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Lecture 4 - Outline Introduction The Neo-Darwinian s...

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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 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 C R C R genotype C W C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes Come together at random 25% C R C R 50% C R C W 25% C W C W Alleles 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
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2 Flower color in this example is a qualitative trait with three phenotypic classes: red, pink, and white . Figure 23.4 Generation 1 C R C R genotype C W C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes Come together at random 25% C R C R 50% C R C W 25% C W C W Alleles 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 Fraction of progeny 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 Evolution requires variation The ultimate source of variation is mutation.
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