Download Chapter 15 Recent Human Evolution Overview • The first part of this

Chapter 15
Recent Human Evolution
Overview
• The first part of this chapter concentrates on the history of populations
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Biological anthropologists use information on biological variation to make inferences concerning the evolutionary
history of populations and of specific traits.
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Case studies in this chapter focus on the Asian origin of Native Americans, the genetic impact of population settlement
and invasion in Ireland, and the biological history of African Americans.
• The second part of this chapter looks at the history of different traits and the influence of natural selection.
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Data on biological variation can also inform us about natural selection in recent human history.
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Perhaps the best-documented example of natural selection in human populations is the relationship between
hemoglobin alleles and two selective forces: sickle cell anemia and malaria
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Skin color is another example of a trait that shows a strong environmental correlation, in this case with latitude.
• Natural selection in human populations is often affected by culture change. Are we still evolving?
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Cultural evolution in the last 10,000 has been rapid
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There are examples in this chapter that point to natural selection:
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Changes in sickle cell allele frequencies in response to malaria
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Evolution of lactase persistence
Genetics and Population History 1
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Genetics and population history
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Often concerned with population history
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How are different populations related?
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Did they have different histories in terms of settlement or contact with other groups?
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Did invaders leave traces of their genes behind?
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Were populations always the same size or did some grow and expand, shrink and become isolated?
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There is the need to remember both gene flow and genetic drift affect populations.
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The origin of Native Americans
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One of the biggest controversies in anthropology is over the timing of New World migration.
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The main reason for the controversy is the poor archaeological record in Alaska.
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Range of dates is from 50,000 years ago to 12,000 years ago.
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These dates are based on geological ice ages:
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Glaciers began retreating between 18,000 and 14,000 years ago in North America
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Significant climatic amelioration occurred 14,000 to 11,000 years ago
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The landmass which connected Asia and North America when sea levels were lower at the height of the last
glaciation is called Beringia
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When sea levels are 300 feet lower, the exposed land mass is approximately 1,000 miles wide.
Genetics and Population History 2
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The origin of Native Americans (continued)
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We will talk about the traditional migration story (called Clovis First) and then talk to the newer ideas
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Clovis First
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In the past, scientists generally believe that people crossed between Siberia and Alaska as part of a general
faunal exchange that occurred during the Wisconsin glacial period (This is called “Clovis First” after the
dominant tool tradition)
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The dominant view is that Native Americans migrated into the Americas via the Bering Land Bridge and
followed game herds.
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They eventually moved into North, Central, and South America.
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Pre-Clovis ideas are abundant and suggest many routes
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Today, many think the geographic origin is the same, but that the route was by sea, in boats (earlier in time
(15-20 kya and is called “Pre-Clovis”)
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Coastal Route (walked) The idea that some early Americans may have used a coastal route to enter North and
South America from Asia
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Coastal Route (boated) The latest idea is that some of the earliest inhabitants came by boat.

Europeans also may have boated.
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There are problems with this view:

The archaeological data that would support this view is submerged.
Relethford Chapter 15 Page 1
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There is little evidence of a marine-adapted population on the coast of northeastern Asia
One computer simulation suggests that the coastal route is not enough to explain all the data.
Genetics and Population History 3
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The genetic evidence for an Asian origin
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Since the late sixteenth century, people have argued for an Asian origin for Native Americans.
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Physical traits support this (such as shovel-shaped incisors), but the best evidence is DNA analysis.
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A comparison of 9 geographic areas for 120 allele frequencies shows a close genetic distance. The Native
Americans are genetically closest to Northeast Asia.
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Mitochondrial DNA from Native Americans belong to a haplogroup (related haplotype) found in Northeast Asia
confirming their descent to migrants from there.
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Relethford adds mtDNA data that suggests there are 5 haplogroups (A, B, C, D, X).
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A, B, C, D are all NE Asian origin, X is also found in Europe
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Did NE Asians migrate to Europe at some time?
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The Y-chromosome and mtDNA patterns support a NE Asian origin
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How many migrations?
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There was a single source of the DNA markers in Siberia and may be a single migration event.
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New data suggests that the initial founders were isolated from other populations in the Bering Straits, rather than
making a quick trek to the New World.
Genetics and Population History 4
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The population history of Ireland
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The population history of Ireland is fascinating from the perspective of genetic variation due to the numerous
possibilities for gene flow.
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It has been invaded/settled by peoples from Scotland, England, Wales, and Scandinavia.
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There was a large-scale study of anthropometric measurement of thousands of Irish men in the 1930s.
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Genetic impact of gene flow from England and Wales
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There is an east-west gradient for many traits
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Genetic similarity follows a west to east gradient found for body size, blood groups and the Y-chromosome
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Most likely reflects differences in past immigration into Ireland from England and Wales in the north, east, and
southeast.
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The northern and eastern regions are more similar to England and aligns with the historical data.
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Great website on migration of Anglo-Saxons in England http://www.ucl.ac.uk/tcga/presentations/ASdemo/AS-2611-03b.html
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Genetic impact of the Viking invasion
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Not all traits exhibit the east-west gradient.
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For instance, craniofacial heights shows show the four midland counties are dissimilar to all others.
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One suggestion this that Viking invasion and settlement may be responsible for gene flow. Settlements were
on the coast but substantial movement into the midland counties occurred.
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Genetic similarities of the Irish midlands to Norway and Denmark show a strongest evidence of Viking
immigration.
Genetics and Population History 5
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Genetic ancestry of African Americans
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Between the 1600s and the 1800s, an estimated 380,000 to 570,000 Africans were involuntarily moved to the US.
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The name of this forced migration was the Middle Passage.
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Even more Africans were forced to migrate to Central and South Americas and the Caribbean; most migrants
were from W and W-Central Africa.
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The primary reason for enslaving large numbers of people was the sugar industry required cheap labor.
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The fathering of children by the European-Americans was extensive.
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Allele frequencies in African Americans tend to be between those of Europeans and Africans, but remain closer to
Africa.
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Analysis of allele frequencies allow us to estimate the overall proportions of African and European ancestry using
DNA markers.
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The gene flow is estimated to range from a 4% contribution among the Gullah of SC to 23% in New Orleans.
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Differences have also been found when comparing maternal (mitochondrial DNA) and paternal (Y-chromosome
polymorphs) ancestry.
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In both cases, the amount of European ancestry was greater on the father’s side
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Means it was more common for European men to father children with African American women that for European
women to have the children of African American men.
Given the miscegenation laws this is an expected finding.
Who Are the Lemba?
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The Lemba are a cultural group located in southern Africa.
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They insist that they are ‘Black Jews”.
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Westerners dismissed this claim out of hand.
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Tudor Parfitt studied the traditions of the Lemba and brought their story to a larger audience.
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The story of the Lemba can be seen in a NOVA documentary called “Lost Tribes of Israel”.
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Later he organized a group of geneticists to study the DNA of the Lemba.
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The findings show there was contact by Middle Eastern (probably Jewish) men.
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The problem is that in Jewish tradition there are only two ways to be considered Jewish: your mother was Jewish or you
converted.
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This example illustrates the complexity of cultural and genetic identities.
Natural Selection in Human Populations 1
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Natural selection has been operating throughout hominin evolution, resulting in a number of major anatomical changes.
This part of the chapter addresses several examples of evolutionary history and natural selection among recent human
evolution.
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Hemoglobin, sickle cell, and malaria
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What is malaria?
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Four species of Plasmodium infect humans and cause malaria (Plasmodium falciparum, P. malariae, P. ovale, and
P. vivax).
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A fifth species was identified in humans in 2007 ([Plasmodium knowlesi [nolls eye]). It is usually only seen in
macaques.
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At first it was seen as rare in humans
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Today can account for most of the infections in some parts of SE Asia.
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It has also been seen to transmit to Western travelers.
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All species are vector borne diseases, being spread by anopheline mosquitoes, and the disease is distributed
throughout much of the world.
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In the human host the parasite is found primarily inside of the red blood cells (RBC).
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The parasite reproduces asexually inside of the RBC, and following this the RBC breaks open releasing many
new parasites.
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These parasites then infect more RBC's, and this ultimately leads to the destruction of massive numbers of
RBC's.
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The characteristic "chill and fever" (paroxysm) associated with malaria occurs when the parasites are released from the
RBC's.
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Since the release of parasites is periodic, the paroxysms are periodic.
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For examples, the paroxysms associated with a tertian malaria (e.g., Plasmodium vivax) occur about every 48
hours, and those associated with a quartian malaria (e.g., Plasmodium malariae) occur about every 72 hours (view
a diagram of the life cycle).
Source: http://ryoko.biosci.ohio-state.edu/~parasite/plasmodium.html
Malaria Life Cycle
Natural Selection in Human Populations 2
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Hemoglobin, sickle cell, and malaria (continued)
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Distribution of the sickle cell allele and malaria
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The RBC protein, hemoglobin, functions to carry oxygen to body tissues.
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The normal beta chain of the hemoglobin is called HbA
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HbS is known as the sickle cell allele.
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Hemoglobin variants
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There are 1546 hemoglobin and thalessemia variants (http://globin.bx.psu.edu/cgi-bin/hbvar/counter)
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The most widely studied mutations include the hemoglobin S , HbC and HbE groups (there are several variants of
each)
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A person who has two S alleles has sickle cell anemia.
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As homozygous for HbS is harmful it is selected against but mutation introduces even as natural selection
eliminates it.
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The balance point between A and S is between 10-20% Hb S
Distribution of the sickle cell allele and malaria
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The distribution of the allele is related to the prevalence of certain forms of malaria and is caused by a virus or
parasite.
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There are 300-500 million cases each year and 1-3 million deaths.
The evolution of the S allele
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In a malarial environment, heterozygous people have an advantage.
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Equilibrium is one in which the fitness of the entire population is at a maximum.
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The cost of adaptation, however, is an increased proportion of individuals with sickle cell anemia.
Maps of HbS and P. falciparum (not pictured)
Natural Selection in Human Populations 3
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The CCR5 gene
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A mutant allele of the CCR5 gene (CCR5-Δ32 ) is found on chromosome 3
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It is a deletion of 32 base pairs
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Found in moderate levels in Europeans (0.03-0.14) and is absent in Africans, East Asians, and Native
Americans
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It has been linked to resistance to HIV (human immunodeficiency virus), the virus that causes AIDS.
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People who are homozygous for the CCR5-Δ32 allele are almost completely resistant to HIV-1 infection.
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It appears that there was a selection for the CCR5-Δ32 allele for resistance to some other infectious disease in
the past, and HIV resistance today is a byproduct of that previous selection.
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In one case using a bone marrow transplant from a person with both copies of the gene to a sick person ALSO
eliminated traces of AIDS in the patient
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Resistance to what in the past?
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Bubonic plague (the “Black Death”) has been suggested as one possibility. But the gene’s presence precedes
the plaque
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Another suggestion is resistance to smallpox.
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Could be a different disease or environmental factor yet to be proposed.
Natural Selection in Human Populations 4
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The biology of skin color
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The pigment melanin is responsible for the majority of variation in lightness and darkness in skin color, controlled by at
least 6 genes.
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The light skin of Europeans and of Asians may be due to different genes, suggesting independent occurrences.
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It is secreted by cells in the bottom layer of the skin.
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Hemoglobin is another pigment affecting skin color which gives oxygenated blood cells their red color What are the
factors that create differences in skin color?
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Color is produced by melanin, hemoglobin and carotene.
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Melanin
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All people have about the same number of melanocytes.
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The primary difference in skin color is in the amount of melanin produced by the melanocytes
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The pigment melanin is responsible for the majority of variation in lightness and darkness in skin color.
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There are two types: eumelanin (dark or brown) and pheomelanin (light or red)
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Hemoglobin is another pigment affecting skin color which gives oxygenated blood cells their red color.
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Carotene provides a yellow to orange factor.
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Thickness of the outer skin layers also alters appearance
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Age and sex can create differences in skin color.
Natural Selection in Human Populations 5
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The distribution of skin color
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Skin color is darkest at the equator and tends to be lighter with the increasing distance from the equator, north or south.
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There is also a hemispheric difference—skin color tends to be darker in the Southern Hemisphere (below the
equator) than in the Northern Hemisphere.
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Ultraviolet radiation is strongest at the equator and dims in strength farther from the equator.
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It tends to be greater in the Southern Hemisphere than in the Northern Hemisphere.
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The distribution of human skin color in the world today suggests past evolutionary events relating to natural selection.
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Dark skin evolved among our early ancestors in Africa as a means of protection against damaging effects of
ultraviolet radiation.
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Later, as some human beings moved out of Africa, lighter skin color evolved farther away from the equator.
Natural Selection in Human Populations 6
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The evolution of dark skin
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People with darker skin suffer fewer cases of skin cancer. The problem with this hypothesis is that it tends to kill
people AFTER their reproductive years (natural selection issue.
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Sunburn has been suggested as a potential factor in natural selection.
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Severe sunburn can lead to infection and the body’s ability to sweat effectively.
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Dark skin could protect from these effects and be selected for in areas of high ultraviolet radiation.
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Dark skin can also protect against folate deficiency (vitamin B9), which affects reproductive capabilities. Was
proposed by John Eaton (reported in Jablonski and Chapin article).
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Folate deficiency alters the sperm and leads to male sterility
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One symptom is anemia (low RBC count)
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Can cause spina bifida also (incomplete development of the brain and/or spine)
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Dark skin may have evolved for one or more of these reasons.
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The evolution of light skin
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Areas of low levels of ultraviolet radiation away from equatorial regions have decreased risk.
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Vitamin D hypothesis: It is widely accepted that the evolution of light skin is related to the synthesis of vitamin D from
the sun and stimulation of chemical compounds caused by ultraviolet radiation.
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Lack of vitamin D leads to rickets
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There are several studies of darker-skinned, recent migrants to northern clines who suffer from rickets.
Natural Selection and Culture Change 1
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Horticulture and the sickle cell allele in Africa
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Prior to the development of horticulture in Africa, the frequency of the sickle cell allele was probably low.
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It was an evolutionary advantage initially with the increase of malaria.
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Human cultural adaptations affected the ecology of other organisms like the mosquito and malaria parasite, also causing
genetic change in the human population.
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This problem has not gone away.
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A modern example of the effects of deforestation in the Amazon is reported here.
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Much of the deforestation is to provide cheap food in the US.
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Rain puddles are another source of disease; in fact, some suggest new forms of drug-resistant malaria reside in the puddleliving mosquitoes.
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Other sources for mosquito breeding are portrayed here.
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Estimates of deaths due to malaria (annually) range from 600,000+ to 1.2+ million
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Most cases in the US are imported.
Natural Selection and Culture Change 2
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Lactase persistence and lactose intolerance
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Lactose intolerance is high in most African and Asian populations and tends to be low in European populations.
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There is a lower prevalence in Africa in populations that have a history of dairy farming.
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Variation in lactase persistence has an evolutionary advantage, and corresponds to the specific cultural history of
populations.
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High lactose intolerance occurs where dairy farming is not practiced
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Lactase persistence occurs where it is practiced.
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Advantages to drinking milk:
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Important source of water
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Overall nutrition
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Sarah Tishkoff from the University of Maryland has found at least 3 different mutations account for lactase persistence
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Cultural practices can also make milk available to those with lactose intolerance, These include yogurt and koumiss
(fermented milk).
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Here is a map
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Lactose intolerance versus milk protein (casein and/or whey) allergy.
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Lactose intolerance symptoms include (to varying degree): diarrhea; nausea; cramps; bloating; and gas
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Allergy symptoms include: Diarrhea; cramps; coughing or wheezing; runny nose; skin rash and (rarely) anaphylaxis.
Frequency of Lactose Intolerance (not pictured)
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