Chapter 7 Beyond alleles: Quantitative Genetics Oldfield Mouse Beyond alleles: Quantitative Genetics Evolution of Phenotypes inland dunes

Continuous traits have a complex genetic basis Polygenic trait: influenced by many genetic loci Human height, skin color Quantitative genetics: study of the genetic mechanisms of continuous phenotypic traits Start with phenotypic distributions in population; then look at how selection and other forces can cause frequencies to change Population geneticists start with alleles at genetic loci and build the genotype and then the phenotype

Continuous traits have a complex genetic basis Polygenic trait: influenced by many genetic loci Interaction between alleles (epistasis) Interaction with environment (phenotypic plasticity) Quantitative genetics: study of the genetic mechanisms of continuous phenotypic traits

Hardy-Weinberg extended to polygenic traits Loci Genotypes Phenotypes 1 2 3 3 9 27 3 5 7

Variance Mean = Variance = How widely dispersed trait values are from the mean. Mean = Variance = n n The more variation there is in a trait – the larger the variance for trait

Standard Deviation

Components of phenotypic variation VP = VG + VE Total phenotypic variance in population Variance due to genetic differences Variance due to environmental differences

Genetic and environmental influences create continuous distribution

Complex Phenotypic Traits Vary continuously within population Yields normal distribution of trait values around the population mean Figure shows three hypothetical populations Each has a mean phenotypic value of 0 and variances range from 1 to 3 As the variance of a sample increases, more and more individuals have trait values located far way from the mean

Complex traits vary continuously Frequency Distance from mean

Broad Sense Heritability Proportion of phenotypic variance explained by genetic differences among individuals Measures relative importance of genetic and environmental effects on trait expression

Problem Broad sense heritability represents all genetic variance as a single value In sexually reproducing organisms, not all genetic effects are transmitted to offspring Some is lost during meiosis when each pair of chromosomes separates, associations between alleles breaks down So - only some genetic variation actually contributes to the phenotypic resemblance between offspring and their parents. Only this portion of the variance enables a population to evolve in response to selection.

Broad Sense Heritability Combining VG and VE with only two loci with two alleles each creates a continuous phenotypic distribution Bars show the five phenotypic trait values produced from additive combinations of alleles at two loci = VG See figure 7.4 Small normal curve over each bar represents the distribution of phenotypes produced by each genotype caused by the environment Small distributions overlap and the result approximates a smooth normal curve (blue line) Combination of genetic and environmental effects yields a continuous distribution of phenotypes

Combining VG and VE with only two loci with two alleles each creates a continuous phenotypic distribution Combining VG and VE with only two loci with two alleles each creates a continuous phenotypic distribution Bars show the five phenotypic trait values produced from additive combinations of alleles at two loci = VG See figure 7.4 Small normal curve over each bar represents the distribution of phenotypes produced by each genotype caused by the environment Small distributions overlap and the result approximates a smooth normal curve (blue line) Combination of genetic and environmental effects yields a continuous distribution of phenotypes Small curves represent distribution of phenotypes produced by each genotype caused by the environment

Narrow sense heritability Proportion of phenotypic variance explained by additive genetic variation Causes offspring to resemble parents h2 = VA / VP = VA / VA + VD + VI + VE Additive Dominance Epistasis

Narrow Sense Heritability Break down VG into smaller components Additive effects Dominance effects Epistatic effects Effect of an allele at one locus depends on which allele is present at another locus Break VG into three sources of genetic variation (I = epistatic) Labrador color VG = VA + VD + VI

Phenotypic Variation VP = VG + VE VP = VA + VD + VI + VE

Narrow Sense Heritability VA is especially important = Proportion of phenotypic variance explained by additive genetic variation- have more genes in common Causes relatives to resemble each other Epistasis Additive Dominance

Narrow Sense Heritability Why doesn’t H2 include Dominance or Epistasis? Traits must be heritable for them to respond to selection Heritability is the genetic component of phenotypic variance Additive effects of alleles cause relatives to resemble each other, contribute to evolutionary response to selection Dominance and epistatic effects of alleles are the interactions among alleles Effect of an allele on the phenotype depends on what it is paired with - context dependent Context breaks down each generation with meiosis

Narrow Sense Heritability Estimate h2 for body mass of fish Randomly pair fish and allow them to breed Weigh parents and offspring The more similar the offspring, the greater the Narrow Sense Heritability Large fish produce large offspring Plot offspring body size against average body size of parents Slope of regression will yield quantitative estimate of h2 for the trait

Narrow Sense Heritability Mean Parental Body Mass Mean Offspring Body Mass Slope = h2

Estimating heritability Slope = h2

Narrow Sense Heritability In asexual plants, animals, protists and bacteria No meiosis, so no loss of context for an allele Epistatic interactions between alleles have an important effect on progeny phenotypes In highly inbred individuals, so many alleles are homozygous and identical by descent that interactions among alleles in offspring are likely to be the same as they were in the parents. Use Broad Dense Heritability H2 to predict the response of the population to selection

Key Concepts When components of variation are additive, genetic and environmental variance sum to total phenotypic variance Heritability is the proportion of phenotypic variance due to genetic differences Narrow sense heritability includes: Additive effects Dominance effects Epistatic effects Maternal/Paternal environmental effects

Modes of selection

Evolutionary Response to Selection 2 ways to calculate Population geneticists – measure the strength of selection as the selection coefficient s The amount, s by which the fitness of a genotype is reduced relative to the most fit genotype in the population Quantitative geneticists measure selection for a trait as the difference in the mean of a trait of reproducing individuals and the mean of the trait for the general population (the selection differential, S)

Evolutionary Response to Selection Selection without evolution If differences in a trait is due solely to the environment, h2 = 0 Offspring will not resemble the parents If all the differences in a trait are due solely to genetics (allele differences among individuals) h2 =1 Offspring track the parents regardless of environmental changes Selection for body size means offspring in next generation will be larger. Strength of selection determines evolutionary response

Selection differential measures the strength of selection

Calculating the evolutionary response to selection The Breeder’s Equation R = h2 x S h2 x S reflects pheotypic variation that influences fitness (S), and the ability to transmit those phenotypic characteristics to offspring (h2) As long as there is some selection. It can lead to a significant evolutionary change These are exactly the points Darwin recognized as necessary for evolution in response to selection

Cumulative effects of directional selection can be large

Disruptive selection

Evolutionary response to selection How much the population changes depends on: Selection differential (S) Heritability

High heritability results in larger change

Key Concepts Selection on quantitative traits can take several different forms Directional Stabilizing Disruptive Evolution and selection are not the same Selection can occur without evolution The magnitude of change depends on: Strength of selection (selection differential) Heritability

Quantitative trait locus (QTL)analysis links traits with genes

QTL analysis of coat color in mice

QTL analysis of coat color in mice

Much of variation in coat color explained by differences in two genes Corin also explains a small amount of variation

Expression of Agouti during development influences coat color

Genetic manipulation of dark mice makes them lighter

Genome-wide association mapping of natural populations groups individuals by phenotype

Key Concepts QTL mapping identifies regions of the genome associated with phenotypic variation First step toward elucidating genes responsible for phenotypic evolution

Environmental influences on quantitative traits VP = VG + VE Total phenotypic variance in population Variance due to genetic differences Variance due to environmental differences

Phenotypic plasticity A single genotype produces different phenotypes depending on the environment

Reaction norm can predict response to environment

All genotypes may not respond to the environment in the same way Genotype x environment interaction

Phenotypic plasticity in Caenorhabditis elegans

Plasticity can evolve

Rapid change can lead to mismatch between plastic traits and environment

Key Concepts Differences in phenotypic plasticity may be heritable and can therefore evolve