Genes Within Populations Chapter 20 1 Nothing in Biology Makes Sense Except in the Light of Evolution The American Biology Teacher, March 1973 (35:125-129). Theodosius Dobzhansky (1900-1975). 2
Genetic Variation & Evolution Definition of Evolution 1) Darwin: descent with modification 2) Evolutionary biologists: changes through time 3) Population geneticists: change in gene frequencies from generation to generation Species accumulate difference a) descendants differ from their ancestors b) new species arise from existing ones 3 Natural Selection Darwin: mechanism of evolutionary change Natural Selection: differential survival & reproduction 1) individuals: show variation & variation inherited 2) individuals: produce more surviving offspring 3) population includes more individuals with these specific variations (characteristics) 4) population evolves & better adapted to its present environment ( struggle for existence )* 5) all above: evolution of new species * not survival of the fittest -Herbert Spencer 4
Charles Robert Darwin (1809-1882) Medical student, Edinburgh, 1825-1828 Divinity student, Cambridge, 1828-1831 Naturalist, H.M.S. Beagle, 1831-1836 Resident in London, 1837-1842 Resident at Down House, Kent, 1842-1882 Barnacle Monographs (1851-1854) an 8 yr project The Structure and Distribution of Coral Reefs: Being the First Part of the Geology of the Voyage of the "Beagle" (1842) On the Origin of Species by Means of Natural Selection; or the Preservation of Favored Races in the Struggle for Life (1859) The Descent of Man and Selection in Relation to Sex (1871) 5 Alfred Russel Wallace (1823-1913) 1848-1852: Amazon Basin 1854-1862: Malay Archipelago 1855: "On the Law that has regulated the Introduction of New Species" 1858: essay introducing concept of natural selection 1870: Contributions to the theory of natural selection 6
Charles Lyell (1797-1875) 1830-1833: Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface, by Reference to Causes now in Operation (3 vols ) uniformitarianism laws of chemistry & physics remained constant throughout earth s history that past geological events occurred by processes that can be observed today 7 Species a group of interbreeding individuals of common ancestry that are reproductively isolated from all other such groups Share: gene pool habitat behavior niche address & occupation of an organism 8
Speciation 1) Allopatric: geographical isolation of original population into two new populations (e.g. Isthmus of Panama) 2) Sympatric: in same geographical area evolution of two new populations from the original population (e.g. nuclear power plant causing genetic mutation in nearby individuals) 9 Gene Theory genotype gene products phenotype blood types Genotypes: AA & AO Phenotypes: A 10
Darwin s theory for how long necks evolved in giraffes 11 Natural selection: mechanism of evolutionary change Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck 1809 Individuals passed on physical & behavioral changes to their offspring Variation by experience not genetic Darwin s natural selection: variation a result of preexisting genetic differences Gene Theory?-No genotype gene products phenotype 12
Lamarck s theory: giraffes long necks evolved 13 Gene Variation in Nature Measuring levels of genetic variation blood groups enzymes Enzyme polymorphism locus with more variation than can be explained by mutation polymorphic Natural populations tend to have more polymorphic loci than can be accounted for by mutation DNA sequence polymorphism 14
Hardy-Weinberg Principle Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician Concluded that: gene frequencies do not change from generation to generation as long as following five assumptions are met: 1. large population size 2. random mating orgies 3. no mutation 4. no migration (no genes transferred to or from other sources) 5. no selection no incest/interbreeding 15 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 16
Hardy-Weinberg Principle 17 Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations 18
Comparative primary structures of the vertebrate insulin 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 20
Agents of Evolutionary Change Mutation: change in a cell s DNA Mutation rates generally so low they have little effect on Hardy-Weinberg proportions of common alleles. Ultimate source of genetic variation Gene flow: movement of alleles from one population to another Powerful agent of change Tends to homogenize allele frequencies 21 22
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 23 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 & gene pool size 24
Founder Effect oceanic islands Galapagos Islands Darwinian Finches Adaptive Radiation 26
Genetic Drift: A bottleneck effect 27 Bottleneck effect: case study 1800s 20 Northern Elephant Seal severe hunting After protection > ~127,000 present 28
Selection Artificial selection: a breeder selects for desired characteristics reproduction uuuumm? 29 Dr. Okazaki
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 31 Selection 32
Selection Pocket mice from the Tularosa Basin 33
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 36
Selection for pesticide resistance Housefly resistance to pesticides 37 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 38
Fitness & its Measurement Body size & egg-laying in water striders 39 Interactions Among Evolutionary Forces Mutation and genetic drift may counter selection The magnitude of drift is inversely related to population size 40
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 relative strengths of gene flow High in birds & wind-pollinated plants Low in sedentary species 41 Interactions Among Evolutionary Forces Degree of copper tolerance 42
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 43 Maintenance of Variation Negative frequency - dependent selection 44
Maintenance of Variation Positive frequency-dependent selection 45 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 46
Maintenance of Variation Fitness of a phenotype does not depend on its frequency Environmental changes lead to oscillation in selection 47 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 48
Maintenance of Variation Homozygous dominant phenotype: no anemia; susceptible to malaria Heterozygous phenotype: no anemia; less susceptible to malaria 49 Maintenance of Variation Frequency of sickle cell allele 50
Maintenance of Variation Disruptive selection acts to eliminate intermediate types 51 Maintenance of Variation Disruptive selection for large & small beaks in black-bellied seedcracker finch of west Africa 52
Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes 53 Maintenance of Variation Directional selection for negative phototropism in Drosophila 54
Maintenance of Variation Stabilizing selection: acts to eliminate both extremes 55 Maintenance of Variation Stabilizing selection for birth weight in humans 56
Experimental Studies of Natural Selection In some cases, evolutionary change can occur rapidly Evolutionary studies can be devised to test evolutionary hypotheses lab & field studies guppy(poecilia reticulata) -- Populations above waterfalls: low predation -- Populations below waterfalls: high predation 57 Experimental Studies High predation environment - Males exhibit drab coloration & tend to be relatively small & reproduce at a younger age Low predation environment - Males display bright coloration, a larger number of spots, & tend to be more successful at defending territories. 58
Experimental Studies evolution of protective coloration in guppies 59 Experimental Studies 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 60
Experimental Studies field experiment Removed guppies from below waterfalls (high predation) Placed guppies in pools above falls 10 generations later, transplanted populations evolved the traits characteristic of low-predation guppies 61 Experimental Studies Evolutionary change in spot number 62
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 #s in left & right eye 63 Experimental Studies Selection for speed in racehorses no longer effective 64
Experimental Studies Phenotypic variation in insect ommatidia 65