1. Genes Within Populations
Chapter 20

2. Genetic Variation and Evolution
Evolution: change in a species through time
Species accumulate differences
Descendants differ from their ancestors
New species arise from existing ones

3. Mechanism of evolutionary change
Lamarck’s theory of evolution
Inheritance of acquired characteristics:
Individuals passed on physical and behavioral changes to their offspring
Variation by experience…not genetic

4. Lamarck’s theory of how giraffes’ long necks evolved

5. Darwin’s Theory of Evolution
Observed land & organisms everywhere they went
Began to notice connections between species

6. Charles Darwin
Served as naturalist on 5 year mapping expedition(HMS BEAGLE) around coastal South America.
Used many observations to develop his ideas
Proposed that evolution occurs by
natural selection

7. Voyage of the Beagle

9. Charles Darwin evolution: modification of a species over generations
-“descent with modification”
natural selection: individuals with superior characteristics are more likely to survive and reproduce than those without such characteristics

10. Tortoises on different islands…
“normal” tortoise
Saddleback allows tortoise to reach higher leaves on drier islands

11. Darwin’s Evidence Similarity of related species
- Darwin noticed variations in related species living in different locations

12. Most famous for his observations of Galapagos finches
Some islands much drier than others
Different islands had their own, slightly different varieties of animals
Darwin hypothesized that new species could gradually appear, much like animal breeders can artificially develop new varieties through selective breeding

14. Explaining his observations: natural selection
Variation: individuals in a population differ from one another
Heritability: variations are inherited from parents
Overproduction: organisms produce more offspring than can survive (survival of the fittest)
Reproductive advantage: some variations allow the organism that possesses them to have more offspring – those variations become more common, and the population changes over time

15. Darwin’s theory for how long necks evolved in giraffes

16. Post-Darwin Evolution Evidence
Fossil record
Mechanisms of heredity
Comparative anatomy
Molecular evidence

17. Why does a peacock have a large tail?
Why are albinos rare?
Why does a peacock have a large tail?

18. Hardy-Weinberg Principle
-Genetic equilibrium: Allele (and genotype) frequencies in a population will remain constant from generation to generation -if equilibrium is upset  evolution (punctuated equilibrium chp 22)

19. Hardy-Weinberg Principle
Requirements to maintain genetic equilibrium:
No mutation
No genes are transferred to or from other sources
Random mating
Very large population
No selection

20. Hardy-Weinberg Principle
Calculate genotype frequencies (p+q)2 = p2 + 2pq + q2
p = frequency of the 1st allele
q = frequency of the 2nd allele
p2 = individuals homozygous for 1st allele
2pq = heterozygous individuals
q2 = individuals homozygous for 2nd allele
because there are only two alleles: p plus q must always equal 1

21. Hardy-Weinberg Principle

22. Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict frequencies in subsequent generations

23. A population not in Hardy-Weinberg equilibrium indicates an agent of evolutionary change is operating in a population (one or more of the 5 conditions are not being met)

24. Agents of Evolutionary Change
Mutation: A change in a cell’s DNA
Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.
Ultimate source of genetic variation

25. Agents of Evolutionary Change
Gene flow: A movement of alleles from one population to another
Powerful agent of change
Tends to homogenize allele frequencies between populations

26. Agents of Evolutionary Change
Nonrandom Mating: mating with specific genotypes
E.G. Sexual Selection – “Peacocks”
Shifts genotype frequencies

27. 3a. Assortative Mating:
mates that are phenotypically similar
does not change frequency of individual alleles
Disruptive selection: forms at both ends of the range of variation are favored over intermediate forms

28. 3b. Disassortative Mating:
phenotypically different individuals mate
Stabilizing selection: intermediate (heterozygous) forms are favored and extremes are eliminated

29. Genetic Drift Small populations
Genetic drift: Random fluctuation in allele frequencies over time by chance
important in small populations
founder effect – When a few individuals start a population (small allelic pool)
Amish
bottleneck effect - drastic reduction in population, and gene pool size

30. Founder Effect

31. Genetic Drift: A bottleneck effect

32. Selection
Natural selection: environmental conditions determine which individuals in a population produce the most offspring
This is the only agent that produces adaptive evolutionary change (selects individuals that are more fit)

33. Selection

34.
Evolution of the eye:

35. Fitness and Its Measurement
Fitness: A phenotype with greater fitness usually increases in frequency
Fitness is a combination of:
Survival: how long does an organism live
Mating success: how often it mates
Number of offspring per mating that survive

36. Body size and egg-laying in water striders

37. Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time
Galápagos Islands ground finches
Wet conditions favor big bills (abundant seeds)
Dry conditions favor small bills

38. Maintenance of Variation
Heterozygotes may exhibit greater fitness than homozygotes
Heterozygote advantage: keep deleterious alleles in a population

39. Maintenance of Variation
Example: Sickle cell anemia
Homozygous recessive phenotype: exhibit severe anemia
Homozygous dominant phenotype: no anemia; susceptible to malaria
Heterozygous phenotype: no anemia; less susceptible to malaria

40. Maintenance of Variation
Frequency of sickle cell allele

41. Maintenance of Variation
Disruptive selection acts to eliminate intermediate types

42. Maintenance of Variation
Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa

43. Maintenance of Variation
Directional selection: acts to eliminate one extreme from an array of phenotypes

45. Maintenance of Variation
Stabilizing selection: acts to eliminate both extremes

46. Maintenance of Variation
Stabilizing selection for birth weight in humans