Clines

• A gradient; Gradual change in genotypes or phenotypes over geographical space

Clines & Human Variation

• Different traits have different clinal distributions

– Reflect evolutionary forces

• Races cross cut clines

• Clines are impractical, yet have biological meaning

Explaining Human Biological Variation

• Skin Color

• Lactose Tolerance

• Adaptation to Temperature

• ABO Blood Groups

Clinal Distribution of Skin Color

Skin Color

Skin Color & Selection

• High Sunlight Environment

– Dark Pigment

• Advantages: Absorbs UV & protects; Disease Resistant

– Light Pigment

• Advantages: Increased Vitamin D synthesis; Less heat stress

• Disadvantages: Increased likelihood of sunburn (cancer, sweating problems, infections); Too much Vitamin D

Skin Color & Selection

• Low Sunlight Environment

– Dark Pigment

• Advantages: Absorbs heat

• Disadvantages: Reduced Vitamin D synthesis (rickets); Increased sensitivity to frostbite

– Light Pigment

• Advantages: Increased Vitamin D synthesis; Less susceptible to frostbite

• Disadvantages: Some heat loss

Skin Color & Selection

• Dark skin generally favored in tropics

• Light skin generally favored at high latitudes

• Huge amount of variation in between (clines)

• But, cultural factors: Diet, Shelter, Clothing, etc.

Lactose Tolerance / Intolerance

• Lactose = sugar in mammalian milk

• Lactase = enzyme that digests lactose

• All baby mammals have lactase but adults usually do not

• Some human adults do…why?

Lactose Tolerance / Intolerance

• Varies in different populations

• Lactose Intolerance (% without Lactase)

– US / European: 2-19%

– Fulani (African): 22%

– African-American: 70-77%

– Asian-American: 95-100%

– Thai: 99%

– Native American: 85%

Adaptation to Temperature

Adaptation (Natural Selection)

• Allen’s Rule: Arms & Legs Longer at Higher Temperatures

• Bergmann’s Rule: Body Size Within a Species Larger at Low Temperatures

Clinal Distribution of O-Gene of ABO Blood Group

Race: A Summary

• Race: Historically problematic & biologically inaccurate

• Cline: Better way of describing human biological variation

• Explain biological differences based on evolutionary forces

• Race a social category that may have very real consequences