1 Genes Within Populations Chapter 20. 2 Darwin: Evolution is descent with modification Evolution:...

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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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• 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

Gene Variation in Nature

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Godfrey H. Hardy: English mathematicianWilhelm 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 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


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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


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Using Hardy-Weinberg equation to predict frequencies in subsequent generations


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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

24Pocket 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


31Degree of copper tolerance


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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

34Positive 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


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• Fitness of a phenotype does not depend on its frequency

• Environmental changes lead to oscillation in selection


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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


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• Homozygous dominant phenotype: no anemia; susceptible to malaria

• Heterozygous phenotype: no anemia; less susceptible to malaria


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Frequency of sickle cell allele

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Disruptive selection acts to eliminate intermediate types


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

west Africa


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


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Directional selection for negative phototropism in Drosophila


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Stabilizing selection: acts to eliminate both extremes


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Stabilizing selection for birth weight in humans


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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


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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


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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


51Evolutionary change in spot number


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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


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Phenotypic variation in insect ommatidia


1 Genes Within Populations Chapter 20. 2 Darwin: Evolution is descent with modification Evolution:...

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