ANTH 1120 Lecture Notes - Thrifty Gene Hypothesis, Lactase Persistence, Vitamin D Deficiency
by OC26715
October 27th 2011
ANTHRO*1120
Contemporary Human Diversity
1. Concept of “race”: a social construct not biologically valid
2. Relationships between evolutionary processes and cultural-environmental
contexts
3. Adaptive significance of human variation
4. As we change/evolve, so do other organisms
Reminder:
– Mutations and genetic variation
– Natural selection
– Polymorphisms: main focus of human variation studies (2 or more alleles)
– How adaptive a gene/trait is depends on the environment
○Dark moths and light moths in industrial England
Types of natural selection
– Directional selection: a particular trait seems to be positively favoured;
average values shift over time
– Normalizing selection: the average values do not change, but extremes
removed
○Birth weight-
– Balancing selection: when a heterozygous combination of alleles is positively
favoured even though a homozygous combination is disfavoured
○Sickle cell allele
Race: social construct, biological fallacy
– Humans classify things (order world around us)
○Classifications are not necessarily without bias
○Linking physical traits with behavioural traits
Overlooks complexities (such as culture)
Draws on the biases of the classifier
– Classifications can be essentializing , typological
○Racial classifications: product of social/cultural beliefs re:
inferiority/superiority
○Historical and contemporary discrimination
– anthropologists : concept of “race” cannot be applied to human variation
– AAPA (quote (pg.228)
Why?
– Different populations not clearly classifiable by discrete traits
For example: skin tone
– No clear breaks between skin tones and no other physical traits that
correspond with skin tones at all times
– Also: epicanthic fold (pg. 233)
Thus
– No single group of traits always applies to a group of people all the time
there is variation within “groups”
○Amount of genetic variation accounted for by differences between
groups is less than variation that exists within groups
The point?
– Human are a polytypic species
– Biological traits show clines
○Clines: gradual differences, rather than sharp, discrete differences, for
example:
Skin tone
Nose shape
– “race” classifications attempt to make clear boundaries where they aren’t
any (and are therefore divisive)
We can ask:
– If race is not a useful device for classifying humans, why is it so widely used
today?
Ideas of “biological determinism”
– The (incorrect) belief that there is an association between physical
characteristics and intelligence, behaviours, morals, values, abilities, social
and economic status
○Makes it possible to persecute others (a justification used to
perpetuate inequalities)
○Historical examples
Biological determinism
– Incorrect, but still prevalent today in various forms
○Racism
○So-called “research” that links ideas of intelligence and behaviours of 3
“races”
Evolutionary Processs + Adaptive significance
– Some human diversity not just due to physical stressors like UV radiation
○Also part of a response to the environment we have modified with our
own actions
○Biology shapes culture and vice versa
Examples
– Malaria, sickle-cell allele and the rise/spread of agriculture in Africa
○Mosquito breeding grounds, parasite spreads
○Malaria as “a powerful selective force”
Another aspect to malaria
– Food:
○Traditional crops: cassava, millet, sorghum, can sugar, dark lima beans
○Contain cyanate and thiocyanate, which seem to inhibit the sickling of
red blood cells- might reduce the symptoms of sickle cell anemia
Dietary changes thus can affect the sickle-cell situation
Examples:
– Lactase persistence (continued production of lactase enzyme after childhood;
allows milk digestion)
○Pastoralism and dependence on herd animals and milk changes in
the frequency of the lactase-persistent allele (selective pressure)
The point?
– Our actions, decisions, shape environment around us, and can also shape our
biology
– A dialectic
○Dialectic: dynamic tension within a given system and the process by
which change occurs on the basis of that tension
Human variation + adaptive significance
– Physical stressors
○ Extreme cold, extreme heat, UV radiation…
○Adapting to different environments
Altitudes
Variations in skin tone- why adaptive (advantageous)?
– First discussed in terms of exposure to solar radiation
○Darker tones closer to the equator greater protection against skin
cancers
– Vitamin D
○Liver, fish oils, butter, yolks, cream
○But can be produced by body too through exposure to UV radiation
○UV radiation weaker further from equator lighter skin would be
advantageous in these regions
○Rickets- vitamin D deficiency- larger forehead, wide bones, large
abdomen, odd shaped legs
– Folate degradation
○Folate: important in fetal neural tube development
Neural tube: the anatomical structure that forms brain and
spinal cord
Must be protected by bone
Spina bifida
○Folate susceptible to degradation by UV radiation
Darker skin would offer some more protection (a selective
advantage closer to Equator)