UCSB EEMB 2 - Lec #13 EEMB 2 SM17 1S (Selection) (15 pages)

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Lec #13 EEMB 2 SM17 1S (Selection)



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Natural Selection The evolutionary process A blend of chance and sorting 1 Variation Variation 2 Natural selection 3 Genetic Divergence 4 Reproductive Isolation Natural Reproductive Genetic isolation Selection Divergence 5 Speciation Speciation Natural 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 environment Natural Selection Evolutionary fitness The contribution an individual makes to the gene pool of the next generation relative to the contribution of other individuals Ex Wildflowers Variants color shape fragrance differ in reproductive success variants have adaptive advantages Relative fitness The contribution of a genotype to the next generation compared to the contribution of alternative genotypes for the same locus Ex Wildflowers red flower AA variants produce 5 offspring pink flower Aa variants produce 4 offspring and white flower aa variants produce 1 offspring 1 Fitness of AA WAA 1 0 highest reproductive success set to 1 2 Fitness of Aa WAa 4 5 0 8 3 Fitness of aa Waa 1 5 0 2 Natural Selection 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 expressed Reproductive success Survival and Fecundity 1 Survival does not guarantee reproductive success 2 Organisms must be fecund fertile producing viable offspring Must survive and be fecund to be fit Mean fitness of population The average reproductive success Continuous Variation greater contribution well adapted high fitness less contribution not well adapted low fitness of individuals of members of a population 0 X 50 Measured trait 100 Natural Selection Types 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 Basic conditions 1 environment remains stable over time 2 organisms have obtained a high state of adaptiveness Natural Selection 0 X 50 Measured trait X time long of individuals of individuals Stabilizing Selection 0 100 50 Measured trait 100 Individuals that are well adapted cluster around the mean Ex Human birth weight of individuals low or high weight leads to high mortality X 6 6 lbs 8 8 lbs Measured trait Natural Selection 2 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 adaptive Basic conditions 1 Environmental and or biological conditions are changing over time 2 Species can not be completely adapted to the changing conditions Natural Selection 0 X time long 50 Measured trait 100 X of individuals of individuals Directional Selection 50 75 Measured trait Individuals at one end of the range for some phenotypic character become more common than intermediate forms Ex European Brown Bear 125 Natural Selection Directional Selection Ex Peppered Moth Biston betularia morphology ranges from light gray to nearly black behavior coloration wing pattern help camouflage moths from birds 1 Pre industrial England Light 2 Industrial revolution 1850 Light 3 Clean air act 1952 1954 Light Dark Dark Dark Natural Selection Directional Selection Peppered Moth Biston betularia Experiment Kettlewell 1950 s used mark release recapture method released light and dark moths in polluted and unpolluted areas Results 1 more dark moths were caught recaptured in polluted areas 2 more light moths were caught recaptured in unpolluted areas Conclusions 1 Birds captured more light moths around polluted areas more dark moths around unpolluted areas 2 Directional selection was operating Natural Selection Directional Selection Pesticide Resistance Chemical pesticide in agriculture has resulted in directional selection If resistance is heritable it becomes more common in next generation Chemicals are agents of selection favoring the most resistant for Today 450 species of pest are resistant to one or more pesticides Pest Resurgence pesticides also kill natural predators of the pests 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 Natural Selection 3 Disruptive Selection Phenotypic characters at opposite extremes of character distribution are favored and intermediate forms are selected against Basic conditions 1 Fitness values for a particular genotype are higher in one environment and lower in a different environment 2 Occurs in diverse and shifting environments Natural Selection 0 X 50 Measured trait time short X of individuals of individuals Disruptive Selection 100 0 X 50 Measured trait Positive selection tends to effect the extremes not the mean The number of organisms is often reduced Ex black bellied seedcracker finches Pyrenestes ostrinus Display two different beak sizes 1 small billed finches feed on soft seeds 2 large billed finches feed on large seeds 100 Natural Selection Disruptive Selection black bellied seedcracker finches If the bill pattern is unrelated to gender or geography what causes it Causes 1 Seed cracking ability directly affects survival 2 Disruptive selection may be eliminating birds with intermediate sized bills 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 seeds During the dry season soft seeds small bill competitive disadvantage mortality During the wet season hard seeds large bill competitive disadvantage mortality Birds of intermediates sized bills are inefficient at cracking both types of seeds Have lower relative fitness than small or large billed birds Natural Selection If natural selection is producing organisms best fit to their environment can it engineer perfect organisms NO 1 Selection can


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