CHAPTER 12

Human Variation and Adaptation

Chapter Outline

◻ The Concept of Race

◻ Contemporary Interpretations of Human Variation

◻ The Adaptive Significance of Human Variation

Historical Views of Human Variation

◻ Biological determinism - cultural and biological variations are inherited in the same way.

◻ Since the 1600s, race has been used to refer to culturally-defined groups.

◻ Race is used as a biological term but is inapplicable to humans

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.

◻ 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.”

Clinal Distributions

◻ A cline is a gradual change in the frequency of a trait or allele in populations dispersed over geographical space.

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

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.

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

Selective Advantages of Dark Skin

◻ Protects from folate degradation

◻ Especially important for pregnant mothers

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.

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.

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)

Population Genetics

◻ The study of the frequency of alleles, genotypes, and phenotypes in populations

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

◻ 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).

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.

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.

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.

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.