Transcript Document
Genes Within Populations Chapter 20
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Genetic Variation and Evolution
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Darwin
: Evolution is descent with modification.
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Evolution
: changes through time.
1. Species accumulate difference; 2. Descendants differ from their ancestors; 3. New species arise from existing ones.
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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 to its present environment.
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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 evolved.
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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: 7
Hardy-Weinberg Principle
Five assumptions
: 1.
2.
3.
4.
5.
No mutation takes place No genes are transferred to or from sources Random mating is occurring The population size is very large No selection occurs other 8
Hardy-Weinberg Principle Calculate genotype frequencies with a binomial expansion:
(p+q)
2 = p 2 + 2pq + 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 (that is the total population)
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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 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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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.
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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 13
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 14
Genetic Drift: A bottleneck effect 15
Selection •
Artificial selection
: characteristics.
a breeder selects for desired 16
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 Pocket mice from the Tularosa Basin 18
Selection to match climatic conditions
Selection for pesticide resistance 19
Fitness and Its Measurement
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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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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.
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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: phenotypes are favored by selection.
rare – Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population.
• Strength of selection changes through time.
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Maintenance of Variation
Negative frequency dependent selection Positive frequency dependent selection.
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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; Heterozygotes may exhibit greater fitness than homozygotes.
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Maintenance of Variation •
Homozygous dominant phenotype:
no anemia; susceptible to malaria.
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Heterozygous phenotype:
no anemia; less susceptible to malaria Frequency of sickle cell allele.
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Maintenance of Variation Disruptive selection : acts to eliminate intermediate types.
Disruptive selection for large and small beaks in black bellied seedcracker finch of west Africa.
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Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes Directional selection for negative phototropism in
Drosophila.
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Maintenance of Variation Stabilizing selection: acts to eliminate both extremes.
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 30
Experimental Studies •
High predation environment
- Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
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Low predation environment
- Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.
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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 32
Experimental Studies Selection for increased speed in racehorses is no longer effective.
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Experimental Studies Phenotypic variation in insect ommatidia.
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