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UCSB EEMB 2 - Lec #13 EEMB 2 SM17 1S (Selection)

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Natural SelectionThe evolutionary process: A blend of chance and sorting 1. Variation2. Natural selection3. Genetic Divergence4. Reproductive Isolation5. SpeciationNatural Selection: From the range of available variations natural selection increases the frequencies of certain genotypes that confer reproductive success (fitness).Organisms are fit when adapted to their environmentVariationNatural SelectionReproductive isolationGenetic DivergenceSpeciationNatural SelectionEvolutionary fitness: The contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individualsEx: Wildflowers: Variants (color, shape, fragrance) differ in reproductive success, variants have adaptive advantagesRelative fitness: The contribution of a genotype to the next generation compared to the contribution of alternative genotypes for the same locusEx: Wildflowers: red flower (AA) variants produce 5 offspring, pink flower (Aa) variants produce 4 offspring and white flower (aa) variants produce 1 offspring1. Fitness of (AA): WAA= 1.0 (highest reproductive success set to 1) 2. Fitness of (Aa): WAa= 4/5 = 0.83. Fitness of (aa): Waa= 1/5 = 0.2Natural SelectionReproductive success: Survival and Fecundity 1. Survival does not guarantee reproductive success2. Organisms must be fecund (fertile - producing viable offspring) (Must survive and be fecund to be fit)Natural selection acts on phenotypes not genotypes- The relative fitness of an allele depends on the entire genetic and environmental context in which it is expressedMeasured trait# of individualsX++++------0 50 100Continuous Variation:+ = greater contribution (well adapted, high fitness)- = less contribution(not well adapted, low fitness)Mean fitness of population: The average reproductive success of members of a populationNatural SelectionBasic conditions: 1. environment remains stable over time 2. organisms have obtained a high state of adaptivenessTypes of Natural selection:1. Stabilizing Selection:(Intermediate forms of traits are favored and alleles that specify extreme forms are selected against)- reduces variation by counteracting effects of sexual recombination, mutation, and migration - Favors the survival and reproduction of the intermediate phenotype (phenotypic character)Stabilizing Selection:Measured trait# of individualsXMeasured trait# of individuals+-+++-----time = long0 50 1000 50 100Natural SelectionXIndividuals that are well adapted cluster around the mean++++------Ex: Human birth weight low or high weight leads to high mortalityMeasured trait# of individualsX6.6 lbs 8.8 lbs2. Directional Selection:Favors survival and reproduction of phenotypes at one extreme or the other of phenotypic distribution- Allele frequencies in a range of phenotypic characteristics shift in one direction or another- Individuals that deviate from the average are favored- occur when mutations appear and prove adaptiveBasic conditions: 1. Environmental and/or biological conditions are changing over time2. Species can not be completely adapted to the changing conditionsNatural SelectionDirectional Selection:Measured trait# of individualsX+-+++----time = long0 50 100Natural SelectionMeasured trait# of individuals50 75 125XIndividuals at one end of the range for some phenotypic character become more common than intermediate formsEx: European Brown BearDirectional Selection:Ex: Peppered Moth (Biston betularia)Natural Selection>><- morphology ranges from light gray to nearly black- behavior, coloration, wing pattern help camouflage moths from birds1. Pre industrial England Light Dark2. Industrial revolution (1850) Light Dark3. Clean air act (1952-1954) Light DarkDirectional Selection: Peppered Moth (Biston betularia)Experiment: Kettlewell (1950’s)- used mark-release-recapture method- released light and dark moths in polluted and unpolluted areasNatural SelectionResults:1. more dark moths were caught (recaptured) in polluted areas2. more light moths were caught (recaptured) in unpolluted areasConclusions:1. Birds captured more light moths around polluted areas & more dark moths around unpolluted areas2. Directional selection was operatingDirectional Selection: Pesticide ResistanceNatural SelectionPest Resurgence: pesticides also kill natural predators of the pests- Chemicals are agents of selection favoring the most resistant for (Today, 450 species of pest are resistant to one or more pesticides)- Chemical pesticide in agriculture has resulted in directional selection - If resistance is heritable it becomes more common in next generation Antibiotic Resistance- Overuse & misuse of antibiotics has resulted in directional selection - If resistance is heritable it becomes more common in next generation - Antibiotics are agents of selection favoring the most resistant forms (Today even last line of defense antibiotics are failing)3. Disruptive Selection:Phenotypic characters at opposite extremes of character distribution are favored and intermediate forms are selected againstBasic conditions: Natural Selection1. Fitness values for a particular genotype are higher in one environment and lower in a different environment2. Occurs in diverse and shifting environmentsDisruptive Selection:Measured trait# of individualsX- Positive selection tends to effect the extremes not the mean - The number of organisms is often reducedMeasured trait# of individuals+-+++-----time = short0 50 1000 50 100++XX-++Natural SelectionEx: black-bellied seedcracker finches (Pyrenestes ostrinus) - Display two different beak sizes:1. small billed finches feed on soft seeds2. large billed finches feed on large seedsDisruptive Selection: black-bellied seedcracker finches What Factors cause the disruptive selection pressure on feeding performance?1. Wet season: flooded forest - soft seeds dominant 2. Dry season: forest burns - hard seeds dominant- small billed birds better at utilizing soft seeds- large billed birds better at utilizing hard seedsBirds of intermediates sized bills are inefficient at cracking both types of seeds (Have lower relative fitness than small or large billed birds)Natural SelectionIf the bill pattern is unrelated to gender or geography, what causes it?Causes: 1. Seed cracking ability directly affects survival2. Disruptive selection may be eliminating birds with intermediate sized billsDuring the dry season:soft seeds (small bill) competitive disadvantage mortality During


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UCSB EEMB 2 - Lec #13 EEMB 2 SM17 1S (Selection)

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