Lecture 7 BIO 311D 1st EditionOutline of Last Lecture I. The Smallest Unit of EvolutionII. The Hardy-Weinberg EquationIII. Genetic VariationIV. Variation Within and Between PopulationsV. Sources of Genetic VariationVI. Altering Gene Number or PositionOutline of Current Lecture I. Hardy Weinberg EquilibriumII. Conditions for the Hardy-Weinberg EquilibriumIII. Apply the Hardy-Weinberg PrincipleIV. Natural selection, genetic drift, and gene flow can alter allele frequencies in a populationV. Genetic Drift (The founder effect and the bottleneck effect)VI. The Key Role of Natural Selection in Adaptive EvolutionVII. Sexual SelectionCurrent LectureHardy-Weinberg Equilibrium:1. Which of these populations are in hardy Weinberg equilibrium? The original population has 36% red, 48% roan, and 16% white.Population A36% red 48% roan16% whitePopulation B40% red40% roan20% white- Answer: Population A because the Hardy-Weinberg equilibrium is when a population that has not yet evolved**Chapter 22 will not be on the examConditions for Hardy-Weinberg Equilibrium:1. No mutations2. Random mating3. No natural selection4. Extremely large population size5. No gene flow*We as humans do not fit into the hardy Weinberg equilibrium because we (whether consciously or unconsciously) naturally select our partnersApplying the Hardy-Weinberg Principle:• We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:• The PKU gene mutation rate is low• Mate selection is random with respect to whether or not an individual is a carrierfor the PKU allele• Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions • The population is large• Migration has no effect as many other populations have similar allele frequencies• The occurrence of PKU is 1 per 10,000 births• q2 = 0.0001 (found by 1/10,000)• q = 0.01 (found by taking the square root of 0.0001)• The frequency of normal alleles is• p = 1 – q = 1 – 0.01 = 0.99• The frequency of carriers is• 2pq = 2 ´ 0.99 ´ 0.01 = 0.0198 or approximately 2% of the U.S. populationNatural selection, genetic drift, and gene flow can alter allele frequencies in a population:• Three major factors alter allele frequencies and bring about most evolutionary change:– Natural selection• Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions• Example: an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture• The only mechanism that consistently causes adaptive evolution• New genetic variations includes both chance and sorting– Genetic drift• When certain allele frequencies are reduced from one generation to the next due to chance occurrences or something that happens within a population to cause that • The smaller a sample, the greater the chance of deviation from a predicted result• Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next• Genetic drift tends to reduce genetic variation through losses of alleles– Gene flow• Consists of the movement of alleles among populations• Important agent of evolutionary change in human populations• Alleles can be transferred through the movement of fertile individuals or gametes• Gene flow tends to reduce variation among populations over time• Can decrease the fitness of a population• Example, the great tit (Parus major) on the Dutch island of Vlieland• Mating causes gene flow between the central and eastern populations• Immigration from the mainland introduces alleles that decrease fitness• Natural selection selects for alleles that increase fitness• Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness • Can also increase the fitness of a population• Example, the spread of alleles for resistance to insecticides• Insecticides have been used to target mosquitoes that carry West Nile virus and malaria• Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes• The flow of insecticide resistance alleles into a population can cause an increase in fitnessA high proportion of the cats on Key West have extra toes (polydactyl) what is most likely explanation?A. High rate of mutationB. Founder effectC. Bottleneck effect D. Directional Selection for extra toesAnswer: The founder effect Genetic Drift (The founder effect and the bottleneck effect):The Founder Effect:- Occurs when a few individuals become isolated from a large population- Common in small populations- Allele frequencies in the small founderpopulation can be different from those inlarger parent population- Because a certain population of thosefounders had this syndrome, they passed iton to their offspring- Their population has a higher percentage ofindividuals with the disease The Bottleneck Effect:- This is a sudden reduction in population size due to a change in the environment- The resulting gene pool may no longer be reflective of the original populations gene pool- If the population remains small, it may be further affected by genetic drift- The bottleneck effect can increase understanding of how human activity affects other species- Example: Elephant population decreased at a fast rate due to the human demand in their tusks, so eventually we had to stop in order to allow the elephant population to stabilize againRelative Fitness:- The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals- Selection favors certain genotypes by acting on the phenotypes of certain organisms- Before industrial revolution, mice were skewed towards light colored furs due to the clean air and better blending into the environment for survival. After the revolution due to coal, mice had darker colored fur due to blending in the environment- Three modes of selection:o Directional selection: favors individuals at one end of the phenotypic rangeo Disruptive selection: favors individuals at both extremes of the phenotypic rangeo Stabilizing selection: favors intermediate variants and against extreme phenotypesThe Key Role of Natural Selection in Adaptive Evolution:• Striking adaptations have arisen by natural selection– For example, cuttlefish can change color rapidly
View Full Document