My 9-year-old daughter has stumped me with a question. She asked why the shape of eyelids differ in different races. She used the example of Japanese versus Caucasian people. Any thoughts?
鈥� The shapes of eyelids, along with other physical features, are the result of evolution 鈥� the selecting and deselecting of various traits through time.
A mutation, or random change, in a gene can result in an inheritable physical characteristic that can be beneficial, harmful or neutral. A beneficial mutation may be selected for, so that it spreads from parents to children through a population in a relatively short time in evolutionary terms 鈥� as little as 50 generations or 1000 years. A mutation that gives only minor benefit may take longer to become common, 1000 generations or 20,000 years.
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Harmful mutations would be selected against 鈥� a euphemistic way of saying that the carrier would die before reaching reproductive age or be less likely to pass on the mutated gene to future generations.
A neutral mutation, which confers no benefit or hindrance, is most interesting. It could survive in the gene pool, neither increasing nor decreasing in prevalence in a population. But if that population moved to a different environment, or the environment changed 鈥� for example, because of climate change 鈥� then that mutation could be selected for and the gene, with the trait it confers, would become more common.
Some hypothesise that the that causes the almond-shaped eyes in east and central Asian populations evolved to protect the eye from extreme weather conditions or high levels of ultraviolet light, such as you might get on the high Mongolian steppes. Whatever the truth of this hypothesis, a population鈥檚 physical features are no fluke. The genes that are expressed through these features have been tested and selected for over thousands of generations and this continues.
David Muir, Portobello High School, Edinburgh, UK
鈥� The epicanthic fold of the upper eyelid conceals the inner corner of the eye, called the medial canthus. It is usually accompanied by a lower nose bridge and is most prominent in women and children.
It is thought that the fold helps to protect the eyes from bitterly cold winds, such as those blowing across Siberia, while the narrow nasal passages associated with the lower nose bridge would help warm and moisten inhaled air before it reached the lungs.
鈥淓picanthic eye folds may help protect the eyes from bitterly cold winds, like those blowing in Siberia鈥�
This fold also makes it easier to narrow the eyes so that the eyelashes can act like a veil, trapping warmer air near the cornea and reducing the intensity of light reflected off any ice or snow. This may explain why the epicanthic fold is seen in north-east Asia, while migration might explain why it has spread to South-East Asia.
But the epicanthic fold is also observed in disparate groups across the globe. In the bushmen of southern Africa and indigenous Americans, it may have afforded protection against dust storms: the fold shields the lacrimal glands, which produce tear fluid to lubricate the eye and cleanse it of dust, and nasal hairs in narrower nostrils may trap more dust.
The trait can also be seen among Scandinavians, Sami, Poles and Germans, as well as Irish and British people, which may be the result of migration.
Mike Follows, Sutton Coldfield, West Midlands, UK