Evolution of phenotypic traits

Transcription

1 Quantitative genetics Evolution of phenotypic traits Very few phenotypic traits are controlled by one locus, as in our previous discussion of genetics and evolution Quantitative genetics considers characters that are controlled by more than one locus Chapter 13 Genetic architecture Refers to the genetic basis of a trait and its relationship to other traits Number of loci Allelic effects Additive effects Dominance at loci Epistasis Pleiotropic effects Multilocus systems Definitions In multilocus systems, a phenotypic trait is influenced by more than one locus Each locus might have alternative alleles Diverse alleles and loci produce phenotypic polymorphism Different loci can be on one chromosome ( linked ) or scattered Quantitative genetics Most phenotypes lack clear categories The study of traits affected by many genes and the environment Polygenic traits show continuous variation 1

2 Living histogram Living histogram What are quantitative traits (QT)? Evolution occurs in quantitative traits Continuous variation (lack of discrete categories) Influenced by: genotype at many different loci environmental (developmental) influences gene-environment interactions Examples Behavioral Physiological Morphological Evolution in behavior Critical photoperiod for entering diapause has changed in introduced populations Evolution in physiology Metal tolerant plants Cost of tolerance detected

3 Evolution in morphology Evolution caused by harvesting Soapberry bugs How are genes influencing quantitative traits identified? Juenger et al., 005. Quantitative trait loci mapping of floral and leaf morphology traits in Arabidopsis thaliana.... Evolution & Development 7: QTL mapping More complex than on-off or single-gene approach Measures association between variation in a phenotype and a genetic marker Reveals Number of loci influencing a quantitative trait Magnitude of their effects on the quantitative trait Location in the genome Genomic positions of QTL for floral and leaf characters Juenger et al. 005 General conclusions about QTL Variation may be influenced by dozens of loci 53 QTL for Drosophila bristle number Loci vary in intensity of effect Diverse genetic mechanisms for affecting variation Additive effects Epistatic effects Gene-environment interactions 3

4 Frequency 11/3/014 Variation in phenotype is due to many factors Phenotype (trait) V P. = V G. + V E. + V G *V E Partitioning variation by influence Phenotypic variation (V P ) Variation associated with genes (V G ) Variation associated with additive genetic variability (V A ) Variation associated with non-additive genetic variability (V NA ) V NA is composed of dominance effects (V D ), and interaction between genes (V I ) Variation associated with environmental variability (V E ) Variation associated with gene-environment interactions (V GE ) For one locus, Midpoint between two homozygotes as a point of reference Mean phenotype of A 1 A 1 individuals is A+a, and mean phenotype of A A individuals is A a. a is the additive effect of an allele If the inheritance of the phenotype is completely additive, then the heterozygote is exactly in between that of the two homozygotes Additive genetic variance Component of genetic variance denoted V A V A depends on magnitude of a and genotype frequencies Variance is lower if one genotype is most common At one locus, V A = pqa Phenotypic variance within populations is the environmental variance, V E 4

5 At several loci, Additive genetic variance Average phenotype of any particular genotype is the sum of the phenotypic values of each loci V A is the sum of the additive genetic variance contributed by each of the loci The additive effects of alleles are responsible for the degree of similarity between parents and offspring Therefore, are the basis for response to selection within populations Expected average phenotype of a brood of offspring equals the average of their parents phenotypes Additive genetic variance allows a response to selection Broad-sense heritability The fraction of phenotypic variability attributable to non-environmental effects More-or-less genetic vs. environmental effects Broad sense heritability = H = V G /V P = V G /(V G + V E ) Variation due to dominance and interactions not truly heritable H is seldom used, heritability is usually heritability in the narrow sense (h ) Heritability in the narrow sense Explicitly specifies additive, genetic influences Heritability is determined by the additive genetic variance (depends on allele frequencies) and environmental variance (depends on environment) h N = V A /V P = V A /(V G + V E ) = V A /(V A + V NA + V E ) h N h N is the slope of the regression of offspring phenotypes on the average of the parents of each brood of offspring How can one separate V A and V E? Problem: offspring can resemble parents through sharing environments Transplant experiments are the classic approach to sorting out different influences 5

6 Regression analysis approach Controlling for environmental influences Can we quantify the response of QT to selection? Selection differential: the difference in average phenotype between the general population and the surviving subset Selection gradient: the shift in relative fitness for the starting population and surviving subset Selection experiments Only particular individuals allowed to breed Difference between mean phenotype of population and mean of selected group = selection differential, S Selection experiments Selection differential vs response: high h N The change in offspring phenotype between selected group and unselected population is the response to selection, R 6

7 Selection differential vs response: low h N Selection differential vs. response: high h N and high S Response to Selection The response to selection will depend on: The selection differential (S) The heritability of the trait (h ) R = h N S Directional selection Mean of a polygenic character shifts beyond the original range of variation Due to favorable gene combinations that effectively did not exist Bristle number original mean was 9 Natural selection in natural populations Intensity of selection i = z a z b sqrt (V P ) Selection gradient slope (b) of the relationship between phenotype values and fitness of these values If selection is stabilizing or diversifying, the change in phenotypic variance is important j = V A V B /V B 7

8 Directional selection: Darwin s finches Grant s study Disruptive selection Disruptive selection Curvilinear fitness function, lowest in center, highest at extremes Broadens distribution, does not change mean Example: African finches Artificial selection on wing venation in Drosophila Stabilizing selection Stabilizing selection Curvilinear fitness function, highest in center, lowest at extremes Narrows distribution, does not change mean 8

9 Gallflies How is variability maintained in populations, given that selection removes genotypes? Frequency-dependent selection Varying selection Gene flow (variation within population) New mutations (mutation-selection balance) Linkage disequilibrium Correlated characters Correlated selection Genetic correlation Correlation between two characters Phenotypic correlation body size and fecundity Genetic correlation Linkage disequilibrium Pleiotropy Effect of environment Genetic correlation is

10 Evolution and genetic correlation Genetic correlation is 0.89 Can selection act on characters that respond to different environments? Selection against eating leeches greater than selection for eating leaches Selection for eating slugs is greater than selection against eating leeches What are the questions? Can flexibility be an adaptation? What is phenotypic plasticity? How does phenotypic plasticity contrast with canalization? What aspects of phenotypic plasticity are heritable? How can we test the hypothesis of adaptive plasticity? Phenotypic plasticity The capacity of an organism to develop any of several phenotypic states, dependent on environment Antonym: canalization A developmental process that produces the same phenotype in spite of environmental variability (= homeostasis) Reduces the effect of environmental noise Variability in a trait in a population represents a balance between plasticity and canalization Phenotypic plasticity example Behavioral change due to changed stimuli in Daphnia Morphological difference between clones grown in different environments 10

11 Phenotypic plasticity Phenotypic plasticity Morphological variation within an organism with continuous development Measuring phenotypic plasticity Norms of Reaction Norm of Reaction Different phenotypes produced by the same genotype in different environments X- axis represents range of environmental conditions Y-axis represents the resulting phenotype Is plasticity adaptive? Light availability and elongation in Impatiens Polyphenism in tadpoles adaptive? Relyea, R. A The heritability of inducible defenses in tadpoles. J. Evol. Biol. 18:

12 Morphological measurements Tadpole norms of reaction Heritability of plasticity 1