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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 Blending inheritance: like mixing paint Lets say the population is yellow, when a favored blue variant arises Favored blue variant mates with yellow to make a green offspring But when green mates with green or yellow, you never get the favored blue form back. Mendelian inheritance Refuted the idea of blending inheritance because white and red forms could be recovered from crosses among pink flowers. 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 Darwins ideas on Natural Selection together with genetics Provided a description of how the frequencies of genes in population change due various forces Dealt with both qualitative and quantitative traits 2 Some giants of the neo- Darwinian synthesis T. Dobzhansky Sewell Wright R.A. Fisher Qualitative (Discrete) traits Can be classified on an either-or basis Eye color, blood type, flower color in Mendels 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 in plants Variation due to the combined effects of many genes 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 Two genetic polymorphisms in humans Figure 14.14 A, B Ww ww ww Ww ww Ww Ww ww ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles)...
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