1. The Evolution of Populations
Chapter 23
The Evolution of Populations

2. Western Historical Context
Gregor Mendel ( )
Austrian monk whose breeding experiments with peas shed light on the rules of inheritance
Mendel was a contem- porary of Darwin, but his work was overlooked until the 20th century
With hindsight we can say that Darwin’s theory of evolution by natural selection was correct, but he didn’t know the mechanism of inheritance.

3. Western Historical Context
The Modern Synthesis (early 1940s)
A conceptual synthesis of Darwinian evolution, Mendelian inheritance, and modern population genetics

4. Potential for rapid population growth when resources are not limiting
Resource availability generally limits population size
Competition for resources
(“struggle for existence”)
Phenotypic variability (morphology, physiology, behavior, etc.)
Natural Selection: Survival and reproduction of the “fittest” individuals
Some variability results from heritable genotypic differences
The Modern Synthesis goes like this…

5. Phenotype vs. Genotype

6. Phenotype vs. Genotype
Phenotype: all expressed traits of an organism

7. Phenotype vs. Genotype Phenotype: all expressed traits of an organism
Genotype: the entire genetic makeup of an individual (i.e., its genome – it’s full complement of genes and the two alleles that comprise each locus), or a subset of an individual’s genes

8. Population = all of the individuals of a species in a given area
Evolution
A change in allele frequency in a population (a change in the gene pool)
Population = all of the individuals of a species in a given area
The Modern Synthesis also identified other mechanisms of evolution besides natural selection…

9. Potential for rapid population growth when resources are not limiting
Resource availability generally limits population size
Competition for resources
(“struggle for existence”)
Phenotypic variability (morphology, physiology, behavior, etc.)
Natural Selection: Survival and reproduction of the “fittest” individuals
Some variability results from heritable genotypic differences
Adaptive evolution: A change in the phenotypic constitution of a population owing to selection on heritable variation among phenotypes that changes the genotypic constitution of the population

10. Population Genetics
Examines the frequency, distribution, and inheritance of alleles within a population

11. Hardy-Weinberg Equilibrium
The population genetics theorem that states that the frequencies of alleles and genotypes in a population will remain constant unless acted upon by non-Mendelian processes (i.e., mechanisms of evolution)

12. See Figs. 23.4 & 23.5 – An example Let’s dissect an example…
See Figs & 23.5 for a textbook example in which the heterozygotes have a distinct phenotype.

13. See Figs & 23.5 – An example
Let’s dissect an example…

14. See Figs & 23.5 – An example
Let’s dissect an example…

15. See Figs & 23.5 – An example
Let’s dissect an example…
This means that 80% of sperm & eggs will carry R,
and 20% of sperm & eggs will carry r

16. Allele Frequencies
Under strict Mendelian inheritance, allele frequencies would remain constant from one generation to the next (Hardy-Weinberg Equilibrium) R
80% (p=0.8)
80% (p=0.8) R
Sperm
Eggs RR
p2=0.64 r
20% (q=0.2)
20% (q=0.2) r rR
qp=0.16 Rr
pq=0.16 rr
q2=0.04
Genotype frequencies: p2=0.64 (RR) 2pq=0.32 (Rr) q2=0.04 (rr)
Allele frequencies: p=0.8 (R) q=0.2 (r)

17. Allele Frequencies
At a later date, you determine the genotypes of 500 individuals, and find the following:
280 RR
165 Rr
55 rr
Frequency of R (a.k.a. “p”): = 725 R alleles in the pop / 1000 = 0.725
Frequency of r (a.k.a. “q”): = 275 r alleles in the pop / 1000 = 0.275

18. The population has EVOLVED!
Allele Frequencies
The frequencies of alleles R and r have changed:
320 RR
160 Rr
20 rr
280 RR
165 Rr
55 rr
T1:
T2:
p=0.8, q=0.2
p=0.725, q=0.275
The population has EVOLVED!

19. Hardy-Weinberg Equation
For a two-allele locus:
Let p = the frequency of one allele in the population (usually the dominant)
Let q = the frequency of the other allele
Notice that:
p + q = 1
p = 1 – q
q = 1 – p
Genotypes should occur in the population according to:
p2 + 2pq + q2 = 1

20. Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
p2 = proportion of population that is homozygous for the first allele (e.g., RR)
2pq = proportion of population that is heterozygous (e.g., Rr)
q2 = proportion of population that is homozygous for the second allele (e.g., rr)

21. Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
Given either p or q, one can solve for the rest of the above equation
What would q be if p = 0.6?
What would 2pq be if p = 0.5?

22. Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
Given the frequency of either homozygous genotype, the rest of the equation can be solved
What would q be if p2 = 0.49?
Hint: q = q2

23. Hardy-Weinberg Equilibrium
Is a null model…
like Newton’s first law of motion:
Every object tends to remain in a state
of uniform motion (or stasis),
assuming no external
force is applied to it
The Hardy-Weinberg Equation will be satisfied, as long as all the assumptions are met…

24. Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size
Because genetic drift affects smaller populations more than larger pops.
Genetic drift = allele frequency change due to chance
Genetic drift reduces genetic variability

25. See Fig. 23.7 Genetic drift in a small population of wildflowers

26. See Fig. 23.7 Genetic drift in a small population of wildflowers

27. See Fig. 23.7 Genetic drift in a small population of wildflowers

28. Genetic drift often results from populations passing through a population bottleneck

29. Genetic drift often results from populations passing through a population bottleneck

30. The founder effect is an example of a population bottle neck
Mainland
population

31. The founder effect is an example of a population bottle neck
Colonists from the mainland colonize an island
Mainland
population

32. The founder effect is an example of a population bottle neck
Colonists from the mainland colonize an island
Island gene pool is not as variable
as the mainland’s
Mainland
population

34. Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
Gene flow = transfer of alleles among populations
Emigration transfers alleles out of a population and immigration transfers them in

35. Gene flow connects populations
time
Population
at t2
(after immigration)
Island gene pool is not as variable
as the mainland’s
Population
at t1

36. Gene flow connects populations
Island gene pool is not as variable
as the mainland’s
Population
at t1

37. Gene flow connects populations
time
Population
at t2
(after immigration)
Island gene pool is not as variable
as the mainland’s
Population
at t1

38. Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations

39. Mutations generally boost genetic diversity
time
Population
at t2
(after immigration)
Island gene pool is not as variable
as the mainland’s
Population
at t1

40. Mutations generally boost genetic diversity
time
Population
at t2
(after a mutation event)
Island gene pool is not as variable
as the mainland’s
Population
at t1

41. Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
4) Random mating with respect to genotypes
E.g., imagine what would happen if RR males mated only with rr females
Those particular matings would result in no RR or rr offspring, thereby altering population-wide genotype frequencies

42. Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
4) Random mating with respect to genotypes
5) No natural selection
E.g., imagine what would happen if rr flowers were the only ones that ever attracted pollinators (even though the population contains RR and Rr individuals as well)

43. Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
4) Random mating with respect to genotypes
5) No natural selection

44. Variation within Populations
Let’s briefly review…
Adaptive evolution: A change in the phenotypic constitution of a population owing to selection on heritable variation among phenotypes that changes the genotypic constitution of the population

45. Variation within Populations
Since selection acts on phenotypes, yet evolution requires population-level genotypic change, it is important to understand intraspecific variation
Note: If all individuals were phenotypically identical, there would be no opportunity for selection
Note: If all individuals were genotypically identical, there would be no opportunity for evolution

46. Variation within Populations
Phenotypic variation results from both environmental and genetic influences
Consider identical vs. fraternal twins

47. Variation within Populations
Phenotypic variation results from both environmental and genetic influences
Phenotypic variation within populations is either discrete or quantitative/continuous
Discrete variation: polymorphism = mutiple phenotypes that are readily placed in distinct categories co-occur (e.g., our red and white flowers result from a polymorphic locus)
E.g., a “bar graph” trait like ABO blood type

48. Variation within Populations
Phenotypic variation results from both environmental and genetic influences
Phenotypic variation within populations is either discrete or quantitative/continuous
Continuous variation: quantitative characters = multiple loci produce a trait (e.g., flower size), and the trait varies continuously in the population
E.g., a “bell curve” trait like human height

49. Variation within Populations
Phenotypic variation results from both environmental and genetic influences
Phenotypic variation within populations is either discrete or quantitative/continuous
Phenotypic variation also exists among populations
E.g., geographic variation
Heliconius species A
Heliconius species B

50. Variation within Populations
How is genetic variation maintained?
1) Diploidy provides heterozygote protection
2) Balanced polymorphism
Heterozygote advantage
E.g., A locus for one chain of hemoglobin in humans has a recessive allele that causes sickle- cell anemia in homozygotes, but provides resistance to malaria in heterozygotes

51. Variation within Populations
How is genetic variation maintained?
1) Diploidy provides heterozygote protection
2) Balanced polymorphism
Heterozygote advantage
Frequency-dependent selection
3) Neutrality

52. Fitness
Darwinian fitness = an individual’s reproductive success (genetic contribution to subsequent generations)
Relative fitness = a genotype’s contribution to subsequent generations compared to the contributions of alternative genotypes at the same locus

53. Effects of Selection See Fig. 23.12 Coat color
This could be how the long neck of the giraffe evolved.

54. Directional selection consistently favors phenotypes at one extreme
Effects of Selection
Directional selection consistently favors phenotypes at one extreme
See Fig
Coat color
This could be how the long neck of the giraffe evolved.
Coat color

55. Stabilizing selection favors intermediate phenotypes
Effects of Selection
Stabilizing selection favors intermediate phenotypes
See Fig
Coat color
Coat color

56. Effects of Selection
Diversifying (disruptive) selection simultaneously favors both phenotypic extremes
See Fig
Coat color
Coat color

57. Directional, diversifying (disruptive), and stabilizing selection
Effects of Selection
Directional, diversifying (disruptive), and stabilizing selection
See Fig
Coat color
Coat color
Coat color
Coat color

58. Intrasexual selection, usually male-male competition

59. Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition
Dynastes tityus
Often leads to sexual dimorphism & exaggerated traits

60. Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition
Dynastes hercules
Often leads to sexual dimorphism & exaggerated traits

61. Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition
Lucanus elaphus
Often leads to sexual dimorphism & exaggerated traits

62. Intersexual selection, usually female mate choice

63. Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intersexual selection, usually female mate choice
Often leads to sexual dimorphism & exaggerated traits

64. Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intersexual selection, usually female mate choice
Often leads to sexual dimorphism & exaggerated traits

65. Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intersexual selection, usually female mate choice
Often leads to sexual dimorphism & exaggerated traits