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BIOL 3350, week 8, ch. 9

by: Morgan

BIOL 3350, week 8, ch. 9 3350

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These notes include in class lecture material as well as major vocabulary and concepts from the textbook.
Evolutionary Biology
Dr. Michael Sears
Class Notes
quantitative, Genetics
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This 5 page Class Notes was uploaded by Morgan on Sunday April 17, 2016. The Class Notes belongs to 3350 at Clemson University taught by Dr. Michael Sears in Spring 2016. Since its upload, it has received 5 views.


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Date Created: 04/17/16
Chapter 9 9.1 Nature of Quantitative Traits  Qualitative traits- traits whose phenotypes come in discrete categories that can be assigned just by looking at them or by using a simple genetic test  Quantitative traits- traits whose phenotypes are continuously distributed o Determined by the genotype at many different loci, and the environment o Ex. Height, athletic ability, intelligence  Quantitative traits are consistent with Mendelian genetics. o Influenced by the combined effects of the genotyp at many loci o Confirmed by Edward East’s data from experiments with longflower tobacco  Quantitative traits are also influenced by the environment- small differences in environment produce small differences in phenotype o Ex. Genetically identical yarrow plants grown at different elevations caused variation in plant heights 9.2 Identifying Loci That Contribute to Quantitative Traits  QTL (Quantitative Trait Loci)- portions of the genome that influence quantitative traits; may contain one or more genes  QTL mapping- collective name for suite of related techniques that employ marker loci to scan chromosomes and identify regions containing genes that contribute to a quantitative trait o Can detect the presence and location of loci influencing a quantitative trait by crossing parents from populations with fixed differences. Among the grandoffspring, we look for associations between phenotype and genotype at marker loci  Marker locus- known site in the genome where the nucleotide sequence varies among chromosomes and where a simple genetic test will identify different alleles  Adaptive evolution often involves the selective fixation of alleles with large phenotypic effects  The logic of QTL mapping- (Figure 9.7, pg 337)  By looking for associations with multiple marker loci, reasearchers can estimate the number of QTLs, their locations, and the strength of the influence on phenotype  Results from QTL mapping studies are often summarized with plots of LOD score  To detect a QTL and find its location, we look for an association between a marker locus genotype and phenotype*  Ex. Monkeyflowers- M. lewisii and M. cardinalis  Candidate Loci- to learn the identity of a QTL and the protein it encodes, we have to look for associations between a coding- locus genotype, the structure and function of the locus’s gene product, and phenotype 9.3 Measuring Heritable Variation  Recall Darwin’s theory of evolution by natural selection: if there is heritable variation among the individuals in a population, and if there are differences in survival and/or reproductive success among the variants, then the population will evolve o Quantitative genetics allows us to analyze evolution by natural selection in traits controlled by many loci  The first step in a quantitative genetics analysis is to determine the extent to which the trait in question is heritable. That is, we must partition the total phenotypic variation (V ) iPto a component attributable to genetic variation (V ) anG a component attributable to environmental variation (V ) E  Heritability- fraction of the total variation in a trait that is due to variation in genes, always a number between 0 and 1  Phenotypic variation- total variation in a trait  Broad sense heritability- degree of genetic determination o = V /G = P /(VG+ VG) E Estimating Heritability from Parents and Offspring  If the variation among individuals is due to variation in their genes, then offspring will resemble their parents o To check if they do, make a scatterplot with offspring trait values on vertical axis (Y) and their parents’ trait values on the horizontal axis (X)  Two parents for every offspring so use an average = midparent value  If more than one offspring per family use an average = midoffspring value  Draw the best-fit line through the points  Narrow-sense heritability- slope of best fit line is an estimate of the heritability (h ); allows us to predict how a population will respond to selection 2 o h = V /VA= P /(V A V A V ) D E o Offspring do not resemble parents- slope near 0 o Offspring strongly resemble parents- slope near 1  Total genetic variation- sum of the additive and dominance genetic variation o V =GV + VA D o Additive genetic variation (V )-Adue to additive effects of genes o Dominance genetic variation (V )- dDe to gene interactions such as dominance Estimating Heritability from Twins  Monozygotic twins develop from a single zygote and thus share all their genes. Dizygotic twins develop from separate zygotes and share half their genes. If the heritability of a trait is high, monozygotic twins will resemble each other more strongly than dizygotic twins 9.4 Measuring Differences in Survival and Reproductive Success  The second step in quantitative genetic analysis is to measure the strength of selection on the trait in question  Selection differential (S)= difference between the mean of the selected individuals and the mean of the entire population  Selection gradient- slope of the regression of relative fitness on phenotype o To calculate-  Assign absolute fitness to individuals in the population  Convert the absolute fitnesses to relative fitnesses  Make a scatterplot of relative fitness as a function of the trait, and calculate the slope of the regression line. The slope of this best fit line is the selection gradient o Can be calculated for any measure of fitness, not just survival o If analyzing selection on a single trait- selection gradient= selection differential divided by the variance 9.5 Predicting the Evolutionary Resp2nse to Selection  Once we know heritability (h ) and the selection differential (S), we can use them to predict the evolutionary response to selection as 2 o R = h S o Ex. Alpine skypilots flower size and bumblebees; thoroughbred race horses  Selection in nature often acts on several traits at once o Traits may be genetically correlated because the same genes influence both traits, or because selection in the past has favored particular combinations of alleles  Selection on one can drag the other along for the ride o To look at both characteristics at once- measure the strength of selection as the slope of the regression plane relating fitness to both characteristics  Slope is 2-D selection gradient  Can follow evolution of a population by tracking the position of the average individual  Population is expected to evolve so as to move up the steepest slope from present location 9.6 Modes of Selection and the Maintenance of Genetic Variation  Directional and stabilizing selection tend to reduce the amount of variation in a population and are more common; disruptive selection tends to increase the amount of variation and is more rare  Directional selection o Consistently increases (or decreases) with the value of a trait o On continuous trait- changes average value of trait in population o Reduces variation in population  Stabilizing selection o Individuals with intermediate values of a trait have highest fitness o On continuous trait- does not alter the average value of the trait in the population o Reduces variation by reducing the number of individuals in the tails of the trait’s distribution  Disruptive slection o Individuals with extreme values of a trait have highest fitness o On continuous trait- does not alter the average value of the trait o Increases variation by trimming off top of traits distribution  All three modes of selection cull individuals with low fitness and preserve individuals with high fitness o All increase mean fitness of population  Hypotheses of how variation is maintained* (pg 359) 9.7 The Bell-Curve Fallacy and Other Misinterpretations of Heritability  Any estimate of heritability is specific to a particular population living in a particular environment o Heritability tells us nothing about the causes of differences between populations living in different environments  Only way to determine the cause of differences between populations is to rear individuals from each of the populations in identical environments  Why measure heritability? o If heritability is > 0 – selecting on the trait will cause a population to evolve o If heritability is < 1 – altering the environment can shift a populations trait distribution


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