Chapter 9- Quantitative Genetics
Chapter 9- Quantitative Genetics •
Quantitative Genetics- the branch of evolutionary biology that provides tools for analyzing the evolution of multi-locus traits 9.1- The Nature of Quantitative Traits • Qualitative Traits- characteristics that we can assign to individuals by just looking at them, or perhaps by conducting a simple genetic test • Traits with discrete phenotypes are special examples; most traits in most organisms show continuous variation o Such as height, athletic ability, and intelligence o Also beak length in soapberry bugs and bill depth in medium ground finches • Traits with continuous variation cannot assign individuals to discrete phenotypic categories by simple inspection o Measurements must be taken o Characters with continuously distributed phenotypes are called quantitative traits Are determined by the combined influence of: • Genotype at many different loci • & The environment • Quantitative traits are traits for which the distribution of phenotypes is continuous rather than discrete • Quantitative traits are consistent with Mendelian genetics. They are influenced by the combined effects of the genotype at many loci. Quantitative traits are also influenced by the environment. 9.3- Measuring Heritable Variation • Basic tenets of Darwin’s theory of evolution by natural selection: o 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. • Quantitative genetics includes tools to measure heritable variation, tools for measuring differences in survival and/or reproductive success, and tools for predicting the evolutionary response to selection • Quantitative genetics allows us to analyze evolution by natural selection in traits controlled by many loci • The first step in a quantitative genetic analysis is to determine the extent to which the trait in question is heritable. That is, we must partition the total phenotypic variation (Vp) into a component due to genetic variation (VG) and a component due to environmental variation (VE) • Heritability- fraction of the total variation in a trait that is due to variation in genes o In the broad sense, that fraction of the total phenotypic variation in a population that is caused by genetic differences among individuals; in the narrow sense, that fraction of the total variation that is due to the additive effects of genes. • Phenotypic Variation (VP)- total variation in a trait • Genetic Variation (VG)- variation among individuals that is due to variation in their genes • Environmental Variation (VE)- variation among individuals due to variation in their environments • Broad-Sense Heritability or Degree of Genetic Determination: Heritability = VG/ VP = VG/ (VG + VE) Estimating Heritability from Parents and Offspring • If the variation among individuals is due to variation in their genes, then offspring will resemble their parents • Figure 9.13: o [9.13a] If offspring do not resemble their parents, then the slope of the best-fit line through the data will be near -; this is evidence that the variation among individuals in the population is due to variation in their environments, not variation in their genes o [9.13c] If offspring strongly resemble their parents, the slope of the best-fit line will be near 1; this is evidence that variation among individuals in the population is due to variation in their genes, not variation in their environments o [9.13b] Most traits in most populations fall somewhere in the middle, with offspring showing a moderate resemblance to their parents; this is evidence that the variation among individuals is partly due to variation in their environments and partly due to variation in their genes • Narrow-sense Heritability (h2)- that fraction of the total phenotypic variation in a population that is due to the additive effects of genes o Is an estimate of the fraction of the variation among the parents that is due to variation in their genes h2= VA/VP = VA/ (VA + VD + VE) • Heritability, h2, is a measure of the (additive) genetic variation in a trait • Additive genetic variation (VA) is variation among individuals due to the additive effects of genes • Dominance genetic variation (VD) is variation among individuals due to gene interactions such as dominance o total genetic variation is the sum of the additive and dominance genetic variation
VG = VA + VD Narrow sense heritability, h2, allows us to predict how a population will respond to selection Estimating Heritability from Twins • Monozygotic (identical) twins share their environment and all of their genes • Dizygotic (fraternal) twins share their environment and half of their genes • If heritability is high, and variation among individuals is due mostly to variation in genes, then monozygotic twins will be more similar to each other than are dizygotic twins • If heritability is low, and variation among individuals is due mostly to variation in environments, then monozygotic twins will be as different from each other as dizygotic twins •
9.4- Measuring Differences in Survival and Reproductive Success • Second tenet of Darwin’s theory: o There are differences in survival and/or reproductive success among individuals • When measuring the differences in success among individuals, it usually means measuring the strength of selection • If we can measure heritable variation and strength of selection, we will be able to predict evolutionary change in response to selection • On average, individuals with some values of a trait survive at higher rates, or produce more offspring, than individuals with other values of a trait o To measure strength of selection, one must note who survivies or reproduces and who fails to do so Then we quantify the difference between the winners and the losers in the trait of interest • The second step in a quantitative genetic analysis is to measure the strength of selection on the trait in question. One measure is the selection differential, S, equal to the difference between the mean of the selected individuals and the mean of the entire population. • A second measure of the strength of selection is the selection gradient o Read the three steps of Selection Gradient on bottom of Pg 338-339. • Selection gradient and the selection differential are closely related, and each can be converted into the other • if analyzing a single trait, then selection gradient is equal to the selection differential divided by the variance in tail length o an advantage of the selection gradient is that we can calculate it for any measure of fitness, not just survival o selection gradient can be calculated for a wider variety of fitness measures • The selection gradient, β, for trait t is equal to the selection differential, S, divided by the variance: β = S / var(t) 9.5- Predicting the Evolutionary Response to Selection • R = h2S • R= predicted response to selection; h2= heritability; S= selection differential 9.6- Modes of Selection and the Maintenance of Genetic Variation • Directional Selection- that which occurs when individual fitness tends to increase or decrease with the value of phenotypic trait; can result in steady evolutionary change in the mean value of the trait in the population o Fitness consistently increases (or decreases) with the value of a trait o Directional selection on a continuous trait changes the average value of the trait in the population o Directional selection also reduces the variation in a population, although often not dramatically • Stabilizing Selection- That which occurs when individuals with intermediate values of a trait have higher fitness; can result in reduced phenotypic variation in a population and can prevent evolution in the mean value of the trait o Individuals with intermediate values of a trait have highest fitness o Stabilizing selection on a continuous trait does not alter the average value of the trait in the population o Stabilizing selection does, however, trim off the tails of the trait’s distribution, thereby reducing the variation So the individuals that are unique gets killed off and the common individuals survive • Disruptive Selection- occurs when individuals with more extreme values of a trait have higher fitness; can result in increased phenotypic variation in a population o Individuals with extreme values of a trait have the highest fitness o Disruptive selection on a continuous trait does not alter the average value of the trait in the population o Disruptive selection does, however, trim off the top of the trait’s distribution, thereby increasing the variance So the trait containing a lot of individuals are cut down
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All three modes of selection eliminate individuals with low fitness and preserve individuals with high fitness o Therefore all three modes of selection increase the mean fitness of the population o both directional and stabilizing selection reduce the phenotypic variation present in a population if the trait in question is heritable, then these modes of selection will reduce the genetic variation in the population as well o eventually, the genetic variation in any trait related to fitness should be eliminated altogether, and the population should reach an equilibrium at which the mean value of the trait, the variation in the trait, and the mean fitness of the population will all cease to change Read Pages 349-350
Quantitative Genetics- the branch of evolutionary biology that provides tools for analyzing the evolution of multi-locus traits 9.1- The Nature of Quantitative Traits • Qualitative Traits- characteristics that we can assign to individuals by just looking at them, or perhaps by conducting a simple genetic test • Traits with discrete phenotypes are special examples; most traits in most organisms show continuous variation o Such as height, athletic ability, and intelligence o Also beak length in soapberry bugs and bill depth in medium ground finches • Traits with continuous variation cannot assign individuals to discrete phenotypic categories by simple inspection o Measurements must be taken o Characters with continuously distributed phenotypes are called quantitative traits Are determined by the combined influence of: • Genotype at many different loci • & The environment • Quantitative traits are traits for which the distribution of phenotypes is continuous rather than discrete • Quantitative traits are consistent with Mendelian genetics. They are influenced by the combined effects of the genotype at many loci. Quantitative traits are also influenced by the environment. 9.3- Measuring Heritable Variation • Basic tenets of Darwin’s theory of evolution by natural selection: o 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. • Quantitative genetics includes tools to measure heritable variation, tools for measuring differences in survival and/or reproductive success, and tools for predicting the evolutionary response to selection • Quantitative genetics allows us to analyze evolution by natural selection in traits controlled by many loci • The first step in a quantitative genetic analysis is to determine the extent to which the trait in question is heritable. That is, we must partition the total phenotypic variation (Vp) into a component due to genetic variation (VG) and a component due to environmental variation (VE) • Heritability- fraction of the total variation in a trait that is due to variation in genes o In the broad sense, that fraction of the total phenotypic variation in a population that is caused by genetic differences among individuals; in the narrow sense, that fraction of the total variation that is due to the additive effects of genes. • Phenotypic Variation (VP)- total variation in a trait • Genetic Variation (VG)- variation among individuals that is due to variation in their genes • Environmental Variation (VE)- variation among individuals due to variation in their environments • Broad-Sense Heritability or Degree of Genetic Determination: Heritability = VG/ VP = VG/ (VG + VE) Estimating Heritability from Parents and Offspring • If the variation among individuals is due to variation in their genes, then offspring will resemble their parents • Figure 9.13: o [9.13a] If offspring do not resemble their parents, then the slope of the best-fit line through the data will be near -; this is evidence that the variation among individuals in the population is due to variation in their environments, not variation in their genes o [9.13c] If offspring strongly resemble their parents, the slope of the best-fit line will be near 1; this is evidence that variation among individuals in the population is due to variation in their genes, not variation in their environments o [9.13b] Most traits in most populations fall somewhere in the middle, with offspring showing a moderate resemblance to their parents; this is evidence that the variation among individuals is partly due to variation in their environments and partly due to variation in their genes • Narrow-sense Heritability (h2)- that fraction of the total phenotypic variation in a population that is due to the additive effects of genes o Is an estimate of the fraction of the variation among the parents that is due to variation in their genes h2= VA/VP = VA/ (VA + VD + VE) • Heritability, h2, is a measure of the (additive) genetic variation in a trait • Additive genetic variation (VA) is variation among individuals due to the additive effects of genes • Dominance genetic variation (VD) is variation among individuals due to gene interactions such as dominance o total genetic variation is the sum of the additive and dominance genetic variation
VG = VA + VD Narrow sense heritability, h2, allows us to predict how a population will respond to selection Estimating Heritability from Twins • Monozygotic (identical) twins share their environment and all of their genes • Dizygotic (fraternal) twins share their environment and half of their genes • If heritability is high, and variation among individuals is due mostly to variation in genes, then monozygotic twins will be more similar to each other than are dizygotic twins • If heritability is low, and variation among individuals is due mostly to variation in environments, then monozygotic twins will be as different from each other as dizygotic twins •
9.4- Measuring Differences in Survival and Reproductive Success • Second tenet of Darwin’s theory: o There are differences in survival and/or reproductive success among individuals • When measuring the differences in success among individuals, it usually means measuring the strength of selection • If we can measure heritable variation and strength of selection, we will be able to predict evolutionary change in response to selection • On average, individuals with some values of a trait survive at higher rates, or produce more offspring, than individuals with other values of a trait o To measure strength of selection, one must note who survivies or reproduces and who fails to do so Then we quantify the difference between the winners and the losers in the trait of interest • The second step in a quantitative genetic analysis is to measure the strength of selection on the trait in question. One measure is the selection differential, S, equal to the difference between the mean of the selected individuals and the mean of the entire population. • A second measure of the strength of selection is the selection gradient o Read the three steps of Selection Gradient on bottom of Pg 338-339. • Selection gradient and the selection differential are closely related, and each can be converted into the other • if analyzing a single trait, then selection gradient is equal to the selection differential divided by the variance in tail length o an advantage of the selection gradient is that we can calculate it for any measure of fitness, not just survival o selection gradient can be calculated for a wider variety of fitness measures • The selection gradient, β, for trait t is equal to the selection differential, S, divided by the variance: β = S / var(t) 9.5- Predicting the Evolutionary Response to Selection • R = h2S • R= predicted response to selection; h2= heritability; S= selection differential 9.6- Modes of Selection and the Maintenance of Genetic Variation • Directional Selection- that which occurs when individual fitness tends to increase or decrease with the value of phenotypic trait; can result in steady evolutionary change in the mean value of the trait in the population o Fitness consistently increases (or decreases) with the value of a trait o Directional selection on a continuous trait changes the average value of the trait in the population o Directional selection also reduces the variation in a population, although often not dramatically • Stabilizing Selection- That which occurs when individuals with intermediate values of a trait have higher fitness; can result in reduced phenotypic variation in a population and can prevent evolution in the mean value of the trait o Individuals with intermediate values of a trait have highest fitness o Stabilizing selection on a continuous trait does not alter the average value of the trait in the population o Stabilizing selection does, however, trim off the tails of the trait’s distribution, thereby reducing the variation So the individuals that are unique gets killed off and the common individuals survive • Disruptive Selection- occurs when individuals with more extreme values of a trait have higher fitness; can result in increased phenotypic variation in a population o Individuals with extreme values of a trait have the highest fitness o Disruptive selection on a continuous trait does not alter the average value of the trait in the population o Disruptive selection does, however, trim off the top of the trait’s distribution, thereby increasing the variance So the trait containing a lot of individuals are cut down
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All three modes of selection eliminate individuals with low fitness and preserve individuals with high fitness o Therefore all three modes of selection increase the mean fitness of the population o both directional and stabilizing selection reduce the phenotypic variation present in a population if the trait in question is heritable, then these modes of selection will reduce the genetic variation in the population as well o eventually, the genetic variation in any trait related to fitness should be eliminated altogether, and the population should reach an equilibrium at which the mean value of the trait, the variation in the trait, and the mean fitness of the population will all cease to change Read Pages 349-350
