CHAPTER 12

Human Variation and Adaptation

Chapter Outline

◻ Historical Views of Human Variation

◻ The Concept of Race

◻ Contemporary Interpretations of Human Variation

◻ Human Biocultural Evolution

◻ Population Genetics

◻ The Adaptive Significance of Human Variation

◻ The Continuing Impact of Infectious Disease

Polytypic

◻ A polytypic species is composed of local populations that differ in the expression of one or more traits.

◻ Even within local populations, there is a great deal of genotypic and phenotypic variation between individuals.

◻ All contemporary humans are members of the same polytypic species, Homo sapiens.

Human Polymorphisms

◻ Characteristics with different phenotypic expressions are called polymorphisms.⬜ A genetic trait is polymorphic if the locus that

governs it has two or more alleles.

⬜ Geneticists use polymorphisms as a tool to understand evolutionary processes in modern populations.

Clinal Distributions

◻ A cline is a gradual change in the frequency of a trait or allele in populations dispersed over geographical space.⬜ Example: The distribution of the A and B

alleles in the Old World.

◻ Clinal distributions are thought to reflect natural selection and/or gene flow.

◻ Consequently, clinal distributions are explained in evolutionary terms.

Polymorphisms at the DNA Level◻ Molecular biologists have recently uncovered

DNA variability in various regions of the genome.

◻ Scattered through the human genome are microsatellites, sites where DNA segments are repeated.

◻ Each person has a unique arrangement that defines their distinctive “DNA fingerprint.”

Human Biocultural Evolution

◻ Humans live in cultural environments that are continually modified by their activities.

◻ Evolutionary processes can be understood only within this cultural context.

Slash-and-burn Agriculture

◻ A traditional land-clearing practice whereby trees and vegetation are cut and burned.

◻ In many areas, fields are abandoned after a few years and clearing occurs elsewhere.

Sickle-Cell Allele

◻ With the spread in Africa of slash-and-burn agriculture, in the last 2,000 years, penetration and clearing of tropical forests occurred. As a result, rain water was left to stand in stagnant pools that provided mosquito breeding areas close to human settlements.

◻ DNA analyses have confirmed such a recent origin and spread of the sickle-cell allele in a population from Senegal, in West Africa.

Sickle-Cell Allele

◻ A recent study estimates the origin of the HbS mutation in this group at 1,250 to 2,100 ya.

◻ At least in some areas, malaria began to have an impact on human populations only recently. ⬜ But once it did, it became a powerful selective force.

◻ The increase in the frequency of the sickle-cell allele is a biological adaptation to an environmental change.

Evolutionary Interactions Affecting the Sickle-cell Allele

Thinking About Race

◻ Sickle-cell anemia has often been called a “racial” disease as it is more common in the United States among African-Americans.

◻ From what you’ve just learned, how is that generalization correct and incorrect?

Lactose Intolerance

◻ In all human populations, infants and young children are able to digest milk.

◻ In most mammals, including humans, the gene that codes for lactase production “switches off” in adolescence.

◻ The geographical distribution of lactose tolerance is related to a history of cultural dependence on fresh milk products.

Frequencies of Lactose Intolerance

Population Group Percent

U.S. whites 2–19

Finnish 48

Swiss 12

Swedish 4

Frequencies of Lactose Intolerance

Population Group PercentU.S. blacks 70–77

Ibos 99Bantu 90Fulani 22Thais 99

Asian Americans 95–100Native Americans 85

Population Genetics

◻ The study of the frequency of alleles, genotypes, and phenotypes in populations from a microevolutionary perspective.

◻ A gene pool is the total complement of genes shared by the reproductive members of a population.

◻ Breeding isolates are populations that are isolated geographically and/or socially from other breeding groups.

◻ Human populations are complicated by cultural and social structures.

Hardy-Weinberg Equilibrium

◻ The mathematical relationship expressing the predicted distribution of alleles in populations; the central theorem of population genetics.⬜ Provides a tool to establish whether allele

frequencies in a human population are changing.

Factors that Act to Change Allele

Frequencies

1. New variation (i.e., mutation)

2. Redistributed variation (i.e., gene flow or genetic drift)

3. Selection of “advantageous” allele combinations that promote reproductive success (i.e., natural selection).

Adaptive Significance of Human Variation

◻ Human variation is the result of adaptations to environmental conditions.

◻ Physiological response to the environment operates at two levels:

1. Long-term evolutionary changes characterize all individuals within a population or species.

2. Short-term, temporary physiological response is called acclimatization.

Homeostasis

◻ A condition of stability within a biological system, maintained by the interaction of physiological mechanisms that compensate for changes.⬜ Stress is the body’s response to any factor that

threatens its ability to maintain homeostasis.

Acclimatization

◻ Physiological responses to changes in the environment.

⬜ Responses may be temporary or permanent, depending on the duration of the environmental change and when it occurs.

⬜ Because it is under genetic influence, acclimatization is subject to natural selection and genetic drift.

Distribution of Skin Color in Indigenous Populations

Ultraviolet Rays

◻ Ultraviolet Rays penetrate the skin and can eventually damage DNA within skin cells. ⬜ The three major types of cells that can be

affected are squamous cells, basal cells, and melanocytes.

Ultraviolet Rays

UV Radiation

◻ Considering the cancer-causing effects of UV radiation from an evolutionary perspective:⬜ Early hominins lived in the tropics, where solar

radiation is more intense than in temperate areas to the north and south.

⬜ Unlike modern city dwellers, early hominins spent their days outdoors.

⬜ Early hominins did not wear clothing that would have protected them from the sun.

Selective Advantages of Dark Skin

◻ Protects from folate degradation

◻ Especially important for pregnant mothers

Rickets

◻ Insufficient amounts of vitamin D during childhood result in rickets, a condition that often leads to bowing of the long bones of the legs and deformation of the pelvis.

Neural Tube

◻ In early embryonic development, the anatomical structure that develops to form the brain and spinal cord.

Spina Bifida

◻ A condition in which the arch of one or more vertebrae fails to fuse and form a protective barrier around the spinal cord.

Thermal Environment

◻ Mammals and birds have evolved complex physiological mechanisms to maintain a constant body temperature.

◻ Humans are found in a wide variety of thermal environments, ranging from 120° F to -60° F.

Human Response to Heat

◻ Long-term adaptations to heat evolved in our ancestors:⬜ Sweat Glands

⬜ Vasodilation

◻ Bergmann's rule - body size tends to be greater in populations that live in cold environments.

Bergmann’s Rule

◻ In mammalian species, body size tends to be greater in populations that live in colder climates.

◻ As mass increases, the relative amount of surface area decreases proportionately.

◻ Because heat is lost at the surface, it follows that increased mass allows for greater heat retention and reduced heat loss.

Allen’s Rule

◻ In colder climates, shorter appendages, with increased mass-to-surface ratios, are adaptive because they are more effective at preventing heat loss.

◻ Conversely, longer appendages, with increased surface area relative to mass, are more adaptive in warmer climates because they promote heat loss.

Bergmann and Allen’s Rules

◻ (a) These Samburu women have characteristic of many inhabitants of sub-Saharan Africa.

◻ (b) The Inuit women are shorter and stockier.

◻ These individuals serve as examples of Bergmann’s and Allen’s rules.

Human Response to Cold

◻ Short-term responses to cold:

⬜ Metabolic rate and shivering

⬜ Capillaries near the skin’s surface widen to permit increased blood flow to the skin, vasodilation

⬜ Narrowing of blood vessels to reduce blood flow from the skin, vasoconstriction.

⬜ Increases in metabolic rate to release energy in the form of heat.

High Altitude

◻ Multiple factors produce stress on the human body at higher altitudes:⬜ Hypoxia (reduced available oxygen)

⬜ Intense solar radiation

⬜ Cold

⬜ Low humidity

⬜ Wind (which amplifies cold stress)

Infectious Disease

◻ Caused by invading organisms such as bacteria, viruses, or fungi.

◻ Throughout evolution, disease has exerted selective pressures on human populations.

◻ Disease influences the frequency of certain alleles that affect the immune response.

Impact of Infectious Disease

◻ Before the 20th century, infectious disease was the number one limiting factor to human populations.

◻ Since the 1940s, the use of antibiotics has reduced mortality resulting from infectious disease.

Impact of Infectious Disease

◻ In the late 1960s, the surgeon general declared the war against infectious disease won.

◻ Between 1980 and 1992 deaths from infectious disease increased by 58%.⬜ Increases in the prevalence of infectious disease

may be due to overuse of antibiotics.

Vectors

◻ Agents that serve to transmit disease from one carrier to another.

◻ Mosquitoes are vectors for malaria, just as fleas are vectors for bubonic plague.

Endemic

◻ Continuously present in a population.

◻ Sufficient numbers of people must be present.⬜ Small bands of hunter-gatherers were not faced

with continuous exposure to endemic disease because their population numbers were low.

Pathogens

◻ Any agents, especially microorganisms such as viruses, bacteria, or fungi, that infect a host and cause disease.

Smallpox

Pandemic

◻ An extensive outbreak of disease affecting large numbers of individuals over a wide area; potentially a worldwide phenomenon.

The Continuing Impact of Infectious Disease

◻ Humans and pathogens exert selective pressures on each other.

◻ Microorganisms evolve.

◻ A pathogen must not be so virulent as to kill its host too quickly.

◻ Selection acts to produce resistance in host. populations and/or to reduce the virulence of disease organisms.

Why It Matters

◻ One goal of the human genome project is to find DNA variants associated with disease and to design treatments that target those genes.

◻ Because some of these variants cluster in certain populations, there have been efforts to identify ancestry to predict risks.

◻ This has been referred to as race-based medicine.

Why It Matters

◻ The result has been the use of the imprecise term race to design treatment protocols.

◻ There are several problems with this effort:

⬜ Clinically observed or self-identification of race or ethnicity is often not congruent with genetic profiles.

⬜ If treatment is assigned for a person based on self-reported race rather than a genetic test, serious illnesses may be missed.

⬜ The use of race as a basis for treatment ignores real differences that lead to ill health.

Why It Matters

Example:

◻ A drug is designed to treat hypertension in African-Americans.

◻ If the drug is automatically prescribed and other drugs are not considered, the patient may not benefit at all.

◻ If the drug is the best choice for a white person, but is viewed as an “African-American drug,” it may not be prescribed as needed.

Why It Matters

◻ Categorizing drugs along racial lines is likely to lead to the same problems that resulted from categorizing people into racial groups.

◻ This doesn’t mean that the quest for underlying genetic factors involved in disease should be halted; it just means that the search should focus on gene and gene complexes rather than race.

QUICK QUIZ

1. Which of the following is a true statement?a) Polygenic traits are usually more

straightforward than polymorphic traits.b) Comparing allele frequencies between

populations can tell us nothing about evolutionary events.

c) Distributions of alleles for a single genetic trait do not conclusively demonstrate genetic relationships between populations.

d) The best way to understand patterns of population relationships is to follow a single polymorphic trait.

Answer: c

◻ The following is a true statement:⬜ Distributions of alleles for a single genetic trait

do not conclusively demonstrate genetic relationships between populations.

2. One of the results of DNA research has been the discovery that:

a) Many variations exist in the DNA of the human genome.

b) Everyone has their own unique arrangement that defines their distinctive “DNA fingerprint”.

c) Human variation is being researched at a microevolutionary level.

d) All of the above.

Answer: d

◻ All of the above are result of DNA research.

3. The total complement of genes shared by the reproductive members of a population is known as

a) Biological determinism

b) Gene pool

c) Genetic drift

d) Breeding isolates

e) Population genetics

Answer: b

◻ The total complement of genes shared by the reproductive members of a population is known as the gene pool.

4. The pigment which helps protect against ultraviolet radiation by absorbing it is

a) carotene.

b) lactose.

c) hemoglobin.

d) melanin.

Answer: d

◻ The pigment which helps protect against ultraviolet radiation by absorbing it is melanin.

5. Inuits have a large "globular" body, while the body structure of the Kalahari !Kung is thin and linear. This is explained by

a) Bergmann's rule.

b) Allen's rule.

c) Gloger's rule.

d) Kleiber's rule.

Answer: a

◻ Inuits have a large "globular" body, while the body structure of the Kalahari !Kung is thin and linear. This is explained by Bergmann's rule.

CHAPTER 13Legacies of Human Evolutionary History: Effects on the Individual

Chapter Outline

◻ Evolved Biology and Contemporary Lifestyles—Is There a Mismatch?

◻ Biocultural Evolution and the Life Course

◻ Human Growth and Development Today and in the Past

◻ The Human Life Cycle

◻ Adolescence

◻ Aging and Longevity

◻ Effects of Technology on the Brain

◻ Are We Still Evolving?

Focus Question

◻ How does the flexibility of human behavior potentially pose problems for the species?

Evolved Biology and Contemporary Lifestyles—Is there a Mismatch?

◻ Some aspects of modern human lives are disconnected from our evolved biology

◻ In past 10,000 years, the pace of cultural change has accelerated, changing the context of human evolution

Changing Contexts

Biocultural Evolution and the Life Course

◻ Biological development occurs from embryo to old age

◻ Cultural factors interact with genetically-based characteristics

Human Growth and Development Today and in the Past

◻ Human growth continues through late teens or early 20s

◻ Three major spurts are typical, including first two trimesters in utero, first four years, and the adolescent growth spurt

Adolescent Growth Spurt

◻ Pronounced increase in growth rate at puberty, compared to fairly steady level maintained since about four years⬜ Western teenagers typically grow around 4

inches per year

◻ Followed by decline in rate of growth until adult stature is achieved by late teens

Curves of Growth in Height For a Healthy American

Girl

◻ The distance curve shows the height attained in a given year.

Curves of Growth in Height For a Healthy American

Girl

◻ The velocity curve plots the amount gained in a given year.

Human Brain Growth

◻ 25% of its adult size at birth

◻ 50% at six months

◻ 75% at 2.5 years

◻ 90% at 2 years

◻ 95% at 10 years

Human Brain Growth

◻ Such a small amount of growth before birth is unusual for primates and mammals

◻ Selective advantages of such an underdeveloped brain⬜ Exit through narrow pelvis modified for bipedalism

⬜ Brain develops in stimulating, cultural context

Nutritional Requirements for Growth◻ Nutrients needed for growth, development, and

body maintenance include: proteins, carbohydrates, fats, vitamins and minerals.

◻ The amount we need of these nutrients coevolved with foods available to humans throughout evolutionary history.

◻ Deficiencies during pregnancy can last during a child’s lifetime.

◻ The specific pattern of amino acids required in human nutrition (essential amino acids) reflects an ancestral diet high in animal protein.

Essential Amino Acids

◻ 9 (of 22) amino acids that must be obtained from the food we eat because they are not synthesized in the body in sufficient amounts.

Comparison of Diet (Table 13-1)

Total dietary energy (%)

PreagriculturalDiet

ContemporaryDiet

RecentlyRecommended

Protein 33 12 12

Carbohydrate 46 46 58

Fat 21 42 30

Alcohol ~0 (7–10) —

Cholesterol (mg) 520 300–500 300

Comparison of Diet

Total dietary energy (%)

PreagriculturalDiet

ContemporaryDiet

RecentlyRecommended

Fiber (g) 100–150 19.7 30–60

Sodium (mg) 690 2,300–6,900 1,000–3,300

Calcium (mg) 1,500–2,000 740 800–1,500

Ascorbic acid (mg) 440 90 60

Preagricultural Diet

◻ Prior to 10,000 years ago

◻ Typically high in animal protein and low in fats (particularly saturated fats)

◻ High in complex carbohydrates (including fiber), low in salt, and high in calcium⬜ Human health declined in most parts of the world,

beginning about 10,000 years ago

⬜ This resulted in an “epidemiological transition” marked by rise of malnutrition, drop in life expectancy

Once adaptive, now maladaptive

◻ Ability to store fat, an advantage when food availability often alternated between abundance and scarcity

◻ “Feast or famine” biology incompatible with context of constant feast

◻ 80% of new cases of Type 2 diabetes appearing between now and 2025 will be in developing nations⬜ Type 2 diabetes, linked to poor diet and inadequate exercise,

occurring in children as young as 4

⬜ “epidemiological collision” in countries where malnutrition and infectious diseases collide with obesity

Other Factors Influencing Growth and Development: Genes and Environment

◻ Genetics – set the underlying limitations and potentials for growth and development

◻ Environmental factors can influence growth and development, but an individual can not exceed their genetic potential.

◻ Epigenome-Instructions that determine how genes are expressed in a cell

◻ Epigenetics-Changes in phenotype that are not related to changes in underlying DNA and that may result from the interaction between the genotype and the environment

◻ Hormones – produced by endocrine glands

⬜ Growth hormone has an impact on almost every cell in the body.

⬜ Cortisol, elevated during stress, suppresses normal immune function during high levels

Other Factors Influencing Growth and Development: Genes and Environment

Endocrine Glands

◻ Glands responsible for secretion of hormones into the bloodstream, i.e. pituitary, thyroid, and adrenal glands, ovaries and testes

Variation in Growth Hormones

The Human Life Cycle

1. Prenatal begins with conception and ends with birth.

2. Infancy is period of nursing.

3. Childhood, or juvenile phase, is period from weaning to sexual maturity (puberty in humans.)

4. Adolescence is from puberty to the end of growth.

5. Adulthood is the completion of growth.

6. Menopause beginning one full year after the last menstrual cycle

Life History Theory

◻ Typical developmental patterns shaped by natural selection

◻ Entire life course thought of as a series of trade-offs among various life history traits

Life Cycle Stagesfor Various Animal Species

Pregnancy, Birth and Infancy

◻ Cultural and social factor shape infant’s development in utero

◻ Birth is dangerous event and often surrounded by ritual significance

◻ “Underdeveloped” human infant brains adapted to developing in cultural enviornments

Diameter of Birth Canal and Head Length and Breadth of Newborns

Nursing

◻ Most anthropologists suggest three to four years of nursing was norm for humans in our evolutionary past

◻ Agriculture produced more options for supplemental foods and could quicken weaning

◻ Nursing can act as birth control

Childhood

◻ Humans have unusually long childhoods, which illustrates importance of learning for human adaptation

◻ Humans might be unique in practice of provisioning for juveniles

Providing for Juveniles (Table 13-3)

Percent Who SurviveWeaning Adolescence

Lion 28 15Baboon 45 33Macaque 42 13Chimpanzee 48 38Human Populations!Kung 80 58Yanomamo 73 50Paleoindian 86 50

Adolescence

◻ Rapid growth seen during adolescent growth spurt unique among primates

Adulthood

◻ Women in our evolutionary past likely experienced fewer menstrual cycles throughout life

◻ Most were more often pregnant or nursing

Menopause

◻ Advantages⬜ “programmed” to live 12 to 15 years beyond birth

of last child, since human parenting involves years of post-partum care

⬜ Grandmother hypothesis■ Women freed to provide high-quality care to

grandchildren

The Secular Trend in Age at Menarche in Europe

Aging and Longevity

◻ Attitude towards old age is culturally determined

◻ Top five causes of death in the US are heart disease, cancer, stroke, accidents, and chronic obstructive lung disease

◻ Senescence, the process of physiological decline in all systems of the body occurring toward the end of the life course

◻ The decline is gradual throughout adulthood

Life Spans (Table 13-4)

OrganismApproximate Maximum

Life Span (in years)Bristlecone pine 5,000

Tortoise 170Human 120

Blue whale 80Indian elephant 70

Gorilla 39Domestic dog 34

Rabbit 13Rat 5

Pleiotropic Genes

◻ Genes that have more than one effect.

◻ Genes that have different effects at different times in the life cycle.

◻ May help to explain evolutionary reasons for aging, but do not explain the causes of senescence.

Mitochondrial Theory

◻ Free radicals produced by mitochondria diminish efficiency of cellular energy production⬜ Ultimately leads to organ failure

Teleomere Hypothesis

◻ Repeated sequences of DNA at end of chromosomes

◻ Get shorter as organisms age

◻ Ultimately, impairs healthy cell division

Changes in Life Expectancy Due to AIDS in Seven African Nations

Effects of Technology on the Brain

◻ Our brains coevolved with technology and language development

◻ Contemporary technological change may be much more rapid than evolution can keep up with

◻ But, brains may be developmentally modified by using new technologies

Are We Still Evolving?

◻ Socioeconomic and political concerns have powerful effect on our species today

◻ Anthropologists cannot predict whether humans will eventually become a different species or become extinct