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Genes Within Populations

Chapter 20

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• 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.

Genetic Variation and Evolution

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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 to its present

environment.

Natural selection: mechanism of evolutionary change

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Darwin’s theory for how long necks evolved in giraffes

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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.

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

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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:

Hardy-Weinberg Principle

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Five assumptions :

1. No mutation takes place2. No genes are transferred to or from other

sources3. Random mating is occurring4. The population size is very large5. 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 (that is the total population)

Hardy-Weinberg Principle

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Hardy-Weinberg Principle

Using Hardy-Weinberg equation to predict frequencies in subsequent generations.

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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.

A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a

population.

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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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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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Pocket mice from the Tularosa Basin

Selection

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Selection to match climatic conditions Selection for pesticide resistance

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Fitness and Its Measurement

• 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

Body size and egg-laying in water striders.

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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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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

Positive frequency-dependent selection.

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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

Heterozygotes may exhibit greater fitness than homozygotes.

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• Homozygous dominant phenotype: no anemia; susceptible to malaria.

• Heterozygous phenotype: no anemia; less susceptible to malaria

Maintenance of Variation

Frequency of sickle cell allele.

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

Maintenance of Variation

Disruptive selection: acts to eliminate intermediate types.

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Directional selection for negative phototropism in Drosophila.

Maintenance of VariationDirectional selection: acts to eliminate one extreme from

an array of phenotypes

29Stabilizing selection for birth weight in humans.

Maintenance of Variation

Stabilizing selection: acts to eliminate both extremes.

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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 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