Lecture V - Selection and Evolution

Lecture V - Selection and Evolution - Lecture V Selection...

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Lecture V – Selection and Evolution
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Selection and Darwin Although Darwin's theory of evolution used natural selection as it’s major mechanism In fact, natural selection was the central innovation of Darwin's theory
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Natural Selection Let us reiterate: what is Natural Selection? First step – all organisms produce more offspring than can survive Second step – all organisms show variation in phenotypes Third step – those variations that increase the reproductive success of their bearers will be more highly represented in the next generation
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Selection and Variation Natural selection works on phenotype only For natural selection to exist, variation must exist in the phenotype However, for selection to affect the patterns of variation in subsequent generations, the traits affected by selection must be heritable In other words, for selection to lead to evolution, it has to work on genetically based traits
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Selection, Phenotype, and genotype So, evolutionary processes work via indirect interactions between Genotype Phenotype Environment Genotype Phenotype Environment Reproduction Translation
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Selection and Evolution For selection to be meaningful, evolutionary-wise, selection has to be for traits that have genetic based variation The genetic basis of variation of a trait is seen in it’s hereditability (h 2 ), in cases that this is a meaningful estimate (no epistasis) Non genetic based variability (for example, phenotypic variability based an food availability) will not result in evolutionary processes
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Types of Selection So, does all selection cause evolution? There are, in general three type of selection on phenotypic traits: 1. Directional 2. Stabilizing 3. Disruptive
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Directional Selection Directional Selection occurs when a certain discrete phenotype, or an extreme of a continuous trait, has a reproductive advantage over others This is the “classical” selection type that creates evolution http://www.emc.maricopa.edu/faculty/farabee/BIOBK/directselect.gif
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Stabilizing Selection Stabilizing selection occurs when an intermediate phenotype, either based on heterozygotes, or the median of a continuous phenotype in favored This slows evolutionary processes, by stabilizing the phenotype and reducing long-term change http://www.emc.maricopa.edu/faculty/farabee/BIOBK/stabsel_1.gif
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Disruptive Selection Disruptive Selection occurs when selection favors either extreme of a continuous trait, or homozygotes in discrete traits This creates multiple possible evolutionary results, from random fixation of a character, to isolation of separated populations and speciation http://www.emc.maricopa.edu/faculty/farabee/BIOBK/disrupsel.gif
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Giraffes famous for their long necks. Classical explanation is that long necks evolved to enable giraffes to reach higher browse. Long neck is an adaptation: a trait or set of
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This note was uploaded on 01/23/2010 for the course BIOS 230 taught by Professor Gibbons during the Fall '08 term at Ill. Chicago.

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Lecture V - Selection and Evolution - Lecture V Selection...

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