1. Evolution in Populations (or How Natural Selection Works)

2. To take a step back:
We have talked about
1) Historical views of organic change
2) Darwin’s views of organic change
3) Evidence for change through time
(small and larger scale examples)
Next few lectures : HOW this change through time happens.

3. Definitions Natural Selection
Evolution (from previous slide show)
Changes over time of the proportion of individuals differing genetically in one or more traits
**PATTERN**
Natural Selection
Differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment.
**PROCESS**

4. A Little Basic Genetics
Gregor Mendel
( )

5. A Little Basic Genetics
Imagine a very simple creature in which every cell has one chromosome -

6. A Little Basic Genetics
But every sexually reproducing creature has 2 of each chromosome
In you, there are 23 pairs of chromosomes for a total of 46

7. Each chromosome contains a series of genes but for now we’ll imagine that there is just one on our chromosome
Gene A

8. Now assume that there are two versions of each gene - represented with a
capital and a small letter
Gene A a

9. Another idea - Cell Division
There are 2 kinds of cell division
1. Mitosis - (duplication) - every cell makes a copy of itself
(this is how you grow and replace dead cells)
2. Meiosis - (reduction) - cells divide in such a way as to reduce the number of chromosomes by 50%

10. Meiosis
In one of the stages of meiosis, the chromosomes line up on the midline of the cell and are separated from each other when the cell divides A A a A a a
Chromosomes line up
The cell divides

11. And just to inject a little bit of reality…….
These are
eggs or sperm a

12. And a little arithmetic…….
What are the chances of getting each type of egg or sperm? A A a
50% chance of each a

13. A few more definitions and terms
Trait - any physical characteristic of an organism
(e.g. eye colour, hair colour, height)
(for now you can assume that gene = trait)
Allele - alternate versions of a gene or trait
(e.g. blue, brown, green, or hazel eyes)
Phenotype - the physical appearance of an organism
Genotype - the genetic makeup of an organism

14. In our simple creature
we have one trait A with two alleles of each
A - the dominant allele for trait A
a - the recessive allele for trait A
Probability of getting either a or A = 0.5

15. What about producing offspring
What about producing offspring? - all the possible combinations of eggs and sperm A a a A A A a a

16. Or to put this a little more formally - A Punnett square
Female parent
A a
(0.5) (0.5) A
(0.5) a AA Aa
Male
parent Aa aa
The letters in each box are the GENOTYPE of the offspring

17. Or to put this a little more formally - A Punnett square
Female parent
A a
(0.5) (0.5) A
(0.5) a AA
(.25) Aa
(.25)
Male
parent Aa
(.25) aa
(.25)
The letters in each box are the GENOTYPE of the offspring

18. So we have 3 possible genotypes
A a
(0.5) (0.5) A
(0.5) a AA
(.25) Aa
(.25) Aa
(.25) aa
(.25)
So we have 3 possible genotypes
AA Aa (x2) aa
Ratio

19. So we have 3 possible genotypes
A a
(0.5) (0.5) A
(0.5) a AA
(.25) Aa
(.25) Aa
(.25) aa
(.25)
So we have 3 possible genotypes
AA Aa (x2) aa
Ratio
….and two possible phenotypes
A_ aa
Ratio 3 1

20. We can generalize this idea to be more inclusive
If you say that the frequency of the dominant allele is ‘p’
and the frequency of the recessive allele is ‘q’
In our example, p = 0.5 and q = 0.5
And p + q = 1
(p + q always equals1)

21. And if you mate two organisms, you can mathematically
determine the expected proportion of offspring of each type
p + q
p2 + 2pq + q2

22. In our simple organism, p = q = 0.5
and p2 + 2pq + q2
= (0.5)(0.5) + 2 (0.5)(0.5) + (0.5)(0.5) =
Which is back to our 1:2:1 genotypic ratio p2
2pq q2

23. This idea holds true for any value of p or q.
For example:
If p is very common - say 90% of the genes in the population
Then p = .9 and q = .1
And p2 = .81 (the frequency of the AA genotype)
2pq = .18 (the frequency of the Aa genotype)
q2 = .01 (the frequency of the aa genotype)

24. In the early 1900’s, Hardy and Weinberg used this idea to establish a fundamental idea in the genetic basis of natural selection

25. • In Generation 1 p2 + 2pq + q2 = .36 + .48 + .16 In Generation 2
The Hardy-Weinberg Equilibrium
Assume that p = .6 and q = 0.4
In Generation 1
p2 + 2pq + q2 =
In Generation 2
p2 + 2pq + q2 =
In Generation 3
p2 + 2pq + q2 =
In Generation 4
p2 + 2pq + q2 = •

26. The Hardy-Weinberg Equilibrium
In any population, allelic and genotypic frequencies will remain the same if Mendelian inheritance patterns are the only factors at work
Requires:
Very large population size
2. No gene flow -no movement of genetic material between populations
No mutations
4. Random mating
5. No natural selection

27. The Hardy-Weinberg Equilibrium
In any population, allelic and genotypic frequencies will remain the same if Mendelian inheritance patterns are the only factors at work
Requires:
Very large population size
No gene flow -no movement of genetic material between populations
No mutations
Random mating
No natural selection

28. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Genetic Drift - a statistic consequence of small populations

29. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Genetic Drift - a statistic consequence of small populations

30. The Hardy-Weinberg Equilibrium
1. Large population sizes

31. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Bottlenecks

32. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Bottlenecks
Northern Elephant Seal

33. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Bottlenecks
1700’s - ??
est.
,000
Northern Elephant Seal

34. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Bottlenecks
Assay of 16 genes in
159 individuals
0/16 had any variability
Northern Elephant Seal

35. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Founder effect

36. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Founder effect
1740’s
Amish

37. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Founder effect
Ellis-Van Creveld Syndrome
Polydactyly
congenital heart defects
pre-natal tooth eruption
short-limbed dwarfism, short ribs
cleft palate,
malformation of the wrist bones
Amish

38. The Hardy-Weinberg Equilibrium
1. Large population sizes
What happens if the population isn’t ‘large’?
Founder effect
U.S. Population
Amish
700
600
500
400
300
200
100
500
Incidence in 100,000 Births
1.4

39. ` The Hardy-Weinberg Equilibrium
2. Mutations - source of all new genetic variation
Rates of mutation - typically about
(may be as low as )
OR 1 mutation/100,000 genes/generation
(1 mutation in /generation)

40. 2. Mutations - source of all new genetic variation
How do we model this?
Frequency of aa
Frequency of AA
Frequency of Aa
Imagine that ‘A’ mutates to ‘a’ at a rate of m per generation
Frequency of A after one generation of mutation
Frequency of A after a second generation of mutation

41. For any number of generations (x)
Imagine that ‘A’ mutates to ‘a’ at a rate of m per generation
Frequency of A after one generation of mutation
Substitute
Frequency of A after a second generation of mutation
For any number of generations (x)

42. The Hardy-Weinberg Equilibrium
3. Random mating
This assumes no preferences in mates
Humans: Preferences
Height - we tend to mate with people closer to our own height

43. The Hardy-Weinberg Equilibrium
3. Random mating
1.0 .1
.01
.001
.0001
Humans - distance from home
Probability
of mating
Distance from ‘home’

44. The Hardy-Weinberg Equilibrium
3. Random mating
1.0 .1
.01
.001
.0001
Humans - distance from home
Probability
of mating
40 km
Distance from ‘home’

45. The Hardy-Weinberg Equilibrium
3. Natural Selection
-depends on variability that is heritable
-differences must be passed to the offspring
Key idea :
Fitness: The contribution an individual makes to the gene pool of the next generation relative to other individuals

46. The Hardy-Weinberg Equilibrium
3. Natural Selection
-depends on variability that is heritable
-differences must be passed to the offspring
Key idea :
Fitness: The contribution an individual makes to the gene pool of the next generation relative to other individuals
Higher fitness
Lower fitness

47. Types of Natural Selection
Most traits have a normal (or bell curve distribution)

48. Types of Natural Selection
Most traits have a normal (or bell curve distribution)

49. Types of Natural Selection
1. STABILIZING SELECTION

50. Types of Natural Selection
1. STABILIZING SELECTION
Human birth weight

51. Types of Natural Selection
2. DIRECTIONAL SELECTION 

52. Types of Natural Selection
2. DIRECTIONAL SELECTION 
Salmon fishing - largest fish are taken every year

53. Types of Natural Selection
3. DISRUPTIVE SELECTION 

54. Types of Natural Selection
3. DISRUPTIVE SELECTION • •  • •
Breed together • • •
Negatively geotactic • •
Selection of positively and negatively geotactic Drosophila •
Positively geotactic • • • • • •
Breed together • • • • • • • •
Artificial Selection - Drosophila geotaxis •

55. Types of Natural Selection
3. DISRUPTIVE SELECTION 
Selection for bristle number in Drosophila

56. Modelling Natural Selection
Select against recessive homozygote - aa
AA Aa aa Total
Initial Frequency
p2 2pq q2 1
Fitness s
Next Generation
1 1 q2(1 – s) 1 – sq2
Normalized