Genes Within Populations Chapter 20 1 Genetic Variation

Genes Within Populations Chapter 20 1

Genetic Variation and Evolution • Darwin: Evolution is descent with modification • Evolution: changes through time 1. Species accumulate difference 2. Descendants differ from their ancestors 3. New species arise from existing ones 2

Natural selection: mechanism of evolutionary change Natural selection: proposed by Darwin as the mechanism of evolution • individuals have specific inherited characteristics • they produce more surviving offspring • the population includes more individuals with these specific characteristics • the population evolves and is better adapted 3 to its present environment

Darwin’s theory for how long necks evolved in giraffes 4

Natural selection: mechanism of evolutionary change Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck • Individuals passed on physical and behavioral changes to their offspring • Variation by experience…not genetic • Darwin’s natural selection: variation a result of preexisting genetic differences 5

Lamarck’s theory of how giraffes’ long necks 6 evolved

Gene Variation in Nature • Measuring levels of genetic variation – blood groups – enzymes • Enzyme polymorphism – A locus with more variation than can be explained by mutation is termed polymorphic. – Natural populations tend to have more polymorphic loci than can be accounted for by mutation. • DNA sequence polymorphism 7

Hardy-Weinberg Principle 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 8

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 9

Hardy-Weinberg Principle • • Calculate genotype frequencies with a binomial expansion (p+q)2 = p 2 + 2 pq + q 2 p = individuals homozygous for first allele 2 pq = individuals heterozygous for both alleles q = individuals homozygous for second allele because there are only two alleles: p plus q must always equal 1 10

Hardy-Weinberg Principle 11

Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations 12

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 13

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 14

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 15

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 16

17

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 18

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 19

20

Genetic Drift: A bottleneck effect 21

Bottleneck effect: case study 22

Selection • Artificial selection: a breeder selects for desired characteristics 23

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 24

Selection 25

Selection Pocket mice from the Tularosa Basin 26

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 27

Selection for pesticide resistance 28

Selection for pesticide resistance 29

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 30

Fitness and its Measurement Body size and egg-laying in water striders 31

Fitness and its Measurement Body size and egg-laying in water striders 32

Fitness and its Measurement Body size and egg-laying in water striders 33

Interactions Among Evolutionary Forces • Mutation and genetic drift may counter selection • The magnitude of drift is inversely related to population size 34

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 35

Interactions Among Evolutionary Forces Degree of copper tolerance 36

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 37

Maintenance of Variation Negative frequency - dependent selection 38

Maintenance of Variation Positive frequency-dependent selection 39

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 40

Maintenance of Variation • Fitness of a phenotype does not depend on its frequency • Environmental changes lead to oscillation in selection 41

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 42

Maintenance of Variation • Homozygous dominant phenotype: no anemia; susceptible to malaria • Heterozygous phenotype: no anemia; less susceptible to malaria 43

Maintenance of Variation Frequency of sickle cell allele 44

Maintenance of Variation Disruptive selection acts to eliminate intermediate types 45

Maintenance of Variation Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa 46

Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes 47

Maintenance of Variation Directional selection for negative phototropism in Drosophila 48

Maintenance of Variation Stabilizing selection: acts to eliminate both extremes 49

Maintenance of Variation Stabilizing selection for birth weight in humans 50

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 51

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. 52

Experimental Studies The evolution of protective coloration in 53 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 54

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 55

Experimental Studies Evolutionary change in spot number 56

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 57

Experimental Studies Selection for increased speed in racehorses is no longer effective 58

Experimental Studies Phenotypic variation in insect ommatidia 59