Lecture22_slides_19Oct11 - 10/20/2011 1 THE EVOLUTION OF...

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Unformatted text preview: 10/20/2011 1 THE EVOLUTION OF QUANTITATIVE TRAITS Most observable traits are not discrete (meaning they have few, binary states), they are continuous or quantitative (meaning they take on a range of values). Quantitative Traits opulation ction Purifying (stabilizing) selection Orange arrows indicate phenotypes disfavored by selection. phenotypic value phenotypic value frequency in p before sel e opulation tion Purifying (stabilizing) selection phenotypic value frequency in p after sele c opulation ction Directional selection Orange arrows indicate phenotypes disfavored by selection. phenotypic value frequency in p before sel e opulation ction Directional selection Orange arrows indicate phenotypes disfavored by selection. phenotypic value frequency in p during sel e 10/20/2011 2 opulation ction Directional selection phenotypic value frequency in p after sele c Most observable traits are not discrete (meaning they have few, binary states), they are continuous or quantitative (meaning they take on a range of values). Quantitative Traits Most evolutionarily significant traits are determined by the contributions of many independent genes (they are polygenic traits). Traits are also influenced by the environment. Imagine a hypothetical trait determined by a single gene. where phenotypically BB > Bb > bb. 0.5 BB Bb bb If freq(B) = 0.5 and the freq(b) = 0.5, then: 0.25 frequency phenotype value cy BB 0.25 Now add a little environmental variation that might either increase or decrease the phenotype of individuals. Now each genotype takes on a phenotypic distribution. 0.25 frequenc Bb bb 0.25 phenotype 0.5 0.25 equency Combine the genetic and environmental influence, and our population has a more continuous phenotypic distribution. fre phenotype The same thing happens if our trait is controlled by combined effects of multiple independent genes. Multiple genes + Environment give the classic bell shaped curve Continuous variation for human height (Figure 9.1 in the text) 10/20/2011 3 Components of Phenotypic Variance # of individuals phenotype phenotype V P = V G + V E V P = Phenotypic variance V G = Genetic variance...
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Lecture22_slides_19Oct11 - 10/20/2011 1 THE EVOLUTION OF...

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