1. Chapter 23: The Evolution of a Population

2. Population
Group of individuals in a geographic location, capable of interbreeding and producing fertile offspring
Natural selection acts on individuals,
but only populations evolve

3. Galapagos Islands
Population of medium ground finches on Daphne Major Island
During a drought, large-beaked birds were more likely to crack large seeds and survive
Evolution by natural selection

4. Types of Evolution
Microevolution= change in the gene pool of a population over many generations
4 Methods of Microevolution
Mutations
Natural Selection
Genetic Drift= chance events cause genetic changes from one population to the next
Gene Flow= individuals or gametes move to a different population

5. Genetic Drift
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next
Bottleneck Effect= event kills a large number of individuals and only a small subset of population is left
Founder Effect= Small number of individuals colonize new location

6. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat= severe reduction in the population of greater prairie chickens in Illinois
Low levels of genetic variation, only 50% of their eggs hatched
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken

7. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck
Results showed a loss of alleles at several loci

8. Number of alleles per locus Percentage of eggs hatched Population size
Figure 23.11b
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
1930–1960s
1993
1,000–25,000
<50
5.2
3.7 93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8 99
Figure Genetic drift and loss of genetic variation.
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8 96

9. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Researchers introduced greater prairie chickens from population in other states
Introduced new alleles into population
Increased the egg hatch rate to 90%

10. Genetic Drift: Summary
Genetic drift is significant in small populations
Genetic drift causes allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed

11. Gene Flow Gene flow = movement of alleles among populations
Alleles can be transferred through:
Movement of fertile individuals
Gametes
Gene flow tends to reduce variation among populations over time
Barriers to dispersal can limit gene flow between populations

12. Geographic Variation Most species exhibit geographic variation
Differences between gene pools of separate populations
Mice in Madeira
Island in Atlantic Ocean
Several isolated populations of non-native mice
Mountain range prevents gene flow
Fusion of chromosomes

13. Geographic Variation
Variation among populations is due to drift, not natural selection

14. Geographic Variation
Cline= graded change in a trait along a geographic axis
Mummichog Fish and Cold-Adapted Allele
Effect of natural selection

15. Natural Selection
Evolution by natural selection involves both change and “sorting”
New genetic variations arise by chance
Beneficial alleles are “sorted” and favored by natural selection
Only natural selection consistently results in adaptive evolution

16. Evolution
Evolution by natural selection is possible because of genetic variation in population
Gene pool= all the alleles for all loci in a population
Genetic Variation = differences in DNA sequences
Gene variability
Average heterozygosity= average percent of loci that are heterozygous in a population
Fixed loci= all individuals in a population have same allele
Nucleotide variability
Measured by comparing the DNA sequences of pairs of individuals

17. Evolution
Phenotype= product of inherited genotype + environmental influences
Discrete characters= classified on an either-or basis
Flower color: red or white
Quantitative characters= vary along a continuum within a population
Skin color in humans
Natural selection can only act on variation with a genetic component

18. Evolution Genetic variation primarily comes from 2 sources:
Mutation and Gene Duplication
Original source of new alleles or genes
May be neutral before it becomes an advantage
“raw material” of evolution
Sexual Reproduction= unique combination of genes following crossing over, independent assortment of chromosomes, and random fertilization

19. Evolution
Genetic drift and gene flow do not consistently lead to adaptive evolution
Can increase or decrease the match between an organism and its environment
Natural selection increases the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the match between an organism and its environment increases
Because the environment can change, adaptive evolution is a continuous process

20. Evolution of a Population
Types of Natural Selection
Directional Selection
Highest reproduction in one extreme phenotype
Stabilizing Selection
Highest reproduction of intermediate phenotypes
Disruptive Selection
Highest reproduction of two extreme phenotypes
Generations 1 2 3

21. Frequency of individuals
Figure 23.13
Original population
Frequency of
individuals
Phenotypes (fur color)
Original
population
Evolved
population
Figure Modes of selection.
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

22. Sexual Selection Form of natural selection
Individuals with certain traits are more likely to mate
Sexual Dimorphism= differences in appearance of males and females
Vertebrates= males usually “showier” of sexes or engage in competition for females
Characteristics may be disadvantage
Male birds with bright feathers more obvious to predators

23. Sexual Selection
Intrasexual selection= competition among individuals of one sex (often males) for mates of the opposite sex
Intersexual selection (mate choice)= individuals of one sex (usually females) are choosy in selecting their mates

24. Sexual Selection How do female preferences evolve?
Good genes hypothesis= if a trait is related to male health, both the male trait and female preference for that trait should increase in frequency

25. Example of Sexual Selection
Females select males based on traits indicating defenses against parasites and pathogens
Bird, Mammal, and Fish Species
Female stickleback fish and Major Histocompatibility Complex (MHC)
Higher reproductive success increases frequency of defense trait in next generation

26. Genetic Variation in Populations
Neutral variation= genetic variation that does not confer a selective advantage or disadvantage
Various mechanisms help to preserve genetic variation in a population
Diploidy= maintains genetic variation in the form of hidden recessive alleles
Heterozygotes can carry recessive alleles that are hidden from the effects of selection

27. Genetic Variation in Populations
Balancing selection= natural selection maintains stable frequencies of two or more phenotypic forms in a population
Balancing selection includes
Heterozygote advantage
Frequency-dependent selection

28. Heterozygote Advantage
Malaria caused by protist, Plasmodium
1-2 million people die/yr from disease
Modifies red blood cells to obtain nutrients, escape destruction by spleen

29. Heterozygote Advantage
Sickle-Cell Allele: recessive allele
Produces abnormal hemoglobin proteins
Normal Blood Cell
Sickle-Cell

30. Heterozygote Advantage
Homozygous Dominant
No sickle cell allele
Susceptible to malaria
Homozygous Recessive
Sickle-cell disease
Resistant to malaria
Heterozygotes= co-dominant alleles, both types of blood cells
Heterozygotes have decreased symptoms of malaria and decreased symptoms of sickle-cell disease
Higher survival

31. Frequency-Dependent Selection
Fitness of a phenotype declines if it becomes too common in the population
Selection can favor whichever phenotype is less common in a population
Example: Predators can form a “search image” of their prey
Most common phenotype
Rare phenotypes may avoid detection by predators, increasing survival and reproduction

32. Limits of Natural Selection
Selection can only act on existing variation in a population.
New alleles do not appear when needed
Evolution is limited by historical constraints.
Ancestral structures are adapted to new situations
Adaptations are usually compromises.
One characteristic may be an adaptation in one situation, a disadvantage in another
Natural selection interacts with chance/random events and the environment.
Chance events can alter allele frequencies in population
Environment can change

33. Evolution of Populations
Measured by calculating changes in gene pool over time
Frequency of an allele in a population
Diploid organisms: total number of alleles at a locus is the total number of individuals times 2
Homozygous Dominant= 2 dominant alleles
Heterozygous= 1 dominant, 1 recessive allele
Homozygous Recessive= 2 recessive alleles

34. Frequency of Alleles in Population
For a characteristic with 2 alleles, we can use p and q to represent their frequencies
Frequency of Alleles:
p= frequency of “A” allele (dominant)
Total number of “A” alleles/total number of alleles
q= frequency of “a” allele (recessive)
Total number of “a” alleles/total number of alleles
Frequency of Alleles= p+ q =1

35. Frequency of Alleles in Population
Population of wildflowers that is incompletely dominant for color
320 red flowers (CRCR)
160 pink flowers (CRCW)
20 white flowers (CWCW)
Calculate the number of copies of each allele:
CR  (number of homozygotes for CR X 2) + number of heterozygotes
(320  2)  160  800
CW  (number of homozygotes for CW x 2) + number of heterozygotes
(20  2)  160  200

36. Frequency of Alleles in Population
To calculate the frequency of each allele:
p  freq CR  number of CR alleles/total number of alleles
p = 800 / (800  200)  0.8
q  freq CW  number of CW alleles/total number of alleles
q= 200 / (800  200)  0.2
The sum of alleles is always 1
0.8  0.2  1

37. Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
frequency of alleles will not change from generation to generation
A “null hypothesis” to check for evidence of evolution
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

38. Hardy-Weinberg Principle
Figure 23.7
Hardy-Weinberg Principle
Alleles in the population
Frequencies of alleles
Gametes produced
p = frequency of
Each egg: Each sperm:
CR allele  = 0.8
q = frequency of
80%
chance
20%
chance
80%
chance
20%
chance
CW allele  = 0.2
Figure 23.7 Selecting alleles at random from a gene pool.
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population

39. Hardy-Weinberg Principle
When allele frequencies remain constant from generation to generation, the population is in Hardy-Weinberg equilibrium
Assumptions:
No natural selection
No mutation
No gene flow
Random mating
Large population

40. Hardy-Weinberg Equilibrium
Hardy-Weinberg Equilibrium Equations
Frequency of Alleles: p + q = 1
Frequency of Genotypes: p2 + 2pq + q2 = 1

41. Hardy-Weinberg Equilibrium
The variables “p” and “q” come from Punnett Square for populations
p= frequency of “A” allele (dominant)
q= frequency of “a” allele (recessive)

42. Hardy-Weinberg Equilibrium
Hardy-Weinberg Equilibrium Equations
Frequency of Alleles: p + q = 1
p= 0.7
q=0.3
Frequency of Genotypes: p2 + 2pq + q2 = 1
p2 = frequency of “AA” genotype
Ex: 0.72 = 0.49
2pq = frequency of “Aa” genotype
Ex: 2(0.7)(0.3)= 0.42
q2 = frequency of “aa” genotype
Ex: 0.32 = 0.09
Frequency of Genotypes= = 1

43. Population Variables: Hardy-Weinberg Equilibrium
Once scientists know what p and q are in the population, they can track the population through time and see if population is at equilibrium or changing
If p and q change through time, one of the Hardy-Weinberg Equilibrium assumptions are not being met
Evolution is occurring
Populations can be at equilibrium at some loci, but not at other loci

44. Real-World Example: Hardy-Weinberg Equilibrium and Fisheries Management
Kelp Grouper (Epinephelus bruneus)
Commercial fish species in Korea
Recent declines in landings
2007: IUCN Red List- Vulnerable
Study by An et al. 2012
Genotyped 12 gene loci from 30 fish
3 of 12 loci showed deviations from Hardy-Weinberg Equilibrium