6.1. Variation and inheritance

Transcription

1 Lectures 1. Introduction 2. Basics in population dynamics 3. Intraspecific interactions 4. Structure and dynamics of populations 5. Spatial dynamics and metapopulations 6. Ecological and evolutionary genetics 6.1. Variation and inheritance 6.2. Natural selection 6.3. Breeding systems 6.4. Genetic variation and structure 1

2 6.1. Variation and inheritance Continuous and discontinuous traits: 1. Variation between genotypes: a) Discontinous Floral form of dragon flowers Antirrhinum (leijonankidat): bilateral (dominant) and radially symmetric flowers b) Continous Fruit length in evening primrose Oenothera erythrosepala (helokki) 2. Variation within genotypes c) Discontinous Heteromorphic leaves in common water-crowfoot Ranunculus aquatilis (vesisätkin) d) Continous Size of the velvetleave Abutilon theophrasti (keltaaulio) varies according to the nutrient gradient 2

3 Genotype and phenotype Phenotypic plasticity Adaptation (acclimatization) of a phenotype to its environment Discrete plasticity, such as heteromorphism = polyphenism (same genotype, different phenotype) Continuous plasticity, continuous set of phenotypes expressed by a single genotype " reaction norm (reaktionormi) Environmentally induced alternative phenotypes. (a) Normal (left) and predatorinduced (right) morphs of water fleas, Daphnia cucullata; (b) wet-season (top) and dry-season (bottom) gaudy commodore butterflies, Precis octavia; (c) omnivore (top) and carnivoremorph (bottom) spadefoot toad tadpoles, Spea multiplicata (d) small-horned (left) and largehorned (right) dung beetles, Onthophagus nigriventris; (e) broad, aerial leaves and narrow, submerged leaves (circled) on the same water crowfoot plant, Ranunculus aquatilis (Pfenning et al. 2010, TREE) 3

4 Phenotypic plasticity of a trait can itself be genetically determined and be an adaptation to the variations in the environment (adaptive plasticity) Thale cress Arapidopsis thaliana (lituruoho) grown in different temperatures (Atkin et al. 2006, J. Exp. Bot.) When phenotypic differences between genotypes vary between environments, it is due to interaction between the genotype (G) and environment (E). 4

5 Reaction norm: Interaction between genotype and environment (G x E) Reaction norms for genotypes 1 and 2 (= continuous phenotypic plasticity) If the direction of the reaction norms are different, there is interaction between the genotype and environment (G x E, Figure b) differences in response to different environments are due to the different genotypes 5

6 Example (a): different response of Arapidopsis thaliana (lituruoho) for cold-treatment: After four weeks in cold, rosettes were moved to common warmer environment " time to flowering is reduced more in plants from some parents and less in plants from other parents Example (b): number of leaves in poppy Papaver dubium (ruisunikko) plants grown in 16 environments with differing nutritional status: Genotypes respond in different ways to environmental variation (a) Arabidopsis thaliana (b) Papaver dubium 6

7 Genotypic variation can be distinguished from phenotypic variation by growing/keeping individuals of different genotypes in same conditions (= common garden experiments) G x E interaction can be found by reciprocal transplant experiments Individuals introduced from two or more environments into others (a) Clausen et al. (1948): Common Yarrow Achillea lanulosa, (= A. millefolium siankärsämö) Plants originating from a latitudinal gradient were grown at a common garden (b) McGraw & Antonovics (1983): Mountain Avens Dryas octopetala (lapinvuokko) Reciprocal transplant experiment showing the effect on leave area 7

8 Quantitative traits Any phenotypic trait (P) is defined by the genotype (G) and environment (E): P=G+E It is impossible to tell, if the phenotypic value of an individual is genetic of environmental But it is possible to estimate what proportion of the phenotypic variability in a population is genetic For that, the phenotypic variation of a population can be divided into genetic (V G ) and environmental (V E ) variance components: V P = V G + V E 8

9 And further, the genetic variance can be divided in components: V P = V A +V D +V I + V E Equation = V G Where V A = the additive genetic variance V D = dominance term between the alleles at a same locus V I = interaction term between alleles at different loci If V D = V I = 0, the total genetic variance equals the additive genetic variance V A 9

10 Heritability Heritability is the ratio of genetic to phenotypic variance (broad sense), i.e. the genetic fraction of the total variance 2 = / Equation No genetic variation, V G = 0 " H 2 = 0 No environmental variation, V E = 0 " V G = V P " H 2 = 1 This means that for estimating the heritability, genotypes need to be replicated This way it is possible to estimate the variation that is due to environment Clonal individuals (using clonally reproducing species or clonal techniques) Inbred lines Assumption: all individuals are homozygous at all loci H 2 > 0.5 " trait has high heritability; most of the variance for the trait is genetic. This is not the same as being mostly genetically determined! In case of low V E, H 2 will be high even if there is not much of genetic variance H 2 < 0.2 " trait has low heritability; most of the variance for the trait is environmental. It could be that the environment does not influence the trait much, but if there is almost no genetic variation, then even low V E accounts for most of the variation, giving a low H 2 value. 10

11 How much of the phenotypic variation is transmissible to the next generation? The additive genetic variance is often expressed as the narrowsense heritability h 2 (note the difference to the broad-sense): h 2 = V V A P Equation And can be estimated from response to (artificial) selection S = selection differential = Y -Y P R = response to selection = Y O -Y Realized heritability = h 2 = R / S Equation

12 h 2 can be estimated also as the slope (kulmakerroin) of a regression between parent and offspring means Example: Beak size in Geospiza fortis: y = 0.82x h 2 = 0.82 h H 2 2 V = V = A P V V G P Beak size offspring mean: (y) Non-genetic h 2 = 0 Genetic 0 < h 2 1 Beak size, parent mean: (x) 12

13 h 2 is high, Because V A, the additive genetic variance, is high or Because V P (= V A + V E ) is low due to low V E For example, plants grown in same conditions in a greenhouse have low V E If the plants were grown in different conditions, V E would be high and therefore h 2 (=V A /V P ) would be low h H 2 2 V = V = A P V V G P 13

14 Trade-off and genetic covariance of traits When a beneficial change in one trait means a detrimental change in the other trait = trade-off Trading between the traits Example; the higher the leaves of a plant are located, the more support is needed and the less is afforded to be invested to leave biomass At a genetic level, this would show as a negative covariance (correlation) between different genotypes Allocation to leaf biomass in relation to leaf height in 18 species of grasses 14

15 6.2 Natural selection Darwin s theory on selection: (i) Individuals vary in populations (different trait values) (ii) Survival and reproduction of an individual depend on individual traits R 0 (iii) (ii) (iii) (iv) This leads to different success of individuals (selection) Variation between individuals in inheritable (i) Trait (v) This leads to increase of the most successful traits and changes the genetic structure of the populations (=evolution)

16 Natural selection and evolution (i) Variation Example: Peppered moth Biston betularia (Koivumittari) (ii) Survival (iii) Success (selection) (iv) Variation is heritable Light: 14.6% Dark: 4.7% Light: 13.0% Dark: 27.5% (v) Genetic changes in populations (evolution) Light form gets more common Dark form gets more common 16

17 Types of selection Directional Stabilizing Disruptive Grey = selected against 17

18 Effect of directional selection on alleles of a locus Example: Cyanogenic and acyanogenic alleles in white clover Trifolium repens (valkoapila) (Ennos 1981) Most individuals have an allele Ac, which enables the plant to produce cyanogenic glucose Only plants that have a dominant allele Li in a second locus are able to release toxic cyanide (HCN), when leaves are damaged (herbivory) Only Li/Li- and Li/li-plants release cyanide, when cells are broken in the mouth/gut of a herbivore li/li-genotypes do not, because their enzyme is not active In case of much of herbivory, selection favours Li/Li and Li/li genotypes and the frequency of Li-allele grows 18

19 Genotypic fitness (w ij ) and the frequency of alleles after selection p + q = 1 Hardy-Weinberg equilibrium = p 2 + 2pq + q 2 = 1 Genotype A 1 A 1 A 1 A 2 A 2 A 2 Σ Frequency (before) p 2 2pq q 2 1 Fitness w 11 w 12 w 22 Effect to offspring p 2 w 11 2pqw 12 q 2 w 22 w Frequency (after) p 2 w 11 /w 2pqw 12 /w q 2 w 22 /w 1 w is the mean fitness of the population The frequency of the allele A 1 after selection: p = p 2 w 11 /w + pqw 12 /w The change in frequency can be calculated also directly ( p = p -p): ( w - w ) p + ( w w ) Δp = pq w 11 - q 19

20 Back to the clover example: If the genotype li/li has a fitness of 0.7 compared to genotypes Li/Li and Li/li and the frequency of allele Li is p = 0.2 (q = 1-p = 0.8): Genotype Li/Li Li/li li/li Σ Frequency (before) 0.04 (=0.2 2 ) 0.32 (=2x0.2x0.8) 0.64 (=0.8 2 ) 1 Fitness Effect to offspring 0.04 (=p 2 w 11 ) 0.32 (=2pqw 12 ) (=q 2 w 22 ) Frequency (after) 0.05 (p 2 w 11 /w) (2pqw 12 /w) (q 2 w 22 /w) (w) Li allele frequency (after): p = (1/2)(0.396) = Change in the allele frequency: p - p = = Δp = ( 1-1) ( 1-0.7) = »

21 Selection coefficient s Intensity of selection can be expressed with a selection coefficient (s, valintakerroin): w = 1- ij s ij Where the selection coefficient expresses selection against a genotype In our clover example: w 11 = w 12 = 1 w 22 = 0.7 = " s 22 = 0.3 = selection against the genotype li/li Will lead to fixation of Li (monomorphic population, p " 1, q " 0) 21

22 Most even advantageous alleles do not spread in the population because chance (genetic drift) will remove them from a population Fixation probability frequency of an allele when a neutral allele is considered Selection affects the fixation probability and is related to selection coefficient and effective population size (sn e ) For example, some estimates suggest that only 10% of alleles, which give a fitness advantage of 5%, are successful and increase in the population Mean selection coefficients against nonlocal genotypes in some reciprocal transplant experiments l = survival m = fecundity l = growth rate 21

23 Balancing selection Preserves genetic variation One form is the heterozygote advantage (= overdominance): w 11 = 1-s 11, w 12 =1, w 22 = 1-s 22 Stable equilibrium of allele frequencies Classical example is the Sickle cell anaemia: heterozygote individuals suffer from less severe symptoms of malaria, but homozygote individuals have the Sickle cell anaemia 23

24 Balancing selection Another form of balancing selection is disruptive selection (= diversifying selection), where selection favours the extreme ends of phenotypic distribution Example, lower mandible size in the Black-bellied Seedcracker Pyrenestes ostrinus (loistomurskaajasieppo) a) Fitness surfaces b) Distribution of juveniles, those that did not survive are shown in grey, those that survived in black c) Distribution in adults 24

25 Frequency dependent selection Positive frequency dependent selection Selection favours common forms E.g. Batesian mimicry, where a mimic species benefit from copying warning colours of a poisonous species Negative frequency dependent selection Selection favours rare forms Scarlet kingsnake (Lampropeltis elapsoides) and coral snake (Micrurus sp.) Red next to black, you can pat him on the back; red next to yellow, he can kill a fellow E.g. when a population is exposed to a parasite, the individuals will begin to acquire immunity, leading to increase in the parasite death rates. Those parasites that are not recognized by the immune system will be more successful in continuing to reproduce " The rarer phenotypes has increased fitness 25

26 Stabilizing selection Selection that favours individuals near the mean Example: Flowering of the Spathulate fleaworth Tephroseris integrifolia (pikkukarvakko) is affected by varying selection types in different years; both directional (the decreasing line) and stabilizing (curved line; Widén 1991) 26

27 Purifying selection Selective removal of harmful, deleterious alleles = negative selection, purging Can result to stabilizing selection Drives the deleterious alleles to low frequencies, where they often remain due to mutation-selection balance E.g. hereditary diseases Chondrodystrophy in Californian condors (Gymnogyps californianus); detrimental when homozygous, allele frequency of ~ 0.09 in captive population purging would have increased inbreeding depression Stabilizing and purifying selection = negative selection decreases the prevalence of traits that diminish fitness Balancing and directional selection = positive selection Increases prevalence of adaptive traits 27

28 Summary Continuous and discrete (discontinuous) variation Phenotypic (V P ), genetic and environmental variation Additive genetic variance (V A ) and heritability (h 2 = V A /V P ) Selection coefficient (s) Directional, balancing, frequency-dependent, stabilizing, and purifying selection 28