2 Genetic variations in populations Genetic variation and evolution are both studied in populations.Because members of a population interbreed, they share a common group of genes called a gene pool.Gene pools consist of all the genes, including the different alleles for each gene, that are present in a populationAllele Frequency – the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same geneEVOLUTION, IN GENETIC TERMS, INVOLVES A CHANGE IN THE FREQUENCY OF ALLELES IN A POPULATION OVER TIME!!!
3 Genetic DriftIn small populations, individuals that carry a particular allele may leave more descendant than other individuals, just by chance.Over time, a series of chance occurrences can cause an allele to become more or less common in a population.This kid of random change in allele frequency is called genetic drift.Tends to reduce genetic variationTends to take place in smaller populationsTwo different models for genetic drift: bottleneck effect & founders effectFigure 23.7CRCRCRCWCWCWOnly 5 of10 plantsleaveoffspringOnly 2 ofGeneration 2p = 0.5q = 0.5Generation 3p = 1.0q = 0.0Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3
4 The Bottleneck EffectA sudden change in the environment (ex: natural disaster) may drastically reduce the size of a populationWipes out a random part of the populationThe gene pool may no longer be reflective of the original population’s gene poolFigure 23.8 A(a)Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.OriginalpopulationBottleneckingeventSurvivingpopulation
5 The Founder Effect The founder effect Occurs when a few individuals become isolated from a larger populationMigrate to another areaCan affect allele frequencies in a population
6 5 Agents of evolutionary change MutationGene FlowNon-random matingGenetic DriftSelection
7 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)
8 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
9 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
10 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
11 H-W formulas Alleles: p + q = 1 Individuals: p2 + 2pq + q2 = 1 B b BB
12 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?
13 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?
14 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
15 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…
16 MalariaSingle-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells123
17 Heterozygote Advantage In tropical Africa, where malaria is common:homozygous dominant (normal) die of malaria: HbHbhomozygous recessive die of sickle cell anemia: HsHsheterozygote carriers are relatively free of both: HbHssurvive 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