Option D Evolution D 4 The Hardy Weinberg

Option D: Evolution D 4: The Hardy. Weinberg Principle

D 4. 1 Explain how the Hardy-Weinberg equation is derived. Population Genetics = Foundation for studying evolution • 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

D 4. 1 Explain how the Hardy-Weinberg equation is derived. Hardy-Weinberg Theorem: • 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

D 4. 1 Explain how the Hardy-Weinberg equation is derived. Hardy-Weinberg Theorem: • 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

D 4. 1 Explain how 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

BBHardy-Weinberg Bb equation bbis derived. D 4. 1 Explain how the Hardy-Weinberg theorem • Counting Individuals ▫ frequency of homozygous dominant: p x p = p 2 ▫ frequency of homozygous recessive: q x q = q 2 ▫ frequency of heterozygotes: (p x q) + (q x p) = 2 pq frequencies of all individuals must add to 1 (100%), so: p 2 + 2 pq + q 2 = 1

D 4. 1 Explain how the Hardy-Weinberg equation is derived. Hardy-Weinberg theorem • Alleles: p+q=1 B • Individuals: p 2 + 2 pq + q 2 = 1 BB BB b Bb Bb bb bb

D 4. 2 Calculate allele, 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? p 2=. 36 BB q 2 (bb): 16/100 =. 16 q (b): √. 16 = 0. 4 p (B): 1 - 0. 4 = 0. 6 2 pq=. 48 Bb q 2=. 16 bb Must. What assume population is frequencies? in H-W equilibrium! are the genotype

D 4. 2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. p 2=. 36 Assuming H-W equilibrium 2 pq=. 48 q 2=. 16 BB Bb bb p 2=. 20 =. 74 BB 2 pq=. 64 2 pq=. 10 Bb q 2=. 16 bb Null hypothesis Sampled data How do you explain the data?

D 4. 2 Calculate allele, 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

D 4. 2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. Assuming all the individuals mate randomly SPERMS EGGS A 0. 57 B 0. 43 A 0. 57 AA 0. 32 p*p= p 2 AB 0. 25 p*q B AB 0. 43 0. 25 BB 0. 18 q*q= q 2 p*q

D 4. 2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. • Close enough for us to assume genetic equilibrium Genotypes Expected frequencies Observed frequencies AA p 2 = 0. 32 233 747 = 0. 31 AB 2 pq =0. 50 385 747 = 0. 52 BB q 2 =0. 18 129 747 = 0. 17

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

Sickle cell frequency • High frequency of heterozygotes ▫ 1 in 5 in Central Africans = Hb. Hs ▫ unusual for allele with severe detrimental effects in homozygotes 1 in 100 = Hs. Hs 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…

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

Heterozygote Advantage • In tropical Africa, where malaria is common: ▫ homozygous dominant (normal) die or reduced reproduction from malaria: Hb. Hb ▫ homozygous recessive die or reduced reproduction from sickle cell anemia: Hs. Hs ▫ heterozygote carriers are relatively free of both: Hb. Hs survive & reproduce more, more common in population Hypothesis: In malaria-infected cells, the O 2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele & distribution of malaria

D 4. 3 State the assumptions made when the Hardy-Weinberg equation is used. Conditions for Hardy-Weinberg Equilibrium: Hardy-Weinberg Theorem describes a non-evolving population. 1. 2. 3. 4. 5. Extremely large population size (no genetic drift). No gene flow (isolation from other populations). No mutations. Random mating (no sexual selection). No natural selection.

D 4. 3 State the 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

Main Causes of Microevolution Mutation Genetic Drift Gene Flow Non-random mating Selection

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

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 mixing across regions reduce differences between populations

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

3. Non-random mating • Sexual selection

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 es ch s he fin c fin ee nd Tr ou Gr er Warbl finch

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

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

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

Conservation issues Peregrine Falcon • 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

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