand children and great grand children (cousin marriage was common throughout history) would blue eyes first appear. So to have a sizable population of breeding cousins, you need to have a lot of children initally. A man would have a better chance of having a multitude of offspring, especially if he were a king or ruler. Such a man would have many wives and concubines, and therefore, the potential for many children. Think about how many times in the past, someone might of had the same mutation, yet it never became a permanent trait because the individual never had enough children.
Decades ago, the assumption was that people, over the course of many eons, became blue eyed because they were adapting to conditions as they migrated north. The thinking was that possibly, blue eyes conferred some kind of protection against snow blindness. Even without the finding from the researchers from Copenhagen, this idea was debunked long ago. Brown eyes, because of the melanin, offer much better protection against ultraviolent light. They become even more valuable the higher you climb. Intensity of UV rays increase 4% for every 1000 feet of elevation gain.
Since individuals pushing north were confronted with darker skies and longer nights, it was considered possible that blue eyes offered better eyesight in these conditions. This has been disproved also. Most scientists consider the mutation to be nothing more than a random shuffle. Our human genome is always trying out different variations, looking for better traits. But if brown eyes offer better protection, why didn't blue eyes mutate back to brown ?
Maybe this mutation was the first step in a longer, mutation process. Could our human genome have been intent on developing something like a tapetum lucidum (a layer of tissue in the eye, which lies immediately behind the retina. It reflects some of the escaping light back into the eye, improving vision in low light conditions). Night predators, such as dogs, wolves, cats, etc, have a tapetum lucidum, giving them superior night vision. Often, these animals have light colored eyes (wolves have eyes with pale irises, huskies often have blue eyes, etc).
Or maybe there's another purpose. One of the Copenhagen researchers believed blue eyes became so prominent, because it's such an attractive feature. It gave an advantage in the mating game. And true, many people are attracted to blue eyes. Still, this seems like a stretch. Instead, couldn't blue eyes have been a body's way of signaling to potential mates that it had a valuble, hidden adaptation ? Let's face it, when it comes to the mating game, there's a lot of unconscious signals that go back and forth.