Chapter 15

Modern Human Biology: Patterns of Adaptation

Chapter Outline

The Adaptive Significance of Human Variation

Infectious Disease

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 (genetic) evolutionary changes

characterize all individuals within a population or species.

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

Pigmentation and Geographical Divisions

Before 1500, skin color in populations followed a geographical distribution, particularly in the Old World.– Populations with the greatest amount of

pigmentation are found in the tropics.– Populations with lighter skin color are

associated with more northern latitudes.

Distribution of Skin Color Among Indigenous Populations

Skin Color

Influenced by three substances: 1. Hemoglobin, when it is carrying oxygen,

gives a reddish tinge to the skin.2. Carotene, a plant pigment which the body

synthesizes into vitamin A, provides a yellowish cast.

3. Melanin, has the ability to absorb ultraviolet radiation preventing damage to DNA.

Production of Vitamin D

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.

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.

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.

Small Pox

The only disease considered to be eliminated as a result of medical technology

Smallpox has a higher incidence in those with type A or AB than in those type O blood.

The immune systems of individuals with type A antigen may not recognize the small pox antigen as a threat.

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.

Zoonoses and Human Infectious Disease