CLINAL VARIATION
Gradual change in a trait
(opposite of discordant variation: abrupt change)
overhead 1 hair color and form
eye shape
body shape
arm and leg length
skin color
nose shape
overhead 2
traits vary independently
this makes it difficult to divide humans into RACES
(humans are all one species)
people have tried
physical anthropologists have tried
ask honest question: WHAT ARE DIFFERENCES AND WHY?
3 major discoveries:
1. more differences in groups than obvious SKIN & HAIR differences
2. Differences are good, diversity leads to evolutionary success
3. “races” are arbitrary categories
world is very COMPLEX - not have 4 categories of people
and traits vary independently
like ABO blood groups
earwax
overheads 3 & 4, then 5
CONCLUSION: can classify specific traits,
but not combinations of traits
“race” meaningless to science
0.01% of genes for skin color
is more variation within groups than between groups
WHY VARIATION?
2
Dark skin – lots of sunlight: skin cancer
Light skin – lots of clouds: vitamin D, rickets -> pale skin advantage
Nose shape - dry air, cool air, cold air
Eye lids- cold
So CLINAL VARIATION
BERGMAN’S RULE: Explains body shape variation
To hold heat - more volume, less surface
To radiate heat - more surface, less volume
Most volume for surface area?
Sphere
Most surface, less volume?
Stick
ALLEN’S RULE: Explains variation in extremity length
Longer is better if hot - radiate heat
Shorter is better if cold - conserve heat
Natural Selection operating on a population will produce differences
over time - Reproductive Success of individuals is different
Like genetic adaptation to high altitude:
Low O2 pressure-> those with higher aerobic capacity (bigger lungs)
able to have more children - so Quechua Indians in Andes
have bigger lungs after thousands of generations
(Andes Mountains: 8,000 - 17,000 feet)
NATURAL SELECTION
explains a lot of differences among human population
ADAPTATION TO DIFFERENT ENVIRONMENT