Chapter 12

Chapter 12Human Variation and Adaptation

•How do you define “race” and do you think it’s a useful concept in understanding variation in our species?

•Are observable characteristics (i.e. skin color and color of eyes) as important as the fundamental differences among humans that are defined by evolutionary processes?

Historical Views of Human Variation•Biological determinism - cultural and

biological variations are inherited in the same way.

•Eugenics - "race improvement" through forced sterilization of members of some groups and encouraged reproduction among others.

Traditional Concept of Race

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

•Race is used as a biological term, but has enormous social significance.

•In any racial group, there will be individuals who fall into the normal range of variation for another group for one or several characteristics.▫Polytypic species

Intelligence

•Genetic and environmental factors contribute to intelligence.

•IQ scores change during a person’s lifetime

•Cognitive abilities are polygenic -- measured by many genes

•Nature and Nurture!

Human Polymorphisms

•Genetic traits with different phenotypic expressions▫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.

Polymorphisms at the DNA Level•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.

•Lactose intolerance…

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 ofLactose Intolerance

Population Group Percent

U.S. blacks 70–77

Ibos 99

Bantu 90

Fulani 22

Thais 99

Asian Americans 95–100

Native 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.

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.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. Acclimatization

• 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.

Skin Color

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.

UV Radiation

•Early hominids 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 hominids didn’t wear clothing that would have protected them from the sun.

•Since this is how we evolved, when don’t get enough sun, we may develop…

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.

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 is due to both Culture….and Biology!

▫Biology part (next slide)

Human Response to Heat

•Long-term adaptations to heat evolved in our ancestors:▫Sweat Glands▫Vasodilation – capillaries near skin’s surface

widen

Human Response to Cold

•Short-term responses to cold: ▫Metabolic rate and shivering▫Vasoconstriction – to reduce blood

flow to the skin

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)

▫Bergmann’s and Allen’s rules...

Bergmann’s Rule• Body size tends to be

greater in populations that live in cold environments.▫ 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.