Understanding Population Genetics: Variations, Selection, and Equilibrium

1. Chapter 19: Population genetics Fig. 19-1

2. Population genetics • Population: interbreeding members of a species • Three major principles of Darwinian evolutionary theory: • variation for traits exists within populations • selection applies to a subset of those traits • (selection can act only upon variations) • traits are genetically transmitted

3. Polymorphism: multiple forms of a gene are • commonly found in a population • (all studied populations are “wildly” polymorphic) • chromosomal polymorphisms • immunological polymorphisms • protein polymorphisms • nucleic acid sequence/structure polymorphisms

4. p = (2 x MM) + (1 x MN) = frequency of M allele q = (2 x NN) + (1 x MN) = frequency of N allele Therefore, frequency of MN reflects the genetic variation in a population

5. Fig. 19-2

6. “Factoids” regarding protein polymorphisms • in most species: • structural polymorphisms are displayed by • about one-third of all proteins • typically, about 10% of the individuals in • a large population are heterozygous for • polymorphisms of an average gene • Therefore, enormous protein-level variation • exists in most populations

7. Electrophoretic allelic variants of esterase-5 in Drosophila Fig. 19-2

8. Electrophoretic variants of hemoglobin A in humans Fig. 19-3

9. Enormous naturally-occuring variation (polymorphism) in protein sequence

10. Enormous naturally-occuring variation (polymorphism) in chromosomal rearrangements

11. Restriction sites within Drosophila xdh gene (58 wild chromosomes sampled) 4-base sites in 4.5 kb DNA * Site present in minority of chromosomes 0/1 Site in ½ of chromosomes Enormous naturally-occuring variation (polymorphism) in nucleotide sequence Fig. 19-5

12. Enormous naturally-occuring variation (polymorphism) in tandem repeat arrays (VNTRs)

13. Hardy-Weinberg Equilibrium Random mating within a large population assures a stable equilibrium of genetic diversity in subsequent generations provided that certain assumptions apply: Mating is random (no biased mating, infinite population size) Allele frequencies do not change (no selection, no migration, etc.)

14. Hardy-Weinberg Equilibrium • For a two-allele system, all genotypes exist as a simple • product of the frequency of each allele: • homozygotes = p2or q2 • heterozygotes = 2pq • p2 + 2pq + q2= 1 Box 19-2

15. Box 19-2

16. Allele frequencies determine frequencies of homozygotes and heterozygotes Rare alleles are almost always found in heterozygotes, almost never homozygous Fig. 19-6

17. Another measure of heterozygosity is haplotype diversity Haplotype: combination of non-allelic alleles on a single chromosome

18. MN allele and genotype frequencies reflect Hardy-Weinberg assumptions Allele and genotype frequencies can vary between populations, while exhibiting H-W equilibria within each population

19. Non-random mating: inbreeding (mating among relatives) Positive inbreeding: Mating among relatives is more common than random Increases frequencies of homozygotes in a population Fig. 19-7

20. Extreme inbreeding: self-fertilization results in loss of heterozygosity No change in p or q; change only in heterozygosity and diversity Fig. 19-8

21. Negative inbreeding (enforced outbreeding) -barriers to inbreeding are common attributes of successful populations Positive assortative mating - individuals chose “like” mates (not necessarily relatives) Negative assortative mating - individuals choose dissimilar mates

22. Sources of variation • Mutation – very slow

23. Mutation is the ultimate source of variation But spontaneous mutations occur at extremely low frequencies

24. Mutation frequency is influenced by allele frequency Mutation alone is a very slow evolutionary force and cannot directly account for diversity observed in populations. Box 19-3

25. Sources of variation • Mutation – very slow • Recombination – rapidly mixes genes to • provide new genetic combinations in • a population • Migration – gene flow among different • populations changes gene frequencies

26. Selection: directed change in genotypes in a population Fitness: survival and reproduction success; function of genotype and environment

27. Fitness can be obvious (mortality, sterililty) • HbS/HbS: severe anemia, low survival • HbS/HbA: apparent resistance to malaria • or more subtle/partial/conditional

28. Fitness (viability) of various homozygotes as a function of temperature Drosophila pseudoobscura Fig. 19-9

29. Enhanced fitness of a genotype will enrich those genes in subsequent generations of that population Frequencies of positively selected genes increase over time Frequencies of negatively selected genes decrease over time Change in A frequency (p) is greatest where p = q Fig. 19-11

30. For a two-allele system, mean fitness (W) in a population is the proportional contribution of fitness by each genotype (A/A, A/a, a/a) W = p2WA/A + 2pqWA/a + q2Wa/a WA/A and WA/a > Wa/ap should increase q should decrease Wa/a > WA/A and WA/aq should increase p should decrease

31. Fitness can account for allele frequency changes over time p for malic dehydrogenase electrophoretic mobility variant MDHF where WS/S=1, WS/F=0.75, WF/F=0.4 Fig. 19-12

32. Selection: directed change in genotypes in a population Fitness: survival and reproduction success; function of genotype and environment Frequency independent selection: fitness is independent of genotype frequency Frequency dependent selection: fitness changes as genotype frequency changes

33. Random genetic drift: random changes in gene • frequency that can lead to extinction/fixation • of genes • Requires no selection • Essentially “sampling error” inherent in • each generation in achieving Hardy- • Weinberg equilibrium • Most exaggerated in small populations • (especially “founder effects”) • Allows isolated populations to diverge • without differential selection (each • experiences its own drift history)

34. Model: history of emergence of ten mutations and their drift in a population over time Drift to extinction for nine; drift to p=1 in one Fig. 19-13

35. Drift explains differences in unselected allele frequencies in isolated populations

36. Genetic change is directed by diverse evolutionary forces which tend to increase (blue) or decrease (red) variation

37. Recommended problems in Chapter 19: 3, 5, 12, 17