WEEK 7 Discussion Forum--Anthropology 1( need high quality assignment!! And due on time !!!)

CHAPTER 12 Human Variation and Adaptation

Historical Views of Human Variation

• Biological determinism - cultural and biological variations are

inherited in the same way as genetic variation.

• Eugenics - "race improvement" through forced sterilization of

members of some groups and encouraged reproduction among

others.

• Eugenic science was taken to a new level during the holocaust, but the

root of modern eugenics science began in the U.S., especially here in

California

"It is better for all the world, if instead of waiting to execute degenerate

offspring for crime, or to let them starve for their imbecility, society can

prevent those who are manifestly unfit from continuing their kind…. Three

generations of imbeciles are enough.“ ~Supreme Court Justice Oliver

Wendell Holmes

Traditional Concept of Race

• Since the 1600s, race has been used to refer to culturally

defined groups.

• Groups that are now often confused with nationality or ethnicity

• Race is generally understood to be a “real,” concrete, and definable

aspect of human biology

• Race is used as a biological term, but is a social construct

which has enormous social significance.

• Connections of aspects such as morality or intelligence to race

Modern Racial Theory

• In any racial group, there will be individuals who fall into the

normal range of variation for another group for one or several

characteristics.

• Human phenotypical expression is so varied and overlapping it

is impossible to draw any clear lines that would definitively

categorize all people

• For example light skinned Africans are often more fair than some Indians

and Asians, and there are dark skinned Indian populations. Many

Mediterranean and Middle Eastern populations are darker skinned than

Non-white populations, yet they are racially “white”

• Modern science has discarded the out of date concept of “race”

all together

Intelligence

• Genetic and environmental factors contribute to intelligence.

• IQ scores change during a person’s lifetime; average IQ scores of different populations overlap.

• Complex cognitive abilities, however measured, are influenced by many genes and are thus polygenic .

• Individual abilities result from complex interactions between genetic and environmental factors.

• No convincing evidence exists that populations vary in their cognitive abilities, regardless of what some popular books suggest.

Polytypic

• A polytypic species is composed of local

populations that differ in the expression of one or

more traits.

• Even within local populations, there’s 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.

Population Genetics

 The study of the frequency of alleles, genotypes, and phenotypes in populations from a micro- evolutionary 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 complicated by cultural and social structures

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.

• Developmental Acclimatization happens during growth and leads to

permanent changes such as heart and lung capacity

• Because it is under genetic influence, acclimatization is

subject to natural selection and genetic drift.

• Tibetans carry several mutations that are not found in other populations

which make them fit for their environment, this is adaptation

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.

• For example Slash and burn agriculture and sickle cell

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.

Lactose Intolerance/Persistence

• 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 Percent

U.S. blacks 70–77

Ibos 99

Bantu 90

Fulani 22

Thais 99

Asian Americans 95–100

Native Americans 85

Distribution of Skin Color in Indigenous

Populations

Selective Advantages of Dark Skin

• 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.

• 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 hominids spent their days outdoors.

• Early hominins didn't wear clothing that would have protected them from the sun.

Selective Advantages of Dark Skin

• Protects from folate degradation

• Especially important for pregnant mothers in preventing

significant neural tube (the anatomical structure that

develops to form the brain and spinal cord) birth defects

such as spina bifida

Selective Advantages of Light Skin

• Vitamin D absorption is hindered by darker sink and

greater amounts of clothing necessary in northern climes

• Vitamin D is necessary for bone growth and deficiency

can lead to Ricketts

• Vitamin D is also linked to the production of 125D which

regulates many body functions including immune defense

• How/why were Inuit populations not affected by this

selective pressure?

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.

• This has led to a huge amount of adaptive

variation

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) The Samburu are characteristic of many inhabitants of sub- Saharan Africa.

 (b) The Inuit 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

• Narrowing of blood vessels to reduce blood flow

from the skin, vasoconstriction.

• Increases in metabolic rate to release energy in

the form of heat.

Human Response to Heat

• Long-term adaptations to heat evolved in our

ancestors:

• Sweat glands and lack of hair: evaporation cools our

bodies

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

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.

• 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.

Infectious Disease

• 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

Infectious Disease

• Pathogens

• Any agents, especially microorganisms such as viruses,

bacteria, or fungi, that infect a host and cause disease.

• Pandemic

• An extensive outbreak of disease affecting large

numbers of individuals over a wide area; potentially a

worldwide phenomenon.

Smallpox

The only disease to

date that science has

completely eradicated.

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

Race-based Medicine.

• 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.

The Problems With This • 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.

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

• This means that research should focus on gene and gene complexes that underly genetic factors involved in disease, rather than race.