Unformatted text preview: IB35ac Class 14: Skin color Starting with this lecture, we'll delve into how genes influence the variation we can actually see in humans. First and foremost, the most obvious phenotype of interest is skin color. Why does this vary? Does it mean anything? 10/12/2009 IB35 Human Biological Variation Class #14 Why Study Human Skin Color? It represents the product of over five million years of evolution It is one of the most obvious ways in which people vary It is important to humanity Skin Color: A Tale of Adaptive Compromises Topics for today: Color of skin is determined by how much visible light it reflects 88% of difference in skin color is related to geographic differences Most Scientists Think that Skin Color is an Adaptation to the Environment The pigment that makes skin dark protects us from the harmful rays of sunlight, like ultraviolet radiation (UVR). What is an Adaptation? An adaptation is something that enhances the survival and reproductive success of an organism in a particular environment. Human Skin Color is Adaptive Melanin pigmentation varies by latitude and is related to many latitude-dependent environmental parameters. Darker skin protects against the deleterious effects of UVR (e.g., sunburn, damage to sweat glands, skin cancer). Human Skin Color is Not Adaptive Sunburn and skin cancer rarely affect reproductive success. Ski color i a b Skin l is byproduct of selection d t f l ti on other functions of pigmentation genes. Skin color has evolved as the result of sexual selection. 1 10/12/2009 Skin Pigmentation Regulates the Penetration of UVR into the Skin It determines both the rate of breakdown of UVR-sensitive compounds and the production of UVR-dependent compounds.
People with darker skin are found closer to the equator. Skin Color is Measured by Skin Reflectance What Gives Skin its Color? Melanin Carotene Hemoglobin Different types of Pigments Types of Melanin Eumelanin - Blackish-brown melanin Pheomelanin - Yellowish melanin Melanin in the Skin is an Adaptation to Regulate the Amount of Sunlight Entering the Body 2 10/12/2009 Melanin is a Natural Suncreen Melanin prevents the breakdown of important vitamins while allowing the production of others. Dark Skin was the Original Condition for the genus Homo A high melanin content was needed to protect the naked skin of active people in extremely sunny environments. Possibly because of our lack of fur Eccrine Sweat Gland - Humans have mostly this particular sweat gland; produces watery sweat Apocrine Sweat Gland - Other animals have mostly this kind of sweat gland; produces more milky, viscous sweat Studying Skin Color Requires that You Study UVR data about UVR levels on Earth from NASA. Origin of the UV Data Set NASA Total Ozone Mapping Spectrometer (TOMS) between 1978 and 1993 Data comprises 37,400 readings for each day Higher Elevation = More UV (less atmosphere to block UV rays) UVMED ->UV radiation at the Earth's surface Correlations: Skin Color with Average UVMED (Chaplin, 2001)
Inverse Relationship: negative values
Filter 685 nm (red) 545 nm (green) 425 nm (blue) UVMED (Average) -0.815 -0.903 -0.891 UVR (100-400 nm) Profoundly Affects Biological Systems Damage to DNA Inhibition of photosynthesis in p ytop a to phytoplankton Damage to cell membranes Photolysis of bioactive compounds (folate, vitamin B12) Catalyzes pre-vitamin D3 synthesis Correlation values fall between 1 and 0 Higher correlation = 1 Lower/no correlation = 0 3 10/12/2009 UVR Affects Reproduction by Breaking Down Vitamins F l Folate (f (from the B Vi h Vitamin group) i i ) is broken down by UVA. Folate deficiency slows the production of DNA. Folate is found in leafy vegetables Folate and Human Development Folate is needed for all processes requiring cell division, especially in early embryonic development and in the production of sperm. Potential Danger of Sun Beds Lapunzina (1996) reported the occurrence of NTDs in three subjects whose mothers had been exposed to UVR on "sun beds" during the first three weeks of their pregnancies. Day 21-25 most crucial to neural tube development in foetuses. Neural tube develops into nervous system The Evolutionary Advantage of Dark Skin Dark skin acts as a natural sunscreen, and protects p against breakdown of folate by UVR. Don't lighten your skin! Under What Conditions is Dark Skin NOT an Advantage? The melanin in dark skin slows the production of vitamin D This is a D. problem in areas that don't receive much sunshine. 4 10/12/2009 You Need Some UVR to Make Vitamin D in Your Skin Vitamin D is needed to build and maintain strong bones, and t t b d to maintain a healthy immune system. If You Don't Get Enough Vitamin D .... CHILDHOOD RICKETS, 18TH AND 19TH CENTURY BRITAIN RICKETS Bones become deformed as they are unable to support weight of body For females, reduced area from hip bones may not be able to support pregnancies Why Did Light Skin Evolve? As people moved away from the equator, their skin had to lose pigmentation in order to permit them to continue to make adequate amounts of vitamin D in their skin. The farther from the equator that you live... the harder it is to make vitamin D. Foods that can supplement Vit D in diet: Fish, Lichen, Animals that eat lichen like reindeer Why Women are Lighter They must produce as much vitamin D as possible during their reproductive years. Their skin color has been defined as the "knife's edge of natural selection knife s edge" selection. Genetics of Skin Color At least 26 genes appear to be involved in determining skin color A few recent studies: KITLG (Kit li ligand) d) MC1R Convergent evolution 5 10/12/2009 KITLG (Kit ligand) In stickleback fish, light-skinned populations have a divergent allele of Kit ligand gene In humans, Europeans and East Asians also have derived alleles at the KITLG locus Regulatory regions of KITLG have evidence of strong selection (humans) Convergence or conserved mechanism in fish and humans for lighter pigmentation
Miller et al. 2007. Cell 131:1179-1189. Genes Influencing Variation in Skin Pigmentation The melanocortin 1 receptor (MC1R) locus, which regulates melanin production, shows little variation in African populations, but exhibits extensive allelic variation in European populations (Sturm et al., 2001). MC1R ->Controls the type of Melanin being produced ->80% of humans with red hair or fair skin have dysfunctional variant of MC1R gene MELANOMA -> Type of skin cancer More about MC1R Raimondi et al. 2008. International Journal of Cancer 122:2753-2760. Looked at 9 most-common MC1R variants 5 variants are significantly associated with g y melanoma development AND the red hair/fair skin (RHC) phenotype 2 have are significantly association with melanoma BUT NOT the RHC phenotype 2 variants are not associated with melanoma OR the RHC phenotype Pigment in Neanderthals Lalueza-Fox et al. Science 318:1453-5. Amplified and sequenced a fragment of the MC1R gene from 2 neanderthal remains Both have a mutation not found in the 3,700 humans studied The mutation impairs function of MC1R Evidence of convergence Acquisition of the same biological trait in unrelated lineages Genes Influencing Variation in Skin Pigmentation The DCT gene shows evidence of selection in a Chinese population (Myles et al., 2006) MATP SLC24A5 OCA2 and TYRP1 show MATP, SLC24A5, OCA2, evidence of selection in European population (Myles et al., 2006) Convergent evolution Light skin has evolved several times in our lineage, and at least twice in modern humans. The light skin of Europeans and Asians is an example of evolutionary convergence. 6 10/12/2009 Conclusions Skin pigmentation is an evolutionary adaptation for regulating penetration of UVR into the skin. Skin pigmentation represents a pg p compromise between the demands of protecting the body and making enough vitamin D. Conclusions Skin pigmentation can change in response to environmental conditions over thousands of years. Dark and light y g skins have evolved many times in human history. Implications - Many people live under UVR conditions to which they are poorly adapted, with adverse consequences for their health 7 ...
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- Spring '08
- Ultraviolet, Human skin color