Variation and its response to selection

Transcription

1 and its response to selection Overview Fisher s 1

2 is the raw material of evolution no natural selection without phenotypic variation no evolution without genetic variation Link between natural selection Linking variation, natural selection 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 Mean Z before variation among individuals in some attribute or trait: variation S = mean Z after mean Z before the trait frequency Mean Z after distribution will differ among age classes Mean Z kids the trait distribution of R = h 2 S all offspring will be different from that of all parents 2

3 Linking variation, natural selection The breeder s equation: The paradox of variation Directional selection the response to selection = transmission of withingeneration changes to the next generation R=h 2 S heritability =proportion of var n that is genetic = 0 if no genetic variability selection differential = 0 if no phenotypic variation Brodie et al. (1995) TREE 10: Trait mean changes towards an extreme and variance decreases. i.e. selection + genetic variance should lead to evolution of a phenotypic trait & evolution Fisher s Stabilising selection Trait mean does not changes but the variance decreases. Traits closely related to fitness should be less variable 3

4 The paradox of variation The paradox of variation Fisher s : The rate of increase in fitness of any organism Brodie et al. (1995) TREE 10: & evolution Fisher s Frequency Frequency S Bill size Bill size S The stronger selection, the greater the reduction in variance Fisher (1958) The Genetical Theory of Natural Selection. 2 nd edn. Link between natural selection Fisher s at any time is equal to its genetic variance in fitness at that time (Fisher) In population at equilibrium rate of fitness increase = 0 => genetic variance in fitness = 0 => genetic variance in traits related to fitness = 0 4

5 Types of variation: polymorphisms Types of variation: polymorphisms Genetic polymorphism Skúlason & Smith (1995) TREE 10: Smith (1993) Nature 363: Black-bellied seedcracker, Pyrenestes ostrinus 5

6 Types of variation: polymorphisms Types of variation: polymorphisms Non-genetic polymorphism Atypical morph: bent Typical morph: conic Acanthina angelica Skúlason & Smith (1995) TREE 10: Lively (1986) Ecology 67: Lively (1986) Evolution 40: Acorn barnacle, Chthamalus anisopoma 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

7 The genetic basis of continuous variation Frequency σ X Quantitative trait X = mean σ = standard deviation σ Coefficient of variation, CV = X (i.e. the standard deviation scaled by the mean) 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) Most continuous traits have at least a partly genetic basis Number of genes affecting traits can be large Traits are polygenic cf monogenic or oligogenic e.g. Body size: outcome of complex developmental, biochemical and 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 7

8 QTL analysis QTL analysis X 1. Choose species & identify genetic markers e.g. RAPDs, msats etc Linkage group 1 marker 1 marker 2 marker 3 marker 7 Linkage group 2 marker 8 marker Construct linkage map 2. Cross contrasting populations 5. Raise progeny & score phenotype 3. Genotype progeny QTL for size Linkage group 1 marker 1 marker 2 marker 3 marker 7 Linkage group 2 marker 8 marker Compare phenotype & genotype Schön et al (2004) Genetics 167: Sample size affects QTL detection power, the proportion of genotypic variance explained by QTL, and bias Difficult to identify all genes when many genes of small effect contribute to a trait 8

9 Quantitative geneticss Quantitative genetics Difficult and expensive to explore variation molecularly How can we analyse genetics? Quantitative genetics: partition of variance (Fisher again!) h 2 is estimated from the degree of similarity between relatives e.g. parent-offspring regression Falconer & MacKay (1996) Introduction to Quantitative Genetics. V P = V A + V G + V E Phenotypic variance among individuals V P = Σ(x i x) 2 n - 1 Additive genetic variance (portion of variation due to simple genetic effects) (narrow sense) heritability h 2 = V A V P Environmental and developmental variance Non-additive genetic variance (dominance and epistasis). median of offspring height y = 0.69x mean of parents height The slope = h 2 9

10 Quantitative genetics Quantitative genetics Hansen & Boonstra (2000) OIKOS 89: Weight of offspring at weaning Weight of offspring at weaning r = Weight of mother r = Weight of father Meadow vole Microtus pennsylvanicus Maternal effects: effect of the maternal parent's genotype on the phenotype of her offspring that are unrelated to the offspring's own genotype Smith & Dhondt (1980) Evolution 34: Mean family beak depth True mid-parent beak depth True foster-parent beak depth Song sparrow Melospiza melodia Fostered young do not resemble foster parents but do resemble true parents, showing that there is strong heritability. 10

11 The paradox of variation Genetic variation in nature Mousseau & Roff (1987) Heredity 59: Traits closely related to fitness should be less variable (narrow sense) heritability h 2 = V A V P low? Most continuous traits have non-zero heritability. But 50% of morphological traits h 2 <0.44 and 50% of life history traits h 2 <0.25. i.e. h 2 is often low. 11

12 Genetic variation in nature Genetic variation in nature CV A for life history traits NOT less than for morphology Drosophila pseudoobscura Houle (1992) Genetics 130: h 2 = V AVP = V A (V A + V R ) V R = V G + V E Hubby & Lewontin (1966) Genetics 54: & Low heritability but high additive genetic variance with high residual variance (from other genetic and environmental sources) electrophoresis! 12

13 Maynard Smith (1998) Evolutionary Genetics. 2nd Edn. Genetic variation in nature Average proportion of loci polymorphic per population Insects Drosophila 0.53 wasps 0.24 Others 0.53 Marine invertebrates 0.59 Marine snails 0.18 Land snails 0.44 Fish 0.31 Amphibians 0.34 Reptiles 0.23 Birds 0.14 Rodents 0.20 Large mammals 0.23 Plants 0.46 Overview Link with natural selection Fisher s, Continuous Amount of variation in nature Maintenance of variation Lack of response of variation to selection 13