25. Are we still evolving?

How does variation between human populations compare to that of chimpanzees?

Variation between human populations is HALF as large as variation between chimpanzee populations. There is no subspecies of humans.

Variation of Humans

There is more variation WITHIN populations than BETWEEN populations due to gene flow

What did ‘scientific justification’ lead to?

• Scientific Racism

• Biological Determinism

• Eugenics

Adaptation

When a population experiences a change in its genotypes in response to an environmental stressor across generations.

Acclimatization

When an individual experiences change in its phenotype in response to the environment, on the scale of days to weeks.

Adaptations: Altitude

• Present in areas that are 2500+ meters above sea level.

• Emerged to prevent hypoxia

• Populations that independently adapted:
  - Andean Highlanders
  - Tibetan Highlanders
  - Ethiopian Highlanders

• Result of convergent evolution

Adaptations: Lactase persistence

• Ability to produce lactase

• Emerged from humans frequently consuming dairy products

• Evolved independently (convergent evolution) in several populations

Adaptations: Skin pigmentation

• Human skin varies in pigmentation

• Two forms of melanin: eumelanin (Darker) and pheomelanin (Lighter)

• Skin color gets darker towards the equator (most solar radiation)

• Darker skin is better at protecting Folate from UV radiation

• Lighter skin is more efficient in vitamin D production

Adaptations: Malaria resistance

• Increase in the occurrence of the sickle cell trait (allele) due to living in areas affected by malaria

• Increase in heterozygotes (A person has only one copy of sickle cell)

• Example of stabilizing selection

Adaptations: Arsenic tolerance

• People living in areas with high levels of arsenic adapt to better excrete arsenic from their bodies

• Example of directional selection

Class Takeaways

1. Science is a process of inquiry, not a single event of discovery

2. Biological evolution is not progressive*

3. Humans did NOT evolve from chimpanzees; both species evolved from a shared ape  ancestor

4. We are still evolving in response to selective pressures

5. Multiple mechanisms of inheritance impact modern humans

6. Race is NOT a real biological category,  but racism has real social and biological effects