1. AP Bio Ch 23 part 2

2. Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Only natural selection consistently results in adaptive evolution

3. A Closer Look at Natural Selection
Natural selection brings about adaptive evolution by acting on an organism’s phenotype

4. Relative Fitness
The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals
Reproductive success is generally more subtle and depends on many factors

5. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Selection favors certain genotypes by acting on the phenotypes of certain organisms

6. Directional, Disruptive, and Stabilizing Selection
Three modes of selection:
Directional selection favors individuals at one end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes

7. Frequency of individuals
Fig
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

8. Frequency of individuals
Fig a
Original population
Frequency of individuals
Phenotypes (fur color)
Figure Modes of selection
Original population
Evolved population
(a) Directional selection

9. Frequency of individuals
Fig b
Original population
Frequency of individuals
Phenotypes (fur color)
Figure Modes of selection
Evolved population
(b) Disruptive selection

10. Frequency of individuals
Fig c
Original population
Frequency of individuals
Phenotypes (fur color)
Figure Modes of selection
Evolved population
(c) Stabilizing selection

11. The Key Role of Natural Selection in Adaptive Evolution
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

12. Fig
(a) Color-changing ability in cuttlefish
Movable bones
Figure Examples of adaptations
(b) Movable jaw
bones in
snakes

13. (a) Color-changing ability in cuttlefish
Fig a
Figure Examples of adaptations
(a) Color-changing ability in cuttlefish

14. Movable bones (b) Movable jaw bones in snakes Fig. 23-14b
Figure Examples of adaptations
(b) Movable jaw
bones in
snakes

15. Because the environment can change, adaptive evolution is a continuous process
Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

16. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

17. Fig
Figure Sexual dimorphism and sexual selection

18. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

19. How do female preferences evolve?
The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for

20. Larval growth NSD LC better Larval survival LC better NSD
Fig
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
SC father
Offspring of
LC father
Fitness of these half-sibling offspring compared
RESULTS
Figure Do females select mates based on traits indicative of “good genes”?
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.

21. SC sperm  Eggs  LC sperm
Fig a
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Figure Do females select mates based on traits indicative of “good genes”?
Offspring of
SC father
Offspring of
LC father
Fitness of these half-sibling offspring compared

22. Larval growth LC better Larval survival LC better NSD
Fig b
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
Figure Do females select mates based on traits indicative of “good genes”?
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.

23. The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population

24. Diploidy
Diploidy maintains genetic variation in the form of hidden recessive alleles

25. Balancing Selection
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

26. Heterozygote Advantage
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
Natural selection will tend to maintain two or more alleles at that locus
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

27. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Fig
Frequencies of the
sickle-cell allele
0–2.5%
Figure Mapping malaria and the sickle-cell allele
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
7.5–10.0%
10.0–12.5%
>12.5%

28. Frequency-Dependent Selection
In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population
Selection can favor whichever phenotype is less common in a population

29. “left-mouthed” individuals
Fig
“Right-mouthed”
1.0
“Left-mouthed”
“left-mouthed” individuals
Frequency of
0.5
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
1981
’82
’83
’84
’85
’86
’87
’88
’89
’90
Sample year

30. “Right-mouthed” “Left-mouthed” Fig. 23-18a
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
“Left-mouthed”

31. “left-mouthed” individuals
Fig b
1.0
“left-mouthed” individuals
Frequency of
0.5
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
1981
’82
’83
’84
’85
’86
’87
’88
’89
’90
Sample year

32. Neutral Variation
Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage
For example,
Variation in noncoding regions of DNA
Variation in proteins that have little effect on protein function or reproductive fitness

33. Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact

34. Fig
Figure Evolutionary compromise

35. Original population Evolved population Directional selection
Fig. 23-UN1
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Stabilizing
selection

36. Salinity increases toward the open ocean
Fig. 23-UN2
Sampling sites
(1–8 represent
pairs of sites) 1 2 3 4 5 6 7 8 9 10 11
Allele
frequencies
lap94 alleles
Other lap alleles
Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).
Salinity increases toward the open ocean 7 8 5 6 4 3
Long Island
Sound 2 1 9 N 10
Atlantic
Ocean W E 11 S

37. Fig. 23-UN3

38. You should now be able to:
Explain why the majority of point mutations are harmless
Explain how sexual recombination generates genetic variability
Define the terms population, species, gene pool, relative fitness, and neutral variation
List the five conditions of Hardy-Weinberg equilibrium

39. Apply the Hardy-Weinberg equation to a population genetics problem
Explain why natural selection is the only mechanism that consistently produces adaptive change
Explain the role of population size in genetic drift

40. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection
List four reasons why natural selection cannot produce perfect organisms