I’m glad you asked because a lot of my college students are confused about this.

Big idea: Individuals don’t get adaptations because they want them, need them, or would be better off with them.

Skin color is influenced by more than one gene. That one gene has more than one variety. In any population, people have some variation in skin color, but let’s keep it simple and imagine that there’s only one gene. We’re also ignoring the fact that lighter-skinned people might get a tan because that would not be passed to the next generation.

Big idea: Changes that happen during your lifetime (getting stronger, losing a finger, stretching your arm, learning, etc.) do not get passed to the next generation genetically

Let’s look at how there could have been a change from darker to lighter populations of people in a high-latitude habitat.

What happens if an individual has a mutation that gives him or her a much lighter skin color? In a sunny climate, that individual might have had a high risk of skin cancer. If she were female, her babies would have had a much greater risk of being born with severe brain and spinal cord problems. So this new light-skin variation of the gene probably would not spread to later generations a sunny climate.

What happens when that same mutation happens in a northern population? The lighter-skinned person would be able to make more vitamin D, hold onto more calcium, and avoid the bone disease called rickets. This strong-boned individual may have had more offspring, allowing the gene to be more common in the next generation and the next.

Big idea: Mutations are random. They do not happen because they are needed. Whether they are good or bad (or neutral) depends on the environment an individual is in. The mutation for lighter skin is just as likely to happen in the tropical climate as the high latitude one.

So if a population of light-skinned people migrated to a sunny climate, and only married people within that population, would we expect their offspring to be darker? Well first, there would have to be people carrying a copy of the dark-skin gene in order for it to spread. If someone does not carry it already, would there be a mutation? Nobody knows, it’s random.

In order for it to spread, individuals with the dark-skin variety of the gene would have to leave more offspring. This is unlikely. The risk of brain and spinal cord issues in babies of light-skinned women who are exposed to too much sun can be avoided by taking supplement called folic acid. People spend more time inside now. People realize the danger of the sun now and lighter-skinned people often protect themselves with sunscreen. If a person gets skin cancer now, they have medical treatment available.

The environment of light-skinned people living near the equator has changed due to nutrition, prevention, and treatment options. Now the gene for darker skin probably does not lead to individuals having more offspring, so that variety of the gene is unlikely to spread.

Big idea: For a variation of a gene to spread in a population, the individuals with that variation have to leave more offspring than individuals who don’t. (There are some exceptions, but this is a long answer already.)

So would light-skinned people living at the equator have darker children just because sunburns are painful?

The skin color of the next generations can change through either genetic drift or through a selection pressure. Within the generations following the one couple, even if they mate with only white people, there will be some natural variation in the population. While everyone will be white, some will be slightly lighter or darker, based on the gene combinations they got.

The population will become darker skinned through selection pressure if the whiter skinned people produce fewer children over many generations. Basically, if lighter skin increases an individuals likelihood of death, even by only a small amount, over many generations, the lighter skin genes will slowly be removed from the population, resulting in a darker skinned population.

Alternatively, the population could become darker (or lighter) skinned by genetic drift. This occurs when there is no selection pressure on skin color; all skin colors are equally good or bad in the environment. Again, the population of white skinned people will still have some slight variation of skin color based on the genetic combination they got from their parents. But the skin color of the population can still change due to chance alone. The lightest or the darkest individual may dye by some tragic accident and remove those extreme skin color genes from the gene pool. Or some individuals my have more children than other for reasons unrelated to skin color; there are a variety of random chance events that may result in people having light or dark skinned genes. And because the population size is limited, only some of the genes will be present, while others are lost. As a result, the population could be equally likely to become lighter, or darker, or stay the same.

In addition, in either genetic drift, or with a selection pressure, a mutation could occur randomly within the population that results in an unusually dark (or light) skin. Then this mutation could become more (or less) abundant based on selection or genetic drift. The genetic mutation can explain how a population can become different from any of the genetic types the population started as.