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Transcript
4/5/09 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 4/5/09 Hardy-Weinberg Principle • • • • Calculate genotype frequencies with a binomial expansion (p+q)2 = p2 + 2pq + q2 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 4/5/09 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 4/5/09 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 11 12 4 4/5/09 Genetic Drift: A bottleneck effect 13 Bottleneck effect: case study 14 Selection • Artificial selection: a breeder selects for desired characteristics 15 5 4/5/09 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 4/5/09 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 4/5/09 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 4/5/09 Interactions Among Evolutionary Forces Degree of copper tolerance 25 Maintenance of Variation • 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 Maintenance of Variation Negative frequency - dependent selection 27 9 4/5/09 Maintenance of Variation Positive frequency-dependent selection 28 Maintenance of Variation • 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 Maintenance of Variation • 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 4/5/09 Maintenance of Variation • Homozygous dominant phenotype: no anemia; susceptible to malaria • Heterozygous phenotype: no anemia; less susceptible to malaria 31 Maintenance of Variation Frequency of sickle cell allele 32 Maintenance of Variation Disruptive selection acts to eliminate intermediate types 33 11 4/5/09 Maintenance of Variation Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa 34 Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes 35 Maintenance of Variation Directional selection for negative phototropism in Drosophila 36 12 4/5/09 Maintenance of Variation Stabilizing selection: acts to eliminate both extremes 37 Maintenance of Variation 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 4/5/09 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 41 guppies 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 4/5/09 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 4/5/09 Experimental Studies Selection for increased speed in racehorses is no longer effective 46 Experimental Studies Phenotypic variation in insect ommatidia 47 16