2 5 Agents of evolutionary change MutationGene FlowNon-random matingGenetic DriftSelection
3 Populations & gene pools Conceptsa population is a localized group of interbreeding individualsgene pool is collection of alleles in the populationremember difference between alleles & genes!allele frequency is how common is that allele in the populationhow many A vs. a in whole population
4 Evolution of populations Evolution = change in allele frequencies in a populationhypothetical: what conditions would cause allele frequencies to not change?non-evolving populationREMOVE all agents of evolutionary changevery large population size (no genetic drift)no migration (no gene flow in or out)no mutation (no genetic change)random mating (no sexual selection)no natural selection (everyone is equally fit)
5 Hardy-Weinberg equilibrium Hypothetical, non-evolving populationpreserves allele frequenciesServes as a model (null hypothesis)natural populations rarely in H-W equilibriumuseful model to measure if forces are acting on a populationmeasuring evolutionary changeG.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. HardymathematicianW. Weinbergphysician
6 Hardy-Weinberg theorem Counting Allelesassume 2 alleles = B, bfrequency of dominant allele (B) = pfrequency of recessive allele (b) = qfrequencies must add to 1 (100%), so:p + q = 1BBBbbb
7 Hardy-Weinberg theorem Counting Individualsfrequency of homozygous dominant: p x p = p2frequency of homozygous recessive: q x q = q2frequency of heterozygotes: (p x q) + (q x p) = 2pqfrequencies of all individuals must add to 1 (100%), so:p2 + 2pq + q2 = 1BBBbbb
8 H-W formulas Alleles: p + q = 1 Individuals: p2 + 2pq + q2 = 1 B b BB
9 Using Hardy-Weinberg equation population: 100 cats84 black, 16 whiteHow many of each genotype?q2 (bb): 16/100 = .16q (b): √.16 = 0.4p (B): = 0.6p2=.362pq=.48q2=.16BBBbbbMust assume population is in H-W equilibrium!What are the genotype frequencies?
10 Using Hardy-Weinberg equation p2=.362pq=.48q2=.16Assuming H-W equilibriumBBBbbbNull hypothesisp2=.20p2=.742pq=.102pq=.64q2=.16q2=.16Sampled data 1:Hybrids are in some way weaker.Immigration in from an external population that is predomiantly homozygous BNon-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.bbBbBBSampled dataHow do you explain the data?How do you explain the data?
11 Application of H-W principle Sickle cell anemiainherit a mutation in gene coding for hemoglobinoxygen-carrying blood proteinrecessive allele = HsHsnormal allele = Hblow oxygen levels causes RBC to sicklebreakdown of RBCclogging small blood vesselsdamage to organsoften lethal
12 Sickle cell frequency High frequency of heterozygotes 1 in 5 in Central Africans = HbHsunusual for allele with severe detrimental effects in homozygotes1 in 100 = HsHsusually die before reproductive ageSickle 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 CcWhy is the Hs allele maintained at such high levels in African populations?Suggests some selective advantage of being heterozygous…
13 MalariaSingle-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells123
14 Heterozygote Advantage In tropical Africa, where malaria is common:homozygous dominant (normal)die or reduced reproduction from malaria: HbHbhomozygous recessivedie or reduced reproduction from sickle cell anemia: HsHsheterozygote carriers are relatively free of both: HbHssurvive & reproduce more, more common in populationHypothesis: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
15 Practice QuestionA population called the “founder generation”, consisting of 2000 AA individuals, 2000 Aa individuals, and 6000 aa individuals is established on a remote island. Mating within this population occurs at random, the three genotypes are selectively neutral, and mutations occur at a negligible rate.What are the frequencies of the alleles A and a in the founder generation?Is the founder generation at Hardy-Weinberg Equilibrium?What is the frequency of the A allele in the second generation (that is, the generation subsequent to the founder generation)?What are the frequencies of the AA, Aa, and aa genotypes in the second generation?
17 your work and label each frequency, probability, and allele. For each of the following problems in population genetics use the Hardy-Weinberg equation. Show all ofyour work and label each frequency, probability, and allele.1. Suppose that in a breeding experiment, 7,000 AA individuals and 3,000 aa individuals mate at random. In the first generation of offspring, what would be the frequencies of the three genotypes (AA, Aa, and aa)? What would be the frequencies of the two alleles? What would be the values in the second generation?
18 2. Among African-Americans, the frequency of sickle-cell anemia (which, as you will recall is a homozygous recessive condition) is about What is the frequency of heterozygotes? When on African-American marries another, what is the probability that both will be heterozygotes? If both are heterozygotes, what is the probability that their first child will have sickle-cell anemia?
19 3. If q = 0.3 and there are Hardy-Weinberg proportions, what is the most common genotype and what is its frequency? What is the least frequent genotype and its frequency?
20 4. In a large, randomly mating population with no forces acting to change gene frequencies, the frequency of homozygous recessive individuals for the character extra-long eyelashes is 90 per 1000, or What percentage of the population carries this trait but displays the dominant phenotype, short eyelashes? Would the frequency of the extra-long-lash allele increase, decrease, or remain the same if long-lashed individuals preferentially mated with each other and no one else?