Overview
This lecture discusses the evolutionary origins, adaptive functions, and unique characteristics of different human eye colors, highlighting their physical, genetic, and cultural significance.
Brown Eyes
- Brown eyes contain high levels of melanin, giving strong protection against sunlight and UV radiation.
- Melanin absorbs light, reducing glare and making brown eyes stable in various lighting conditions.
- Brown eyes minimize risk of sun-related conditions like photophobia and macular degeneration.
- Over 70% of the global population has brown eyes, reflecting their evolutionary advantage.
- Brown eyes appear uniform but can show golden tones in specific lighting.
Blue Eyes
- Blue eyes have very little melanin, allowing more light in and improving vision in low-light environments.
- Their blue color results from light scattering, not pigment.
- Blue eyes are sensitive to light and common in northern regions with less sunlight.
- A single mutation 6,000–10,000 years ago led to the first blue eyes.
- Blue eyes can change appearance based on lighting conditions.
Green Eyes
- Green eyes have moderate melanin and lipochrome, balancing light absorption and sensitivity.
- Only about 2% of people have green eyes, making them rare.
- Color results from pigment blend and light scattering, shifting with lighting.
- Their dynamic hue and rarity have made them symbols of mystery and magic.
Hazel Eyes
- Hazel eyes have intermediate melanin and lipochrome, adapting well to variable environments.
- Their color shifts between green, gold, and brown depending on lighting.
- Hazel eyes are found mainly in European or Middle Eastern populations.
Amber Eyes
- Amber eyes are rare, containing high lipochrome and little melanin, giving a stable golden color.
- Their color does not change with lighting and is often seen in people of southern European, Asian, or South American descent.
- Amber eyes are sometimes mistaken for light brown or hazel but are uniquely uniform and intense.
Gray Eyes
- Gray eyes have less melanin than blue eyes, scattering light across multiple wavelengths.
- Most common in Northern Europe, they aid vision in low-light conditions.
- Gray eyes remain neutral and stable in tone, appearing silver or slate.
- Less than 3% of the population has gray eyes.
Black Eyes
- Black eyes are an extreme form of dark brown due to dense melanin, maximizing light absorption and protection.
- Common in Africa, South Asia, and Southeast Asia, black eyes stay visually stable in all lighting.
- Their depth and flat appearance make expressions harder to read.
Heterochromia
- Heterochromia refers to having two differently colored eyes or color variations within one iris.
- Caused by uneven melanin distribution, genetic mutations, or injury; usually benign.
- Less than 1% of people have heterochromia, often linked with mystery in cultural symbolism.
Key Terms & Definitions
- Melanin — dark pigment that provides color and UV protection in eyes, skin, and hair.
- Lipochrome — yellow pigment present in some eye colors like green or amber.
- Photophobia — sensitivity to light.
- Light scattering — process causing blue and gray eye color, not due to pigment.
- Heterochromia — condition of having two different eye colors or uneven iris pigmentation.
Action Items / Next Steps
- Reflect on your own eye color characteristics and any associated sensitivities.
- Prepare for a review on the adaptive significance of eye color in different environments.