The Evolution of Skin Colour

What are the causes of skin cancer? Exposure to abnormally high frequencies of light can potentially alter the DNA of a cell and turn it cancerous, the resulting cancerous cells are known as skin cancer. The most common cause is ultraviolet light emitted by the sun although it has been known to be caused by tanning booths, unusually high levels of x-rays, exposure to some chemicals and in rare cases the abnormal genes that cause skin cancer can be inherited by children from their parents. . Why are Caucasians more at risk of skin cancer than other populations? Melanin is the pigment in the skin that absorbs ultraviolet light and protects the skin from sun damage, so the less pigment in a person’s skin the less protection they have from ultraviolet light. So light skinned people (Caucasians), particularly those with light coloured eyes and light coloured hair are more susceptible to sun damage and skin cancer than people with more pigment in their skin.

People, who have no melanin at all, like people with albinism or vitiligo, are much more susceptible to skin cancer. 3. At what age does skin cancer typically occur? Is the incidence of skin cancer greater in youth or old age? A person’s lifetime sun exposure is a known factor in skin cancer because of this the risk of developing skin cancer increases with age, most diagnoses are made in people over 50 years of age.

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However people who are exposed to large quantities of ultraviolet radiation can develop skin cancer as early as 20 to 30 years of age. The average age of skin cancer diagnosis occurs at 53 years old. 4. Does the amount of UV light reaching the Earth vary in a predictable manner? If so, describe the pattern you observe. The earth’s axis is slightly tilted relative to its orbit, this means that during stages of the earth’s orbit one side of the earth is closer to the sun than the other and becomes exposed to higher levels of ultraviolet light.

Half a year later when the earth is on the opposite side of the sun the other half of the earth is exposed to higher levels of ultraviolet light. This cyclic process of varying ultraviolet light exposure causes the areas exposed to most ultraviolet light to increase and decrease in latitude and the year progresses. The earth’s axis is only slightly tilted, because of this the ultraviolet exposure band only varies in latitude a small amount and never reaches the northern and southern most parts of the earth.

Over a year the mean of daily ultraviolet radiation that hits the earth’s surface generally increases the further away a place is from the north and south poles. The earth’s atmosphere plays a large role in absorbing the ultraviolet light from the sun, the atmosphere is thinner in certain places and causes a greater amount of ultraviolet light to hit the earth’s surfaces beneath it, one such place is over Australia where it receives much larger ultraviolet radiation than it should. . What latitude receives the greatest amount of UV light? The least? On average the equator receives the greatest amount of UV light because it spends the greatest amount of time nearest to the sun and varies from this by a small amount, the UV light also has to pass a smaller distance through the atmosphere as it passes directly above it and the ozone is naturally thinner above the tropics so there is less ozone to absorb the UV light. The equator is at 0° latitude.

The north and south poles receives the least UV light as the sun is usually furthest away from these points, ozone holes have formed above the north and south poles but because they spend much of each year without sun and the parts of the year they spend with constant exposure to sunlight the sun is so low in the sky the UV light does not go through the ozone hole but has to pass a considerable distance through the normal atmosphere and becomes largely absorbed by the ozone. The north and south poles are at 90° latitude. . Based on these data, where might you expect to find the most lightly pigmented and most darkly pigmented people on the planet? Be as specific as you can. The darkest skinned people would live in Guyana, Suriname and possibly some of north Brazil in South America. Northern Nigeria, northern Cameroon, southern Chad, northern Central African Republic, Southern Sudan, South Sudan, Eritrea, Djibouti, Ethiopia, Somalia, Kenya, Uganda, Tanzania and possibly some of Zambia and Angola in Africa and Singapore in Malaysia.

West Sumatra, Riau, Jambi, West Borneo, Central Kalimantan, East Borneo and possibly gulf of Tomini, West Sulawesi, Central Sulawesi, South Sulawesi, Moluccas, West Papua and Papua in Indonesia. The lightest skinned people would live in Northern Canada, Greenland, Iceland, Norway, Sweden, Finland and northern Russia. 7. Provide a rationale to your answer above (i. e. , why did you think that more darkly pigmented people would be found in those areas)?

Two different factors affect the UV light radiation a developing population would be exposed to, the first being the UV light reaching the earth, the equator is exposed to the largest amount of annual UV radiation, the annual UV radiation exposure reduces the further north or south a population develops, UV exposure is the driving force behind the evolution of melanin production so those that developed nearest to the equator would have generally have greater amounts of melanin and thus darker skin.

The other factor is shelter; developing humans in areas with the greatest amount of shade from the UV light have less need to protect themselves from it and thus would not develop darker skin. Tropical rainforest make great shelter from the UV light so any humans who developed in tropical rainforests would have lighter skin. So the darkest skinned people would have developed near to the equator, allowing for ± 20° latitude because of earth’s 23. 5° axial angle and population movement, and in areas uncovered with tropical rainforests.

The lightest skinned people would have developed nearest to the north or south pole, allowing for ± 30° latitude because of the earth’s 23. 5° axial angle and the hazardous conditions and thus low population near the north and south poles, and especially the areas sheltered from the sunlight. It is possible for the darkest skinned and the lightest skinned people to have evolved in these places and then moved anywhere in the world so areas that are predicted to be mostly light skinned people could be completely populated with dark skinned people. 8. Interpret this graph and the trend it describes. a.

Is skin reflectance randomly distributed throughout the globe? If not, how would you describe the pattern? The skin reflectance is not randomly distributed throughout the globe; the further a population is from 0° latitude the greater its skin reflectance. b. Restate your findings in terms of skin colour and UV light (instead of skin reflectance and latitude). The greater the UV light exposure the darker the populations skin colour, the closer the population is to 0° latitude the greater the UV light exposure. c. How closely do these findings match the predictions of your hypothesis (Question 6)?

The new findings suggest that the population with the lowest skin reflectance is slightly above the equator and the population with the greatest skin reflectance is nearest to the North Pole. These findings match the predictions made in question 6 extremely accurately. d. Some populations have skin colours that are darker or lighter than predicted based on their location (their data point falls somewhere outside of the line shown in Figure 2). What might explain the skin colour of these exceptional populations? Propose a few hypotheses.

Genetic drift is the change in the frequency of an allele in a population due to random sampling. This means that even though certain skin colours are better adapted to an environment and the individuals with that skin colour would reproduce more the genes for their skin colour is not inherited by their children, causing the children to have an unexpected skin colour. If the population is able to find shelter from the sunlight or is required to spend an excessive amount of time exposed to sunlight then the population would be required to evolve differently than would be expected from their position.

It is possible that in the past the darker or lighter skinned populations migrated to different areas where they are not expected, this could be caused by many reasons including drought, famine and infrequent Godzilla attacks. 9. Hypothesize why different skin colours have evolved. Based on what you know, what factor is most likely to exert a selective pressure on skin colour? Lighter skin colours tend to burn much easier than darker ones; because of this a light skinned person living in an area with high UV light exposure would suffer frequent and severe burns causing damaged and tender skin and even skin cancer.

Those that get skin cancer would die without modern medical attention and therefore would be incapable of passing down their light skinned genes, however this is not the main cause for the evolution of skin colour as skin cancer usually only affects those over fifty years old so there must be another cause. Natural selection plays a major role in almost all evolution; natural selection occurs when an organism has more desirable phenotypes than others and thus reproduces more.

A possible cause is in the fact that humans with lighter skin would usually suffer from severe sunburn and have damaged and tender skin as a result, because of this they would become less attractive and weaker and therefore a less desirable mate. Therefore the individuals with lighter skin are much less likely to pass down their light skinned genes to the next generation. 10. Review your answer to Question 3. Keeping your answer in mind, how strong a selective pressure do you expect skin cancer (UV-induced mutations) to exert on reproductive success?

Skin cancer generally affects people over the age of fifty and the average life expectancy for any country never made it into the 50’s or above until the 1900’s. Because of this it can be assumed that skin cancer had an extremely minor and almost negligible effect on the evolution of skin colour and reproductive success. 11. Based on this information, does your hypothesis about the evolution of skin colour (Question 9) seem likely? Why or why not? How does skin colour meet, or fail to meet, the three requirements of natural selection outlined above?

The hypothesis in question nine allows skin cancer to play only a very minor role in the evolution of skin colour. It includes the variation as the organisms with lighter skin colour do suffer from damaged skin and become generally weakened. The genes for the lighter and darker skin colour are hereditary and thus allows for the hereditary component. Disfigured and weakened individuals are less desirable mates causing Selective Pressure and Differential Reproductive Success. 12. Based on Branda and Eaton’s results (Figure 3), what is the apparent effect of UV light exposure on blood folate levels?

According to the results the UV light exposure increases the body’s folate levels. 13. What is the apparent effect of UV light on folate levels in these test tubes? The UV light appears to break down the folate exposed to it reducing the folate levels in the test tube exposed to UV light. 14. How is folate linked to natural selection? High folate levels improve an organism’s ability to reproduce but folate levels are only affected by dietary intake and destruction through alcohol consumption or ultraviolet skin exposure.

So for natural selection to be linked to folate one of those factors must be hereditary. The melanin in the skin is the chemical that absorbs the UV light and protects the rest of the body from harm; it can assume that it would also protect the folate in the blood from the UV light. If this is true then individuals with greater quantities of melanin in their skin would also have far greater levels of folate than an individual with less melanin, especially if both are often exposed to UV light. This means that the individuals with greater melanin would be far more capable of reproducing.

This means that the three conditions for natural selection are present, there is a variation in the population, the genes for greater melanin levels are hereditary and those with more melanin have greater reproductive success. 15. All other things being equal, which skin tone would you expect to be correlated with higher levels of folate? Darker skin has greater levels of melanin in it and therefore protects the folate in the blood from the UV light causing the folate levels to be much higher. 16. Based on this new information, revise your hypothesis to explain the evolution of human skin colour.

Even though the skin damage of the lighter skinned individuals would have an effect on the evolution of skin colour it must be assumed that the greater reproductive success of the darker skinned individuals would play the major part in the evolution of skin colour. 17. What would happen to the reproductive success of: a. A light-skinned person living in the tropics? The UV light would break down the folate in their blood and cause spermatogenesis in men or miscarriages in women making them either infertile or to have low reproductive success. . A light-skinned person living in the polar region? There’s little UV light to break down the folate in their blood so their reproductive success would be very high. c. A dark-skinned person living in the tropics? The melanin in their skin protects the folate in their blood from the UV light; however some UV would still get through and break down their folate so they would have slightly reduced folate levels. However it should not have any negative effects on their reproductive success. d. A dark-skinned person living in the polar region?

There’s little UV light to break down the folate in their blood and the little light that does hit them would be absorbed by the melanin in their skin leaving them with exceptionally high folate levels. This would cause their reproductive success to be very high. 18. Predict the skin tones expected at different latitudes, taking folate needs into consideration. Use the world map (Figure 4) to indicate the skin tone expected at each latitude (shade the areas where populations are darkly pigmented). The darker the areas on the map the darker the average skin colour. 19.

Can folate explain the variation and distribution of light- and dark-skinned individuals around the world? Folate levels only explain why dark skinned people evolved; it does nothing to explain why there are still light skinned people. If folate levels and skin cancer are the only factors of reproductive success that UV light affects then every person in the world would have extremely dark skin so there must be another factor. 20. How is vitamin D linked to natural selection? Higher vitamin D levels are required for calcium absorption which is essential for pregnant women.

Having excessively high levels of vitamin D is not toxic to the body however low levels of vitamin D can reduce reproductive success. In areas where there is low amounts of UV light exposure the body becomes less capable of producing vitamin D and experiences lower reproductive success. To account for this the body needs to absorb the absolute maximum amount of UV light available to it. Melanin in the skin protects the body from UV light by absorbing the UV light before it harms anything else however the body needs the UV light to synthesize the vitamin D.

The genes for lower levels of melanin in the skin are hereditary, this means that the three conditions for natural selection are present, there is a variation in the population, the genes for lower melanin levels are hereditary and those with less melanin have greater reproductive success. 21. Which skin tone allows someone to maintain the recommended level of vitamin D? For the body to produce enough vitamin D throughout the entire year the body cannot have too much melanin, the amount they need depends on where they live. The less UV light exposure an area receives the less melanin the skin can have.

Therefore the closer a population is to the north or south poles less melanin and a lighter skin tone is required to produce the lighter levels of vitamin D. 22. Based on this new information, revise your hypothesis to explain the evolution of the variation and distribution of human skin colour. The closer a population is to the equator and therefore the greater the UV light exposure they receive, greater amounts of melanin is required to protect the folate in their blood from the UV light therefore darker skin is required to evolve nearer to the equator.

The further a population is from the equator and therefore the lower the UV light exposure they receive, lower amounts of melanin is required to allow the UV light to assist in the synthesis of vitamin D. Therefore the darkest skinned people live near the equator and the further north or south a population lives from the equator the lighter skin tone they require. 23. Taking only vitamin D into consideration, what would happen to the reproductive success of: a. A light-skinned person living in the tropics?

A light skinned person living in the tropics would have exceptionally high levels of vitamin D and would experience very high reproductive success b. A light-skinned person living in the polar region? They would be capable of producing enough vitamin D to maintain normal reproductive success however there will still be a drop in their vitamin D levels when compared to light skinned people living in more tropical regions. c. A dark-skinned person living in the tropics? A dark skinned person living in the tropics would be able to produce enough vitamin D to maintain normal reproductive success. d.

A dark-skinned person living in the polar region? A dark skinned person living in a polar region would be not be capable of producing enough vitamin D and would suffer from a heavily lowered reproductive success. 24. Predict the skin tones expected at different latitudes, taking only vitamin D needs into consideration. Use the world map (Figure 6) to indicate the skin tone expected at each latitude (shade a region to represent pigmented skin in that population). The darker the areas on the map the darker the average skin colour. 25. Can vitamin D alone explain the current world distribution of skin colour?

Vitamin D levels only explain why light skinned people evolved; it does nothing to explain why there are dark skinned people. If vitamin D levels were the only factors of reproductive success that UV light affects then every person in the world would have extremely light skin. 26. Using principles of natural selection, predict the skin tone expected at different latitudes, taking ultraviolet exposure, vitamin D, and folate needs into consideration. Use the map (Figure 7) to indicate skin tone patterns at different latitudes (shade regions where populations are expected to be darkly pigmented).

The map below incorporates UV light exposure, vitamin D and folate levels, some extra adjustments have been made for lighter skinned people living in areas with rainforests because of constant shelter and darker skinned people living in deserts because of a lack of shelter. 27. Are UV light, vitamin D and folate needs sufficient to explain the current world distribution of skin colour? UV light exposure, vitamin D and folate almost completely explain the world’s distribution of skin colour with the few exceptions, for example the Inuits living in the North Pole and the Northern Europeans. 8. How might you explain that Inuits, living at northern latitudes, are relatively dark-skinned (much more so than expected for their latitude)? Propose a hypothesis. The Inuits live extremely nearby by to the North Pole. The north and south poles aren’t just points representing the northernmost and southernmost points of the earth, the earth has a magnetic field that surrounds the entire earth and passes through the north and south poles. The magnetic field deflects particles carried in solar winds emitted by the sun however some are pulled into the north and south poles.

This creates an amazing spectacle known as the aurora lights, the aurora lights is a beautiful light produced in the sky as the energetic particles interact with the thermosphere. However some of the light produced is outside of our visible spectrum and can actually be harmful to those being constantly exposed to it. Two pictures below show the aurora lights in different wave lengths. The first picture displays the aurora lights in UV light and the second picture displays the aurora lights in X ray light.

The Inuits have had to adapt to be capable of absorbing this extra radiation they are exposed to. They require an increase in melanin to absorb the UV light and because of the extra UV light they don’t require extremely light skin to synthesize vitamin D. Because of this they have evolved with slightly darker skin than would be expected for their latitude. 29. Conversely, Northern Europeans are slightly lighter-skinned than expected for their latitude. Propose a hypothesis to explain this observation.

The Northern Europeans have developed with slightly lighter skin than expected for their latitude for the opposite reason that caused the Inuits to develop darker skin. The magnetic fields of the earth deflect radiation most efficiently near to the poles but not actually at the poles, because of this less UV light is exposed to those who live far north and far south but not actually at the north or south poles. Because of this to gain enough vitamin D Northern Europeans are required to have unexpectedly light skin. 30.

Which populations have insufficient amounts of vitamin D in their blood? The East Asians, South Asians and about 75% of Europeans have insufficient amounts of vitamin D in their blood. 31. What is a common trait to all of these populations? How could this explain their deficiency? All three populations evolved in areas that receive much larger quantities of UV light exposure than Canada. Because of this they would have developed much more melanin than is optimal for living in Canada and therefore are incapable of producing a sufficient amount of vitamin D. 32.

For several decades, milk and cereals have been fortified with vitamin D to help increase intake of this nutrient. Hypothesize why these fortification programs appear to be failing in some ethnic groups (there may be different reasons for different groups). No major ethnic groups ban the consumption of milk however many ethnic groups require a person’s skin to be almost completely covered. This shields them from the UV light and does not allow them to produce enough vitamin D, not even the extra vitamin D in milk and cereals is enough to give them a sufficient amount.

In many cultures drinking milk and eating milk is a fairly uncommon practice so they benefit very little from the added vitamin D. 33. What should some Canadians do to avoid vitamin D deficiency? Canadians have extremely pale skin and therefore have to worry very little about becoming vitamin D deficient; this does not mean it cannot happen. To prevent it from happening they can take supplements or go for a daily 15 minutes walk in the sun, there are many options for people trying to boost their vitamin D levels but the simplest is always to spend some time in the sun. 4. If individuals do not take steps to boost their vitamin D intake, hypothesize what might happen to their descendants in the future. If individuals do not take steps to boost their vitamin D intake then only the paler individuals in the population would be reproductively successful causes the paler genes to pass down. This is called genetic drift and could potentially cause the loss of the dark skinned genes. Eventually the entire world would have extremely pale skin and would be required to stay in the shade and only go outside with significant sun protection. 35.

What might happen to Australians of European descent over time? Australians of European descent would eventually adapt to the Australian climate and be born with darker skin each generation until it finally matches that of the native aboriginals. 36. Think of the forces that affected the evolution of skin colour in the past. Contrast these to the factors that affect skin pigmentation today. Hypothesize about the factors that may affect skin pigmentation in the future. a. Do you think the impact of natural selection on skin colour is as strong today as it was in the past?

The impact of natural selection today isn’t nearly as strong as it has been in the past, humans have developed methods of shielding ourselves from the sun such as with sunscreen and have also developed method of exposing ourselves to extra UV light such as in UV tanning beds. Because of these developments any person with any skin colour can expose themselves to the optimal amount of UV light in any location destroying any differences in reproductive success across the different skin colours which halts natural selection. . What factors may have decreased or increased selection today? Modern methods of optimising the amount of UV light exposure have significantly decreased natural selection. However modern day intercontinental travel has allowed for gene mixing on a global scale that has never before been achievable, the gene mixing has allowed no genes to dominate over another and eventually will allow all races to mix together into a single race. This is the final stage of human natural selection in terms of skin colour. . Predict skin colour distribution in the year 2500. Outline your assumptions and the reasons for your predictions. In the year 2500 all races will have mixed together into a single indistinguishable race. There will of course always be individuals born with lighter or darker skin but the global average skin colour will be a light brown. Those who live near the equator will of course be more tanned than those who live further north or south but their genetic skin colour will remain the same. 37.

During the trip, the lightly-pigmented individual is looking forward to lying on a beach and working on his/her tan. Will this person be adapting to the environment (in a Darwinian sense)? In other words, will evolution take place by developing a tan in a geographical region of intense UV light? The European individual will definitely be adapting to the Caribbean environment by building up a tan as he/she is beginning to produce greater quantities of melanin in his/her skin to produce this change in skin colour.

However this is not an adaption in the Darwinian sense, his genes remain identical  to the way they were before the trip (unless of course if he/she develops a skin cancer), this means that if he/she is to have children then there is no possibility of the tan being passed on to the child. 38. Should the lightly-pigmented person wear sunscreen on the trip? The lightly pigmented person should definitely wear sunscreen, not only will he/she protect his/her skin from sun damage and skin cancer but will also reduce the affect the UV light has on his/her folate levels by 90% 39.

Should this person wear sunscreen when he/she is at northern latitudes (i. e. , at home in Canada)? It is only necessary to wear sunscreen if the person is going out for excessive amounts of time; there are relatively low levels of UV light across Europe so sun damage and skin cancer is a minor concern. 40. Could this observation be the result of sexual selection? Explain your answer. Although it is impossible to test but it is possible that men prefer women with lighter skin, however a result of only a 3-4 % reduction of melanin suggests that this is not the case. 41. Could this observation be the result of “normal” natural selection (i. . , not sexual selection)? Explain your answer. The evolution of women having slightly lighter skin is probably caused by the fact that lighter skin allows for greater calcium absorption. Calcium absorption is essential in pregnancy, not only to assist the development of the child but also to produce milk after the child is born. There it can be assumed that women with slightly lighter skin than would be expected for their location have a higher chance of having a healthy child. Therefore the genes that cause women to have lighter skin are passed down more often than darker skin.

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The Evolution of Skin Colour. (2016, Dec 10). Retrieved from https://graduateway.com/the-evolution-of-skin-colour/