Hardy-Weinberg Principle 4/5/09. Chapter 20. Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician

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1 Chapter 20 1 Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician Concluded that: The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met 2 Hardy-Weinberg Principle Five assumptions : 1. No mutation takes place 2. No genes are transferred to or from other sources 3. Random mating is occurring 4. The population size is very large 5. No selection occurs 3 1

2 Hardy-Weinberg Principle Calculate genotype frequencies with a binomial expansion (p+q) 2 = p 2 + 2pq + q 2 p = individuals homozygous for first allele 2pq = individuals heterozygous for both alleles q = individuals homozygous for second allele because there are only two alleles: p plus q must always equal 1 4 Hardy-Weinberg Principle 5 Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations 6 2

3 A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Five agents of evolutionary change 7 Agents of Evolutionary Change Mutation: A change in a cell s DNA Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. Ultimate source of genetic variation Gene flow: A movement of alleles from one population to another Powerful agent of change Tends to homogenize allele frequencies 8 9 3

4 Agents of Evolutionary Change Nonrandom Mating: mating with specific genotypes Shifts genotype frequencies Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes 10 Genetic Drift Genetic drift: Random fluctuation in allele frequencies over time by chance important in small populations founder effect - few individuals found new population (small allelic pool) bottleneck effect - drastic reduction in population, and gene pool size

5 Genetic Drift: A bottleneck effect 13 Bottleneck effect: case study 14 Selection Artificial selection: a breeder selects for desired characteristics 15 5

6 Selection Natural selection: environmental conditions determine which individuals in a population produce the most offspring 3 conditions for natural selection to occur Variation must exist among individuals in a population Variation among individuals must result in differences in the number of offspring surviving Variation must be genetically inherited 16 Selection 17 Selection Pocket mice from the Tularosa Basin 18 6

7 Selection to match climatic conditions Enzyme allele frequencies vary with latitude Lactate dehydrogenase in Fundulus heteroclitus (mummichog fish) varies with latitude Enzymes formed function differently at different temperatures North latitudes: Lactate dehydrogenase is a better catalyst at low temperatures 19 Selection for pesticide resistance 20 Fitness and Its Measurement Fitness: A phenotype with greater fitness usually increases in frequency Most fit is given a value of 1 Fitness is a combination of: Survival: how long does an organism live Mating success: how often it mates Number of offspring per mating that survive 21 7

8 Fitness and its Measurement Body size and egg-laying in water striders 22 Interactions Among Evolutionary Forces Mutation and genetic drift may counter selection The magnitude of drift is inversely related to population size 23 Interactions Among Evolutionary Forces Gene flow may promote or constrain evolutionary change Spread a beneficial mutation Impede adaptation by continual flow of inferior alleles from other populations Extent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow High in birds & wind-pollinated plants Low in sedentary species 24 8

9 Interactions Among Evolutionary Forces Degree of copper tolerance 25 Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population Negative frequency-dependent selection: rare phenotypes are favored by selection Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population Strength of selection changes through time 26 Negative frequency - dependent selection 27 9

10 Positive frequency-dependent selection 28 Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time Galápagos Islands ground finches Wet conditions favor big bills (abundant seeds) Dry conditions favor small bills 29 Heterozygotes may exhibit greater fitness than homozygotes Heterozygote advantage: keep deleterious alleles in a population Example: Sickle cell anemia Homozygous recessive phenotype: exhibit severe anemia 30 10

11 Homozygous dominant phenotype: no anemia; susceptible to malaria Heterozygous phenotype: no anemia; less susceptible to malaria 31 Frequency of sickle cell allele 32 Disruptive selection acts to eliminate intermediate types 33 11

12 Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa 34 Directional selection: acts to eliminate one extreme from an array of phenotypes 35 Directional selection for negative phototropism in Drosophila 36 12

13 Stabilizing selection: acts to eliminate both extremes 37 Stabilizing selection for birth weight in humans 38 Experimental Studies of Natural Selection In some cases, evolutionary change can occur rapidly Evolutionary studies can be devised to test evolutionary hypotheses Guppy studies (Poecilia reticulata) in the lab and field Populations above the waterfalls: low predation Populations below the waterfalls: high predation 39 13

14 Experimental Studies High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age. Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories. 40 Experimental Studies The evolution of protective coloration in guppies 41 Experimental Studies The laboratory experiment 10 large pools 2000 guppies 4 pools with pike cichlids (predator) 4 pools with killifish (nonpredator) 2 pools as control (no other fish added) 10 generations 42 14

15 Experimental Studies The field experiment Removed guppies from below the waterfalls (high predation) Placed guppies in pools above the falls 10 generations later, transplanted populations evolved the traits characteristic of low-predation guppies 43 Experimental Studies Evolutionary change in spot number 44 The Limits of Selection Genes have multiple effects Pleiotropy: sets limits on how much a phenotype can be altered Evolution requires genetic variation Thoroughbred horse speed Compound eyes of insects: same genes affect both eyes Control of ommatidia number in left and right eye 45 15

16 Experimental Studies Selection for increased speed in racehorses is no longer effective 46 Experimental Studies Phenotypic variation in insect ommatidia 47 16