I agree that it’s natural for people to look
around and wonder why we look different in some
ways. It doesn’t mean we’re implying one way is
better. It’s also important to remember that
there is no scientific basis for classifying
people by race. For example, there’s a range
of eye shapes in any population. I found a source
that said almond-shaped eyes are a dominant trait,
and rounder eyes are recessive, but it’s not as
though all “rounder” eyes are the same shape and
all “almond” eyes are the same shape.
In the case of eye shape, I don’t know whether
there’s selection pressure for different eye
shapes in different environments. Perhaps the
shape and skin fold help to protect the eye in the
cold, windy regions of northern Asia or the hot,
sandy winds of Asian deserts. Fitness differences
are not necessary to explain the differences in
eye shape because microevolution (changes
in gene frequency within a species) can be driven
by more than natural selection.
Founder effects are one way that
populations can wind up looking different.
Imagine that a small group of immigrants made
their way into Asia. The set of genes in that
small group would be different from the genes in
the large population they left behind. Their
descendants would tend to look like the founders.
Genetic drift (read answer #2 for more on this)
can happen in small populations
as a result of random events. For example,
suppose that there were a few people in the early,
small population that had rounder eyes and they
did not leave any offspring. Maybe they all got
killed in a flood together, or maybe the reason
was different for each of them. Those genes would
be lost from the population. Note that the
small population size is the key to genetic
drift. If you have a population of 100, and
only 5 have some gene, it is reasonable that all 5
could just be lost for reasons that had nothing to
do with the fitness of that gene. If there were
5,000 people in a population of 100,000, it seems
unlikely that all 5,000 would be lost randomly.
A bottleneck is an event where most of the
population is lost, so the genes of the
descendants are all from a small group of people.
It is similar to the founder effect.
Sexual selection could also drive changes in gene
frequency. Imagine that for cultural reasons, a
particular shape of eye made an individual a lot
more successful in finding a mate. The gene that
made them attractive would become more common.
So populations that are separated for a long
time tend to become less alike. It could be
due to several causes, including the ones above,
mutation, and gene flow between populations as
individuals find mates in populations that used to
Just to take this full circle, ocean currents
could be important in the story of human
appearance. The currents probably influenced where
people wound up as humans spread across Earth.
Ocean levels allowed people from Asia to spread
into North America and their descendants became
the many groups of indigenous people of the
Americas. The winds and currents determined where
the Europeans ended up in the Americas as well.
Here are a couple of good resources about
variations in human appearance. The HHMI skin
color one gets into natural selection a lot more.
The other one discusses genetics more.
Unfortunately, neither one tackles eye shape.
Thanks for taking your student’s question
seriously and looking for answers.