1. Evolution Examples
Lactase persistence

2. The Cell Biology of Lactase Persistence

3. Biology of the Digestive Tract
Enterocytes (the cells that line the inside of digestive tract) are responsible for breaking down and absorbing nutrients from the food in the stomach and small intestine.

4. First Food: Mother’s Milk
The enterocytes of all infant mammals exhibit high levels of lactase during infancy, when milk is the main source of nutrition.
Photo credit: Jim French, Flickr

5. Lactase Unlocks an Energy Source
Lactose is a disaccharide sugar found in milk.
Lactase breaks down lactose into
two monosaccharides, glucose and galactose.
These simple sugars can be absorbed by cells in the small intestine and used as a source of energy.
Lactase
(enzyme)
Lactose
Glucose
Galactose

6. Location of Lactase
This is the “brush border” of an enterocyte, which is the side of the enterocyte that comes in contact with the contents of the small intestine. Lactase is stained brown.

7. Lactase Breaks Down Lactose
Glucose
Galactose
Lactose
Lactase
INTESTINE CONTENTS ENTEROCYTE BLOODSTREAM
Lactase, PURPLE, is a transmembrane protein on the interior border of the enterocyte, YELLOW. When lactose comes into contact with its active site (depicted as an oddly shaped extension into the intestine contents, GREEN), it is broken down into Glucose and Galactose

8. Lactase Regulation
Almost all known mammals experience a decrease in lactase biosynthesis in the years after weaning. The regulation of lactase biosynthesis after weaning is the main factor that separates Lactase Persistent from Lactase Non-Persistent individuals.
Definition: Weaning: the process of gradually replacing a infant mammal’s diet of breast milk with what will be its adult diet

9. Why does this happen? Why can’t adult mammals digest milk?
Lactase Regulation
Why does this happen? Why can’t adult mammals digest milk?

10. Lactase Regulation
The decrease in lactase production after weaning is likely a matter of energy conservation at a cellular level:
It takes energy to produce any enzyme, including lactase, the enzyme needed to digest milk.
Typically, mammals do not consume milk once they have stopped nursing.
Without milk consumption, energy spent producing lactase would be energy wasted at the cellular level.
Therefore, over time, the more energetically favorable option has been selected for: a decrease in lactase production after weaning.
This is an important point about evolution at the molecular scale. Be sure to stress cellular energetics as the “building blocks” of making the most efficient “machine,” or animal.

11. Lactase not Produced in Adults
If undigested lactose passes into the large intestine, it will trigger the symptoms of Lactose Intolerance.
1. The increased sugar concentration in the large intestine creates an osmotic gradient that draws water into the gut. This causes cramping and diarrhea.
2. Bacteria in the large intestine digest the lactose as food, creating gaseous by-products like methane, carbon dioxide, and hydrogen. This leads to gas build-up and flatulence.

12. Lactose Interolerance
For around 10% of Americans, 10% of Africa's Tutsi tribe, 50% of Spanish and French people, and 99% of Chinese, a tall cold glass of milk means an upset stomach and other unpleasant digestive side effects.

13. Lactase Persistence
Some humans do produce lactase after weaning, and are therefore able to continue to consume milk and other dairy products into adulthood.

14. Discussion 1. How do you think this is possible?
2. What changed in order for these adult humans to be able to digest milk?

15. Discussion 1. How do you think this is possible?
2. What changed in order for these adult humans to be able to digest milk?
This is possible through Lactase Persistence, or the continued production of lactase at high levels throughout adulthood.
The down-regulation of lactase biosynthesis that normally occurs must have been prevented or counteracted.

16. The Neolithic Revolution
Image source:
The Neolithic revolution describes a period of time, between 12,000 and 6,000 years ago, during which humans around the world began transitioning from a hunter-gatherer lifestyle to a farming-herding lifestyle.

17. Pastoralism
Pastoralism, the cultural practice of milking livestock (such as goats, sheep, cows, and camels), was another innovation of the Neolithic Revolution. It was adopted in various cultures between 12,000 and 7,000 years ago.

18. Milking Livestock and Drinking that Milk
The Biocultural Coevolution theory proposes that pastoralism and lactase persistence coevolved. This means that they arose around the same time*, and both changes were reinforced by each other.
*In evolutionary terms, “around the same time” can mean a couple thousands of years. Remember that the timescale for evolution is extraordinarily slow.
Image source:

19. An Apparent Paradox? What was going on?
Anthropological evidence places the advent of pastoralism at 10,500 to 6,500 years ago.
However, genetic research says the lactase persistence trait did not become widespread in Europe until 7,000 to 5,000 years ago.
This suggests that for several thousand years some humans were milking sheep, goats, cows, or camels despite being unable to digest milk.
What was going on?

20. Cheese
It is likely that Neolithic humans fermented milk into cheese, which greatly reduced the lactose content of the dairy product, making it more accessible.
Ancient pottery remnants like this, found in Northern Europe, were likely sieves used to strain and ferment milk into cheese

21. Lactose Tolerant Mutation
The lactose tolerance mutation arose randomly (as all mutations do), but once it arose, it had a distinct advantage in these populations.
Natural selection would have favored individuals carrying the lactose tolerance mutation, spreading it through ancient European populations that depended on dairying.

22. How Did Lactase Persistence Spread?
Lactase persistence is thought to have arisen and spread through two types of natural selection: positive selection (selection for advantageous traits) and negative selection (selection against disadvantageous traits).
Positive Selection and Negative Selection are not the same thing.
Additional material may be required to better explain this if students are having trouble.

23. How Did Lactase Persistence Spread?
Negative Selection
Positive Selection
(disadvantages of Lactose Intolerance)
(advantages of Lactase Persistence)
Individually: Non-LP individuals missed out on a potentially important source of nutrition and hydration
Individually: Milk supplies protein, fat, sugar, and vitamins, and is dependable despite cold weather and/or bad crops
Experiencing painful, dehydrating symptoms upon consuming milk (see slide 15), which could be deadly.
Neolithic women who could digest milk were estimated to produce 32% more offspring.
Culturally: Milk is a more efficient protein source: it does not require killing livestock, yet the milk from one cow nearly equals the caloric value of the meat from a whole cow.
Culturally: Without pastoralism, herders must slaughter their livestock to gain dietary protein from their meat.
Information from The Milk Revolution, Andrew Curry

25. The Natural History of Color Vision and Colorblindness
** Notes for this section:
The following slides are included as an overview of the natural history of the Monkey Opsins case.
For more information visit:
In this section pairs of monkeys are shown so that students can observe the biogeographic pattern that old world monkeys are trichromatic whereas new world monkeys are not.
Two questions are posed that can be used as a basis for peer-peer discussions or other interactive engagement.

26. What is colorblindness?
Reduced ability to interpret light as color. 1 in 12 males are colorblind. < 1 in 100 females are colorblind.

27. Color Vision in Monkeys
Species: Grey Cheeked Mangabey
Lives: Africa
Vision: Trichromatic vision (i.e. like most humans)
Species: White Headed Capuchin
Lives: Central and South America
Vision: Dichromatic vision (i.e. “colorblind”.

28. Color Vision in Monkeys
Species: Japanese Macaque
Lives: Asia
Vision: Trichromatic vision
Species: Black Squirrel Monkey
Lives: Central and South America
Vision: Dichromatic vision

29. Color Vision in Monkeys
Species: Guinea Baboon
Lives: Africa
Vision: Trichromatic vision
Species: White Faced Saki
Lives: Central and South America
Vision: Dichromatic vision

30. Color Vision in Monkeys
Species: Roloway Monkey
Lives: Africa
Vision: Trichromatic vision
Species: Pied Tamarin
Lives: Central and South America
Vision: Dichromatic vision

31. What have you noticed? Is there a relationship between color vision and continent of origin among monkey species?

32. Monkeys of the World

33. Question
Why do most New World Monkeys have dichromatic vision while most Old World Monkeys have trichromatic vision?

34. The Ecology of Color Vision in Monkeys
** Notes for this section:
The following slides are included as an overview of the ecology of the Monkey Opsins case.
For more information visit:
In this section the importance of color vision in selecting fruit is demonstrated.
The results of three studies are documented – one by Caine and Mundy (2000) one by Smith et al (2003) and one by Saito et al
Several questions are posed that can be used as a basis for peer-peer discussions or other interactive engagement.

35. Ecology: Why Trichromatic Vision?
DICHROMATIC VISION
One of the above leaves has black fungus on it. Can you tell which one?

36. Ecology: Why Trichromatic Vision?
Leaf with black fungus

37. Ecology: Why Trichromatic Vision?
DICHROMATIC VISION
In the above picture, the red leaves are not palatable but the green leaves are nutritious.
Which leaves are which?

38. Ecology: Why Trichromatic Vision?
DICHROMATIC VISION
TRICHROMATIC VISION
With trichromatic vision, distinguishing between important colors becomes possible.

39. Food Selection – The Driver of Trichromacy Evolution?

40. Food Selection
Research has linked color vision with the ability to select ripe food when foraging.

41. Food Selection – Summary
Research suggests that trichromatic vision is more likely to be selected for when food is distinguished from non-food by color.
Research suggests that dichromatic vision is more likely to be selected for when food is distinguished from non-food by shape.

42. The Cell Biology of Color Vision in Monkeys
** Notes for this section:
The following slides are included as an overview of the cell biology of the Monkey Opsins case.
For more information visit:
A retinal chromophore opsin molecule is activated by a photon of light. This in turn stimulates the opsin protein that tells the cone cell to send a signal to the brain.

43. How Does Color Vision Work? Cell Biology
The retina has two types of cells: rod cells and cone cells. There are more rod cells than cone cells. Cone cells are responsible for color vision.

44. Three types of Cone Cell
Different kinds of opsin proteins embedded in the membrane of cone cells.
Central Dogma of Molecular Biology:
DNA  RNA  Protein
Genes code for…. proteins.

45. Chromatic Vision: Cone Cells
Cone cells in the retina of the eye allow light of different wavelengths to be interpreted as color in the brain.
The Brain
Light Waves
Color
The Cone cell

46. The Role of Opsins
There are three types of opsins: Short Wave Sensitive (SWS) Medium Wave Sensitive (MWS) Long Wave Sensitive (LWS) An individual possessing only SWS and MWS opsins will have dichromatic vision. An individual possessing SWS, MWS and LWS opsins will have trichromatic vision.

47. The Genetics of Color Vision in Monkeys
** Notes for this section:
The following slides are included as an overview of the genetics of the Monkey Opsins case.
Trichromatic vision arises from gene duplication and gene mutation of the MWS opsin protein, resulting in the creation of the LWS opsin protein.
For more information visit:

48. The Genetics of Color Vision
The section of DNA on a chromosome that codes for an opsin protein is called an opsin gene.

49. Location of Opsin Genes
The gene coding for the SWS opsin protein is located on chromosome #7. The gene coding for the MWS and LWS opsins are located on the X-chromosome.
OPTIONAL ACTIVITY! See activity notes.

50. Evolution of LWS Opsin Gene
The LWS gene arose through gene duplication and gene mutation of the MWS gene on the X-chromosome.

51. Gene Duplication
Gene duplication happens during meiosis in prophase I where unequal crossing over can occur.

52. Unequal Crossing Over (Meiosis, Prophase 1)
For more information see:

53. Phylogenetics – Exploring Relationships Among Species
This tree is a representative sample of new world and old world monkeys. It is not meant to encapsulate all species.
From a genetic perspective, Old World Primates are more closely related to one another than they are to New World Primates (and vice versa). Evidence suggests that they share a common ancestor in whom the gene duplication and mutation events occurred.

54. Primates In New/Old World
Squirrel Monkey
Owl Monkey
Spider Monkey
Wooly Monkey
Chimpanzee
Human
Gorilla
Orangutan
Gibbon
Rhesus
Baboon
Mangabey
Mona
Langur
Colobus
Capuchin
Marmoset
Sakis
Continents Split
50 Million Years Ago
ATTENTION!!! ANIMATED SLIDE!
A non-animated version of this slide follows.
These events can now be mapped onto the phylogenetic tree to give a fuller picture.
Research suggests that color vision evolved around 40 million years ago:
Old World
New World
Color Vision Evolves!
Gene Duplication and Mutation
Primates In New/Old World
55 Million Years Ago
Rise of Primates
75 Million Years Ago

55. What next?
Research indicates that some human females have tetra-chromatic vision. Describe how this can be possible from a cell biology and genetic perspective.