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Chapter 20

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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

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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

Hardy-Weinberg Principle

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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

Hardy-Weinberg Principle

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Hardy-Weinberg Principle

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Using Hardy-Weinberg equation to predict frequencies in subsequent generations

Hardy-Weinberg Principle

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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

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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

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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

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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

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Genetic Drift: A bottleneck effect

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Bottleneck effect: case study

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Selection •  Artificial selection: a breeder selects for

desired characteristics

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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

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Selection

18 Pocket mice from the Tularosa Basin

Selection

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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

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Selection for pesticide resistance

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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

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Body size and egg-laying in water striders

Fitness and its Measurement

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Interactions Among Evolutionary Forces

•  Mutation and genetic drift may counter selection

•  The magnitude of drift is inversely related to population size

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•  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

Interactions Among Evolutionary Forces

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25 Degree of copper tolerance

Interactions Among Evolutionary Forces

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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

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Negative frequency - dependent selection

Maintenance of Variation

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28 Positive frequency-dependent selection

Maintenance of Variation

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•  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

Maintenance of Variation

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•  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

Maintenance of Variation

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• Homozygous dominant phenotype: no anemia; susceptible to malaria

• Heterozygous phenotype: no anemia; less susceptible to malaria

Maintenance of Variation

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Maintenance of Variation

Frequency of sickle cell allele

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Disruptive selection acts to eliminate intermediate types

Maintenance of Variation

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Disruptive selection for large and small beaks in black-bellied seedcracker finch of

west Africa

Maintenance of Variation

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

Maintenance of Variation

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Directional selection for negative phototropism in Drosophila

Maintenance of Variation

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Stabilizing selection: acts to eliminate both extremes

Maintenance of Variation

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Stabilizing selection for birth weight in humans

Maintenance of Variation

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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

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•  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.

Experimental Studies

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The evolution of protective coloration in guppies

Experimental Studies

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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

Experimental Studies

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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

Experimental Studies

44 Evolutionary change in spot number

Experimental Studies

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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

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Selection for increased speed in racehorses is no longer effective

Experimental Studies

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Phenotypic variation in insect ommatidia

Experimental Studies