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Unformatted text preview: 1 Evolution II: Mechanisms Outline • Introduction – The Neo-Darwinian synthesis • Sources of Variation – Mutation – Sexual recombination • Crossing over • Independent assortment • Hardy Weinberg equilibrium – Selection – Drift – Gene Flow – Assortative mating • Maintenance of genetic variation Darwin • Did not read Mendel • Thought of inheritance as a blending of traits of parents • Blending would tend to eliminate differences among individuals, and dilute advantageous traits • Breeding experiments showed that variation could be preserved across generations and selected traits could be enhanced • This refuted the blending hypothesis but there was nothing to replace it 2 • Mendelian inheritance – Refuted the idea of blending inheritance because white and red forms could be recovered from crosses among pink flowers. – Particulate inheritance Figure 23.4 Generation 1 CRCR genotype CWCW genotype Plants mate Al CRCW (al pink flowers) 50% CR gametes 50% CW gametes Come together at random Generation 2 Generation 3 Generation 4 25% CRCR 50% CRCW 25% CWCW 50% CR gametes 50% CW gametes Come together at random 25% CRCR 50% CRCW 25% CWCW Al eles segregate, and subsequent generations also have three types of flowers in the same proportions Neo-Darwinian Synthesis • First half of twentieth century • Brought Darwin’s ideas on Natural Selection together with genetics • Provided a description of how the frequencies of genes in population change due to various forces • Dealt with both qualitative and quantitative traits • Qualitative (Discrete) traits – Can be classified on an either-or basis – Eye color, blood type, flower color in Mendel’s peas (white, pink, red) – Variation usually due to one or two loci • Quantitative traits – Vary along a continuum within a population – Height or weight of individuals, bill size in birds, seed weight – Variation due to the combined effects of many genes 3 • Flower color in this example is a qualitative trait with three phenotypic classes: red, pink, and white. Figure 23.4 Generation 1 CRCR genotype CWCW genotype Plants mate Al CRCW (al pink flowers) 50% CR gametes 50% CW gametes Come together at random Generation 2 Generation 3 Generation 4 25% CRCR 50% CRCW 25% CWCW 50% CR gametes 50% CW gametes Come together at random 25% CRCR 50% CRCW 25% CWCW Al eles segregate, and subsequent generations also have three types of flowers in the same proportions × AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCcAABBCC 20/64 15/64 6/64 1/64 F r a c tio n o f p r o g e n y • Quantitative variation usually indicates polygenic inheritance Figure 14.12 A simple model for skin color Evolution occurs in populations • Individual organisms do not evolve in the Darwinian sense • Natural selection acts on genes or individuals, but populations evolve • Evolution is the change in gene frequencies in populations through time 4 Evolution requires variation • The ultimate source of variation is mutation.• The ultimate source of variation is mutation....
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This note was uploaded on 12/01/2010 for the course BIO BILD 3 taught by Professor Christopherwills during the Fall '10 term at UCSD.

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page - 1 Evolution II: Mechanisms Outline • Introduction...

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