Evolution of Populations

1. Evolution of Populations Chapter 23

2. Macroevolution • Evolution on a large scale • Changes in plants & animals • Where new forms replace old • Major episodes of extinction

3. Microevolution • Changes within a population • Changes in allele frequencies • Leads to adaptation of an organism

4. Variation • Gene variation • Driving force behind evolution • New genes & alleles can arise by mutation or gene duplication • Sexual reproduction

6. Genetic Variation from Sexual Recombination

7. Population genetics • Study of the properties of genes in populations

8. Population • Group of individuals • Same species • Interbreed • Fertile offspring

9. Population • Contains a great deal of variation • Variation-raw material for evolution

10. Gene pool • All the alleles • Of all individuals within a population

11. Hardy-Weinberg Principle • Determines if population is evolving • Frequencies of alleles in population • Used for baseline of genes in a population

12. Hardy-Weinberg • Equilibrium • When proportions of genotypes remain the same • Generation to generation

13. Hardy-Weinberg • Original proportions of genotypes in a population remain constant if • 1. Large population • 2. Random mating • 3. No mutations • 4. No gene flow • 5. No natural selection

14. Hardy-Weinberg • P+q=1 alleles • p=dominant • q=recessive • p2 + 2pq + q2 = 1 genotypes

15. Hardy-Weinberg • 84 black 16 white (100 total)

16. Hardy-Weinberg • p2 + 2pq + q2 = 1 P + q=1 • q2 = .16 • q = .4 • p = .6 • p2 = .36 • 2pq = .48

17. Hardy-Weinberg • If the dominant allele is 30% of the gene pool • What is • % dominant phenotype • % recessive phenotype • % hybrid

18. Hardy-Weinberg • Factors that affect evolutionary change • 1. Mutations • 2. Nonrandom mating • 3. Gene flow • 4. Genetic drift • 5. Natural selection

19. Mutation • Occurs at a low rate • Not a strong influence on evolutionary change

20. Nonrandom mating • Individuals with one genotype mate with another at a greater rate • Not a strong influence on allele frequency

21. Gene flow • Movement of alleles from one population to another • Populations exchange genetic information • Example • New animal comes into population • Mates & survives

22. Gene flow • Bees and pollen • Seeds • Reduces genetic differences between populations

23. Gene flow • Insecticide resistant alleles • Mosquito West Nile & Malaria • Spreading the allele

24. Gene flow • Advantage when a beneficial mutation enters a population • Select for the allele • Disadvantage when an inferior allele enters the population • Select against the allele

25. Genetic drift • Change in allele frequency due to chance alone • Small populations

26. Genetic drift • Only a few possible alleles are present • Example: • Red, blue, yellow seeds • If blue & yellow are isolated from red • Eventually the population will only have blue or yellow and no red

27. Genetic drift • May see a rise in harmful alleles • Lose alleles

28. Fig. 23-8-3 CW CW CR CR CR CR CR CR CR CR CR CW CR CW CR CR CR CR CR CR CR CR CR CR CW CW CW CW CR CR CR CW CR CR CR CR CR CW CR CR CW CW CR CR CR CW CR CR CR CR CR CR CR CW CR CW CR CR CR CW Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 1.0 p = 0.5 q (frequency of CW) = 0.3 q = 0.0 q = 0.5

29. Genetic drift • 1. Founders effects • Few individuals leave a population • New isolated population • Few alleles present • Island populations • Amish (polydactyly)

30. ..\..\..\Desktop\polydactyl.jpg

31. Genetic drift • 2. Bottleneck • Occurs when a few surviving individuals have only a few genes • Loss of genetic variability • Occurs when a natural event happens • Flood, drought, disease etc.

32. Fig. 23-9 Bottlenecking event Surviving population Original population

33. Genetic drift • Northern elephant seal • California • Reduced to few seals in a population due to hunting • Has rebounded in numbers • Organisms with limited genetic variation

35. Fig. 23-10a Post-bottleneck (Illinois, 1993) Pre-bottleneck (Illinois, 1820) Range of greater prairie chicken (a)

37. Selection • Natural selection the process that causes evolutionary change • Adaptive evolution

38. Selection • Natural selection to happen & cause evolutionary change • 1. Must have variation in individuals among population • Enables choice of traits that are better able to survive

39. Selection • 2. Variation causes different number of offspring surviving • 3. Variation must be genetically inherited

40. Selection • Individuals with a certain phenotype • Leave more surviving offspring than other phenotypes

41. Relative fitness • Reproductive success • Number of surviving offspring left for the next generation • Green vs brown frogs • Green leave 4 offspring • Brown leave 2.5 offspring • More green mating eventually lose the brown phenotype

42. Relative fitness • 1. Survival (how long) • 2. Mating success • 3. Number of offspring • Examples: larger organisms mate more • Larger fish or frogs leave more offspring

43. Forms of selection • 1. Disruptive selection • 2. Directional selection • 3. Stabilizing selection

44. Forms of selection • 1. Disruptive selection • Eliminates intermediate type • Favors extremes • Example: • African-bellied seed cracker finch • Large beak Large seeds • Small beak Small seeds

45. Original population Frequency of individuals Phenotypes (fur color) Evolved population (b) Disruptive selection

46. Forms of selection • 2. Directional selection • Favors one extreme

47. Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population (a) Directional selection

48. Forms of selection • 3. Stabilizing selection • Eliminates both extremes • Example: birth weight of newborns • Small & large newborns can be harmful • Increased death rate • Intermediate BW best survival

49. Original population Frequency of individuals Phenotypes (fur color) Evolved population (c) Stabilizing selection

50. Selection • Environment imposes conditions • Determines selection • Cause evolutionary change.