Option D: Evolution D4: The Hardy- Weinberg Principle

1. Option D: Evolution D4: The Hardy- Weinberg Principle

2. D 4.1Explainhow the Hardy-Weinberg equation is derived. • Darwin’s could not explain how inherited variations are maintained in populations - not “trait blending” • A few years after Darwin’s “Origin of Species”, Gregor Mendel proposed his hypothesis of inheritance: Parents pass on discrete heritable units (genes) that retain their identities in offspring

3. D 4.1Explainhow the Hardy-Weinberg equation is derived. • Frequencies of alleles & genotypes in a population’s gene pool remain constant from generation to generation unless acted upon by agents other than sexual recombination (gene shuffling in meiosis) • Equilibrium = allele and genotype frequencies remain constant

4. Hardy-Weinberg Theorem: D 4.1Explainhow the Hardy-Weinberg equation is derived. • Hypothetical, non-evolving population • preserves allele frequencies • Serves as a model (null hypothesis) • natural populations rarely in H-W equilibrium • useful model to measure if forces are acting on a population • measuring evolutionary change G.H. Hardy mathematician W. Weinberg physician

5. D 4.1Explainhow the Hardy-Weinberg equation is derived. Hardy-Weinberg theorem • Counting Alleles • assume 2 alleles = B, b • frequency of dominant allele (B) =p • frequency of recessive allele (b) = q • frequencies must add to 1 (100%), so: p + q = 1 BB Bb bb

6. BB Bb bb D 4.1Explainhow the Hardy-Weinberg equation is derived. Hardy-Weinberg theorem • Counting Individuals • frequency of homozygous dominant: p x p = p2 • frequency of homozygous recessive:q x q = q2 • frequency of heterozygotes: (p x q) + (q x p) = 2pq • frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1

7. B b BB Bb bb D 4.1Explainhow the Hardy-Weinberg equation is derived. Hardy-Weinberg theorem • Alleles: p + q = 1 • Individuals: p2 + 2pq + q2 = 1 BB Bb bb

8. D 4.2Calculateallele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. population: 100 cats 84 black, 16 white How many of each genotype? q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 p2=.36 2pq=.48 q2=.16 BB Bb bb Must assume population is in H-W equilibrium! What are the genotype frequencies?

9. BB Bb bb D 4.2Calculateallele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. p2=.36 2pq=.48 q2=.16 Assuming H-W equilibrium BB Bb bb Null hypothesis p2=.20 p2=.74 2pq=.64 2pq=.10 q2=.16 q2=.16 Sampled data How do you explain the data? How do you explain the data?

10. D 4.2Calculateallele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. • Using the calculated gene frequency to predict the EXPECTED genotypic frequencies in the NEXT generation OR • to verify that the PRESENT population is in genetic equilibrium

11. SPERMS A 0.57 B 0.43 EGGS A 0.57 B 0.43 D 4.2Calculateallele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. Assuming all the individuals mate randomly AA 0.32 AB 0.25 p*p= p2 p*q AB 0.25 BB 0.18 q*q= q2 p*q

12. D 4.2Calculateallele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. • Close enough for us to assume genetic • equilibrium

13. Application of H-W principle • Sickle cell anemia • inherit a mutation in gene coding for hemoglobin • oxygen-carrying blood protein • recessive allele = HsHs • normal allele = Hb • low oxygen levels causes RBC to sickle • breakdown of RBC • clogging small blood vessels • damage to organs • often lethal

14. Sickle cell frequency • High frequency of heterozygotes • 1 in 5 in Central Africans = HbHs • unusual for allele with severe detrimental effects in homozygotes • 1 in 100 = HsHs • usually die before reproductive age Why is the Hs allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous…

15. Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells Malaria 1 2 3

16. Heterozygote Advantage • In tropical Africa, where malaria is common: • homozygous dominant (normal) • die or reduced reproduction from malaria: HbHb • homozygous recessive • die or reduced reproduction from sickle cell anemia: HsHs • heterozygote carriers are relatively free of both: HbHs • survive & reproduce more, more common in population Hypothesis: In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele & distribution of malaria

17. D 4.3Statethe assumptions made when the Hardy-Weinberg equation is used. Hardy-Weinberg Theorem describes a non-evolving population. • Extremely large population size (no genetic drift). • No gene flow (isolation from other populations). • No mutations. • Random mating (no sexual selection). • No natural selection.

18. D 4.3Statethe assumptions made when the Hardy-Weinberg equation is used. • If any of the Hardy-Weinberg conditions are not met  microevolution occurs • Microevolution = generation to generation change in a population’s allele frequencies

19. Mutation Gene Flow Non-random mating Genetic Drift Selection

20. 1. Mutation & Variation • Mutation creates variation • new mutations are constantly appearing • Mutation changes DNA sequence • changes amino acid sequence? • changes protein? • changes structure? • changes function? • changes in protein may change phenotype & therefore change fitness

21. 2. Gene Flow • Movement of individuals & alleles in & out of populations • seed & pollen distribution by wind & insect • migration of animals • sub-populations may have different allele frequencies • causes genetic mixingacross regions • reduce differences between populations

22. Human evolution today • Gene flow in human populations is increasing today • transferring alleles between populations Are we moving towards a blended world?

23. 3. Non-random mating • Sexual selection

24. Warbler finch Tree finches Ground finches 4. Genetic drift • Effect of chance events • founder effect • small group splinters off & starts a new colony • bottleneck • some factor (disaster) reduces population to small number & then population recovers & expands again

25. Founder effect • When a new population is started by only a few individuals • some rare alleles may be at high frequency; others may be missing • skew the gene pool of new population • human populations that started from small group of colonists • example:colonization of New World

26. Bottleneck effect • When large population is drastically reduced by a disaster • famine, natural disaster, loss of habitat… • loss of variation by chance event • alleles lost from gene pool • not due to fitness • narrows the gene pool

27. Cheetahs • All cheetahs share a small number of alleles • less than 1% diversity • as if all cheetahs are identical twins • 2 bottlenecks • 10,000 years ago • Ice Age • last 100 years • poaching & loss of habitat

28. Peregrine Falcon Conservation issues • Bottlenecking is an important concept in conservation biology of endangered species • loss of alleles from gene pool • reduces variation • reduces adaptability Breeding programs must consciously outcross Golden Lion Tamarin

29. 5. Natural selection • Differential survival & reproduction due to changing environmental conditions • climate change • food source availability • predators, parasites, diseases • toxins • combinations of allelesthat provide “fitness” increase in the population • adaptive evolutionary change