Modern Human Variation II Study Notes

Modern Human Variation II Study Notes

Skin Color: An Adaptive Response

• Skin color is an example of human variation that has evolved as an adaptive response to environmental factors.

• Evolutionary Tradeoff:

  • UV Radiation:

  • UV radiation is essential for synthesizing vitamin D, which is necessary for proper skeletal development.

  • However, UV radiation also depletes folic acid, a nutrient essential for DNA synthesis and spinal development.

• Individuals living near the equator tend to have darker skin, which is advantageous because:

  • Dark pigmentation allows for ample production of vitamin D due to high UV exposure.

  • It serves to protect folic acid from depletion due to UV radiation due to high melanin concentrations.

• Conversely, individuals living farther from the equator tend to have lighter skin because:

  • They experience lower UV radiation levels that do not threaten their folic acid levels.

  • Their lighter skin allows effective synthesis of vitamin D in conditions of reduced UV exposure.

Homeostasis and Human Variation

• Homeostasis:

  • Refers to maintaining the normal functioning of the body, regulating internal conditions to sustain health.

  • The maintenance of internal homeostasis is essential for increasing fitness, which leads to improved survival and reproduction rates.

Human Diversity and Adaptations

• Genetic Adaptations:

  • Occur at the population level through the mechanism of natural selection.

  • An example includes variations in melanin levels and skin color across different populations.

Genetic Adaptations: Climate

• Populations with extensive histories in extreme environments exhibit specific body proportions influenced by climate:

  • Bergmann’s Rule:

  • As body volume increases, the surface area increases at a slower rate.

  • This leads to reduced heat loss in colder climates, hence:

    • Large body size is favored in cold climates.

    • Small body size is favored in hot climates.

  • Allen’s Rule:

  • Indicates that longer limbs provide greater surface area for heat venting.

  • In hot environments, longer limbs allow for improved heat dissipation.

  • Conversely, in cold environments, shorter limbs reduce heat loss.

Types of Human Adaptations

• Functional Adaptations (Adjustments): These can be classified further:

  • Developmental Adaptations:

  • Occur at the individual level usually during childhood.

  • Example: Children raised in high altitudes develop increased chest size due to lower oxygen levels.

  • Acclimatization:

  • Occurs at the individual level and can manifest at any point in a person's life.

  • Example: Tan development in response to sun exposure.

  • Cultural Adaptations:

  • Develop at the individual level throughout one's lifetime and are influenced by cultural practices.

  • Example: Wearing insulating clothing in arctic climates.

Functional Adaptations: Cold Stress

• Adaptations to Cold Stress:

  • Cultural Adaptations:

  • Wear warm coats, start fires, build shelters.

  • Acclimatization:

  • Immediate physiological response: Shivering provides heat.

  • After several days in cold conditions, the body acclimatizes:

    • Shivering diminishes.

    • Heat production becomes more efficient.

    • Skin temperatures increase.

Functional Adaptations: High Altitude

• Acclimatization:

  • Increased respiration rates to compensate for lower oxygen availability.

  • Increased production of red blood cells to enhance oxygen transport.

• Developmental Adaptation:

  • Individuals may exhibit larger chest sizes due to prolonged exposure to high altitudes.

Genetic/Cultural Adaptation: Lactase Persistence

• Approximately 65% of the global population is lactose intolerant (cannot properly digest milk during adulthood).

• Lactase Persistence:

  • Represents a genetic adaptation that emerged partly in response to cultural practices of dairy farming.

  • All mammals, including humans, initially produce lactase while being weaned but typically cease production by ages 7-8 years.

• The persistence of lactase is influenced by:

  • Populations historically involved in the domestication of cows or goats.

  • This mutation may provide a survival advantage during food shortages or famine.

  • Independent mutations across different human populations have led to variations of lactase persistence.

Nutritional Adaptation

• Case Study: Inuit Diet:

  • Inuits consume a high-fat diet but show low incidences of heart disease and diabetes.

  • This phenomenon may be linked to specific genetic mutations combined with natural selection processes.

Why Are We All So Similar but Different?

• Variability among humans arises from a combination of:

  • Genetics: Inherent biological differences.

  • Environment: Geographic and situational influences.

  • Culture: Social practices and cultural norms.

  • Behavior: Individual actions and responses to stimuli.

  • Diet: Nutritional habits influenced by availability and cultural practices.