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 be separated.

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.

skin color

ancestral journeys

Thanks for taking your student’s question seriously and looking for answers.

That's a good question, and the answer is "nobody knows".

I suggest that the reason is because of an evolutionary process known as genetic drift: this is where features that have no effect on fitness either good or bad randomly "drift" toward commonness or rarity in a population, with no natural selection to either forbid or maintain them. This is an extremely important evolutionary process and is probably responsible for a large number of traits in humans and other organisms. This (along with mutation) is the random part of evolution that is driven purely by chance. However, I don't know this for a fact, and am not sure how it could be tested, so I could very easily be wrong; I do agree with Nathaniel that there doesn't seem to be an obvious reason why different ethnic groups of humans should vary in eye shape except for one form or another of genetic drift.

I don't think we know why different groups of people have different eye shapes. It might be genetic drift - our genes slowly change, so populations that live far from each other for a long time will look slightly different. We know light skin gets skin cancer more easily and also absorbs Vitamin D better, but for a lot of differences, we don't know whether one type has an advantage or not.