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

## Presentation on theme: "Option D: Evolution D4: The Hardy- Weinberg Principle."— Presentation transcript:

Option D: Evolution D4: The Hardy- Weinberg Principle

Population Genetics = Foundation for studying evolution
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

Hardy-Weinberg Theorem:
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

Hardy-Weinberg Theorem:
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 (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?" G.H. Hardy mathematician W. Weinberg physician

Hardy-Weinberg theorem
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

Hardy-Weinberg theorem
BB Bb bb D 4.1 Explain how 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

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

q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6
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? q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): = 0.6 p2=.36 2pq=.48 q2=.16 BB Bb bb Must assume population is in H-W equilibrium! What are the genotype frequencies?

Assuming H-W equilibrium
D 4.2 Calculate allele, 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 1: Hybrids are in some way weaker. Immigration in from an external population that is predomiantly homozygous B Non-random mating... white cats tend to mate with white cats and black cats tend to mate with black cats. Sampled data 2: Heterozygote advantage. What’s preventing this population from being in equilibrium. bb Bb BB Sampled data How do you explain the data? How do you explain the data?

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. 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

Assuming all the individuals mate randomly
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 B A B 0.43 A 0.57 EGGS AA AB p*p= p2 p*q AB BB q*q= q2 p*q

Close enough for us to assume genetic equilibrium
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 p2 = 0.32 233  747 = 0.31 AB 2pq =0.50 385  747 = 0.52 BB q2 =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 = HsHs 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 = HbHs unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs usually die before reproductive age Sickle Cell: In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite. Cystic fibrosis: Cystic fibrosis carriers are thought to be more resistant to cholera: 1:25, or 4% of Caucasians are carriers Cc 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

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

Conditions for Hardy-Weinberg Equilibrium:
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. 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 Gene Flow Non-random mating Genetic Drift Selection

Not every mutation has a visible effect.
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 Every individual has hundreds of mutations 1 in 100,000 bases copied 3 billion bases in human genome But most happen in introns, spacers, junk of various kind Not every mutation has a visible effect. Some effects on subtle. May just affect rate of expression of a gene.

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 Are we moving towards a blended world?
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 bottleneck
small group splinters off & starts a new colony bottleneck some factor (disaster) reduces population to small number & then population recovers & expands again Warbler finch Tree finches Ground finches 1 family has a lot of children & grandchildren therefore has a greater impact on the genes in the population than other families Genghis Khan tracked through Y chromosome.

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 Small founder group, less genetic diversity than Africans All white people around the world are descended from a small group of ancestors 100,000 years ago (Chinese are white people!)

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 2 bottlenecks
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