Population Genetics

1. Population Genetics

2. Hardy-Weinberg Equilibrium Determination A B both A and B neither A nor B Which of these populations are in Hardy-Weinberg equilibrium?

3. Question 6 – Chap. 23 • Researchers examining a particular gene in a fruit fly population discovered that the gene can have either of two slightly different sequences, designated A1 and A2. Further tests showed that 70% of the gametes produced in the population contained the A1 sequence. If the population is at Hardy-Weinberg equilibrium, what proportion of flies carries both A1 and A2? • A 0.7 B 0.49 C 0.21 D 0.42 E 0.09

4. Question from an earlier edition of Campbell • At a locus with a dominant and recessive allele in Hardy-Weinberg equilibrium, 16% of the individuals are homozygous for the recessive allele. What is the frequency of the dominant allele in the population? • A 0.84 B 0.36 C 0.6 D 0.4 E 0.48

5. Hardy-Weinberg Equilibrium Hardy-Weinberg Equilibrium is based on: 1. A very large population where all genotypes are equally viable 2. Random mating (panmixia) 3. No mutations 4. No gene flow (dispersal of individuals and their genes) 5. No natural selection

6. Evolutionary Change • Evolution is a generation to generation change in a population’s frequencies of alleles – change in proportions of alleles in the gene pool is evolution at its smallest scale and is often referred to as microevolution • The two main causes of microevolution are genetic drift and natural selection

7. Natural Selection

8. Genetic Drift • Random changes in gene frequency in a population – this can lead to losses in genetic diversity – the population becomes more homozygous • this is most important in small populations

9. CRCR CRCR Genetic Drift CRCW CRCR CWCW CRCW CRCR CRCW CRCR CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3

10. 5 plants leave off- spring CWCW CRCR CRCR CRCR Genetic Drift CRCW CRCW CRCR CWCW CWCW CRCR CRCW CRCW CWCW CRCR CRCR CRCW CRCW CRCW CRCR CRCW Generation 1 Generation 2 p (frequency of CR) = 0.7 p = 0.5 q (frequency of CW) = 0.3 q = 0.5

11. 5 plants leave off- spring 2 plants leave off- spring CWCW CRCR CRCR CRCR CRCR Genetic Drift CRCW CRCW CRCR CRCR CRCR CRCR CWCW CRCR CWCW CRCR CRCW CRCW CRCR CRCR CRCR CWCW CRCR CRCR CRCW CRCR CRCR CRCW CRCW CRCR CRCW Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 0.5 p = 1.0 q (frequency of CW) = 0.3 q = 0.5 q = 0.0

12. Population Bottleneck

13. Original population

14. Original population Bottlenecking event

15. Original population Bottlenecking event Surviving population

16. Northern Elephant Seal

17. Northern Elephant Seal Population

18. Post-bottleneck (Illinois, 1993) Pre-bottleneck (Illinois, 1820) Greater prairie chicken Range of greater prairie chicken (a) Percentage of eggs hatched Number of alleles per locus Population size Location Illinois 1930–1960s 1993 5.2 3.7 1,000–25,000 <50 93 <50 Kansas, 1998 (no bottleneck) 750,000 99 5.8 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 (b)

19. Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken (a)

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

21. Founder effect – founder population and three possible new populations

22. Mal de Meleda – founder effect

23. Serial Founder Effect • Serial founder effects have occurred when populations migrate over long distances. Such long distance migrations typically involve relatively rapid movements followed by periods of settlement. The populations in each migration carry only a subset of the genetic diversity carried from previous migrations. As a result, genetic differentiation tends to increase with geographic distance.

24. Movement of mitochondrial genes out of Africa

25. ‘Wisteria vine’ model of human genetic diversity

26. Gene Flow • Gene flow is the movement of alleles in and out of a population • Gene flow occurs because gametes or fertile individuals move from one population to another and take their genes with them

27. Gene flow

28. Gene Flow in Conifers

29. Population in which the surviving females eventually bred 60 Central population Central NORTH SEA Eastern population 50 Eastern Vlieland, the Netherlands 40 2 km Survival rate (%) 30 20 10 0 Females born in central population Females born in eastern population Parus major

30. Non-Random Mating • Hardy-Weinberg assumes random mating – if mating is not random then the population may change in the short term – the most common form of non-random mating is in-breeding – the mating of closely related individuals • In fact inbreeding is very common – many mammals probably mate with first or second cousins in the wild; many plants self-pollinate – the ultimate form of inbreeding • Inbreeding tends to produce homozygous populations

31. Inbreeding and White Squirrels

32. Mutations • Mutations are the ultimate source of new genetic variations – a new mutation that is transmitted in gametes immediately changes the gene pool of a population by inserting a new allele into the gene pool