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
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
If you are interested in questions like this, you
may want to study genetics.
Thanks for asking,