AP Biology Hardy-Weinberg Equation Measuring Evolution of Populations.
Chapter 20 Genes Within Populations AP Biology 2012.
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Transcript of Chapter 20 Genes Within Populations AP Biology 2012.
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Chapter 20Genes Within Populations
AP Biology 2012
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20.1 Genetic Variation and Evolution
Evolution – changes over timeDarwin – descent with modification
Darwin’s mechanism is natural selectionorganisms with a desirable characteristic produce more offspring that live than
those that do not
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Opposing theory – inheritance of acquired characteristics – giraffe example
Based on genetics – which mechanism makes the most sense?
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Enzyme polymorphismpolymorphism – the presence in a population of more than one allele of a gene at a frequency greater than that of newly arising mutations
Common in insects and plants
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20.2 Changes in Allele Frequency
Was once believed that genetic variation was blended from one generation to the next
- blending inheritanceThought that variation was lost due to blending
Question – Wouldn’t all members of a population eventually only exhibit dominant traits?
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Hardy-Weinberg Principle – proportions of a genotype remain constant as long as:
1. No mutation takes place2. No immigration or emigration takes
place3. Random mating is occurring4. The population size is very large5. No selection occurs
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Hardy-Weinberg Equation
(p + q)2 = p2 + 2pq + q2
Where did we see this before?
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Cat Example:If 16% of cats are white and white is recessive Then q2 = 0.16 q = 0.4
p = 0.6 b/c p + q = 1
Genotypic Frequencies would be:BB p2 = (0.6)2 = .36 or 36%Bb 2pq = 2(0.6)(0.4) = .48 or 48%
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HW can be used to find evidence for evolution Populations changeNatural Selection occursMating is not randomImmigration and emigration
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20.3 Five Agents of Evolutionary Change
1. Mutation changes allelesmutation rates are very low
1/100,000 cell divisionshowever – ultimate source of change
*mutation is not a result of natural selection
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2. Gene flow occurs when alleles move between populations
a. New organism with different alleles comes to the area
b. gametes spread – seeds or pollenc. mating between adjacent populations
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3. Nonrandom mating shifts genotype frequencies
a. assortative mating – phenotypically similar individuals mate – increases homozygous
b. disassortative mating – phenotypically different individuals mate – increases heterozygous
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4. Genetic driftchange in genetic frequencies
A. Founder Effect – few individuals “found” a new population, alleles they do not have are lostB. Bottleneck Effect – few individuals survive and then produce the new population
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5. Selection favors some genotypes over othersA. Artificial selectionB. Avoiding predatorsC. Matching climatic conditionsD. Pesticide resistance
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20.4 Fitness and Its Measurement
A phenotype with greater fitness usually increases in frequency
Why?
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Toad Example2 phenotypes of toads – green and brown
Green 4.0 to next generationBrown 2.5 to the next generationG – 4.0/4.0 = 1 B – 2.5/4.0 = 0.625What do you think should eventually happen?
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20.5 Interactions Among Evolutionary Forces
Sometimes drift doesn’t favor the allele favored by selection?
How would you explain this statement?
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Gene flow may promote or constrain evolutionary change
How would you explain this?
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20.6 Maintenance of Variation
Frequency-dependent selection – fitness of a phenotype depends on its frequency within the population
Negative frequency-dependent selection – rare phenotypes are selected
Positive frequency-dependent selection – common phenotypes are selected
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Oscillating selection – favored phenotype changes as the environment changesExample – Ground Finch
A. large bills favored during droughtB. small bills favored during wet seasons
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In some cases heterozygous may exhibit greater fitness than homozygous
Example – Sickle Cell AnemiaHeterozygous individuals are resistant to malaria
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20.7 Selection Acting on Traits Affected by Multiple Genes
Disruptive Selection removes intermediatesExample
African black-bellied seedcracker finchSome individuals have large beaks –
for large seedsSome have small beaks – for small
seedsNo intermediates – no medium seeds
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Directional Selection – selection that eliminates an extreme, causing those genes to become less frequentExample – Fruit flies
Some have genes that cause them to move toward light – often leading to death
Over time less flies are moving toward light
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Stabilizing Selection – selection that eliminates both extremes, increase of the common phenotypeExample – Infant birth weights
Highest survival is found between 6 & 7 lbs
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20.8 Experimental Studies of Natural Selection
In groups• Identify the biology of the guppy• Identify why the guppy is a good organism to
study• Describe the laboratory experiment• Describe the field experiment• What were the results
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20.9 The Limits of Selection
What we cannot do:Chickens cannot lay larger or more eggs
b/c shells would become thinnerRacehorses cannot get any faster
b/c we bred them faster than mutations occur – no faster horses in 50 yrs
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Some selection on one gene can be affected by another -- seen in epistasis
Epistasis – when one gene modifies the effect of another – example – fur color in cats