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Biol 418: Evolutionary Ecology Variation Overview Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation Variation and its response to selection • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow 1 Biol 418: Evolutionary Ecology Variation Linking variation, natural selection & evolution •Natural selection requires variation in a trait that shows fitness differences. It is a process (change in phenotypic distribution within a generation) •Evolution is the outcome (change in phenotypic distribution across generations) •Inheritance is the mechanism that transmits the effects of selection across generations, causing evolutionary change Variation is the raw material of evolution no natural selection without phenotypic variation no evolution without genetic variation Mean Zbefore Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature variation among individuals in some attribute or trait: variation S = mean Zafter – mean Zbefore the trait frequency Mean Zafter distribution will differ among age classes Mean Zkids the trait distribution of 2 R=h S all offspring will be different from that of all parents Jennifer Gow 2 Biol 418: Evolutionary Ecology Variation Linking variation, natural selection & evolution Biol 418: Evolutionary Ecology Variation The breeder’s equation: the response to selection = transmission of withingeneration changes to the next generation Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow R=h2S The paradox of variation Directional selection heritability =proportion of varn that is genetic = 0 if no genetic variability selection differential = 0 if no phenotypic variation i.e. selection + genetic variance should lead to evolution of a phenotypic trait Trait mean changes towards an extreme and variance decreases. Brodie et al. (1995) TREE 10: 313-318. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Stabilising selection Trait mean does not changes but the variance decreases. Traits closely related to fitness should be less variable 3 Biol 418: Evolutionary Ecology Variation The paradox of variation Biol 418: Evolutionary Ecology Variation The paradox of variation Fisher’s fundamental theorem: Frequency ‘The rate of increase in fitness of any organism Brodie et al. (1995) TREE 10: 313-318. S Bill size S Bill size Jennifer Gow The stronger selection, the greater the reduction in variance Frequency Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature at any time is equal to its genetic variance in fitness at that time’ (Fisher) Fisher (1958) The Genetical Theory of Natural Selection. 2nd edn. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature In population at equilibrium rate of fitness increase = 0 => genetic variance in fitness = 0 => genetic variance in traits related to fitness = 0 Jennifer Gow 4 Biol 418: Evolutionary Ecology Variation Types of variation: polymorphisms Biol 418: Evolutionary Ecology Variation Types of variation: polymorphisms Genetic polymorphism Black-bellied seedcracker, Pyrenestes ostrinus Skúlason & Smith (1995) TREE 10:366-370. Smith (1993) Nature 363: 618-620. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Jennifer Gow 5 Biol 418: Evolutionary Ecology Variation Types of variation: polymorphisms Biol 418: Evolutionary Ecology Variation Types of variation: polymorphisms Non-genetic polymorphism Atypical morph: bent Skúlason & Smith (1995) TREE 10:366-370. Lively (1986) Ecology 67:858-864. Lively (1986) Evolution 40:232-242. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Jennifer Gow Typical morph: conic Acorn barnacle, Chthamalus anisopoma Acanthina angelica • Environmental-inducement of bent by presence of predators • Bent confers protection from predator • Bent is less fecund and grows slower • Bent morph is only advantageous in presence of predators but predation is patchy 6 Biol 418: Evolutionary Ecology Variation Types of variation: continuous variation Biol 418: Evolutionary Ecology Variation Types of variation: continuous variation The genetic basis of continuous variation Frequency X = mean σ •Most continuous traits have at least a partly genetic basis σ = standard deviation Coefficient of variation, CV = σ •Number of genes affecting traits can be large X •Traits are ‘polygenic’ cf ‘monogenic’ or ‘oligogenic’ (i.e. the standard deviation scaled by the mean) e.g. Body size: outcome of complex developmental, biochemical and X Quantitative trait Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow •CV allows comparison of variability of traits of very different sizes •Organisms showing determinate growth show restricted CV: •Birds have a CV of 2-4% for most traits •Microtus voles have a CV of about 10 % for body weight •CVs may be larger for organisms with indeterminate growth (e.g. fish) Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature physiological processes: expect that many genes are involved. •How can we analyse such traits from a genetic perspective? •Could try and identify all genes that underlie variation •One way is called ‘Quantitative Trait Locus’ or QTL analysis Jennifer Gow 7 Biol 418: Evolutionary Ecology Variation Types of variation: continuous variation QTL analysis Biol 418: Evolutionary Ecology Variation Types of variation: continuous variation QTL analysis X 1. Choose species & identify genetic markers e.g. RAPDs, msats etc 2. Cross contrasting populations 3. Genotype progeny Schön et al (2004) Genetics 167: 485-498. QTL for size Linkage group 1 Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow marker 1 Linkage group 2 Linkage group 1 marker 8 marker 1 marker 2 marker 3 Linkage group 2 marker 8 marker 2 marker 3 marker 13 marker 13 marker 7 4. Construct linkage map 5. Raise progeny & score phenotype marker 7 6. Compare phenotype & genotype Sample size affects QTL detection power, the proportion of genotypic variance explained by QTL, and bias Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Difficult to identify all genes when many genes of small effect contribute to a trait 8 Biol 418: Evolutionary Ecology Variation Types of variation: continuous variation Quantitative geneticss Biol 418: Evolutionary Ecology Variation Difficult and expensive to explore variation molecularly How can we analyse genetics? Quantitative genetics: partition of variance (Fisher again!) Types of variation: continuous variation Quantitative genetics h2 is estimated from the degree of similarity between relatives e.g. parent-offspring regression VP = VA + VG + VE Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Phenotypic variance Environmental and among individuals developmental variance VP = Σ(xi – x)2 n-1 Additive genetic variance Non-additive genetic (portion of variation due to simple variance (dominance and genetic effects) epistasis). (narrow sense) heritability h2 = VA VP 72 Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature median of offspring height Falconer & MacKay (1996) Introduction to Quantitative Genetics. 70 The slope = h2 68 y = 0.69x + 21.5 66 64 64 66 68 70 72 mean of parents height 74 Jennifer Gow 9 Jennifer Gow Biol 418: Evolutionary Ecology Variation Types of variation: continuous variation Quantitative genetics 72 Meadow vole Microtus pennsylvanicus 70 r = 0.20 68 Song sparrow Melospiza melodia 66 Smith & Dhondt (1980) Evolution 34: 1155-1158. 64 64 66 68 70 Weight of mother 72 74 Maternal effects: effect of the maternal parent's genotype on the phenotype of her offspring that are unrelated to the offspring's own genotype 72 70 r = 0.07 68 66 64 64 66 68 70 Weight of father 72 74 Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Mean family beak depth Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Quantitative genetics Weight of offspring at weaning Hansen & Boonstra (2000) OIKOS 89: 81-94. Types of variation: continuous variation Weight of offspring at weaning Biol 418: Evolutionary Ecology Variation True mid-parent beak depth Fostered young do not resemble foster parents but do resemble true parents, showing that there is strong heritability. True foster-parent beak depth Jennifer Gow 10 Biol 418: Evolutionary Ecology Variation The paradox of variation Biol 418: Evolutionary Ecology Variation Genetic variation in nature Mousseau & Roff (1987) Heredity 59: 181-191. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Traits closely related to fitness should be less variable (narrow sense) heritability h2 = VA VP low? Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Most continuous traits have non-zero heritability . But 50% of morphological traits h2 <0.44 and 50% of life history traits h2 <0.25. i.e. h2 is often low. 11 Biol 418: Evolutionary Ecology Variation Genetic variation in nature Biol 418: Evolutionary Ecology Variation Genetic variation in nature CVA for life history traits NOT less than for morphology Drosophila pseudoobscura Houle (1992) Genetics 130: 195-204. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow h 2 = VA = VA (VA + VR) VP VR = VG + VE Low heritability but high additive genetic variance with high residual variance (from other genetic and environmental sources) Hubby & Lewontin (1966) Genetics 54: 577-594 & 595609. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow electrophoresis! 12 Biol 418: Evolutionary Ecology Variation Maynard Smith (1998) Evolutionary Genetics. 2nd Edn. Variation • Link between natural selection & evolution – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism – Continuous • Genetic variation in nature Jennifer Gow Genetic variation in nature Average proportion of loci polymorphic per population Insects Drosophila wasps Others Marine invertebrates Marine snails Land snails Fish Amphibians Reptiles Birds Rodents Large mammals Plants Biol 418: Evolutionary Ecology Variation Overview Variation • Link with natural selection & evolution 0.53 0.24 0.53 0.59 0.18 0.44 0.31 0.34 0.23 0.14 0.20 0.23 0.46 – The breeder’s equation – The paradox of variation • Fisher’s fundamental theorem • Types of variation – Polymorphism, Continuous • Amount of variation in nature • Maintenance of variation • Lack of response of variation to selection Jennifer Gow 13