Quantitative Inheritance - Pt.2Offspring-parent regression for height in humans (and why it’s called regression) (Fig. 8.11d)Assumptions of offpring-parent regression as an estimate of heritability“Cross-fostering” and heritability of beak length in song sparrows (Fig. 8.12) - 1“Cross-fostering” and heritability of beak length in song sparrows (Fig. 8.12) - 2Estimating heritability from twin studies (Fig. 8.14)The heritability (H2 ?) of “general cognitive ability” as measured in a study of Swedish twins is about 0.62 (Fig. 8.1c)Estimating heritability from crosses between inbred lines: Corolla height in longflower tobacco (see Fig. 8.3)Measuring the strength of directional selection (Fig. 8.15) Selection for increased tail length in miceSelection differential and selection gradientTwo-trait analysis of selection on Geospiza fortis on Daphne Major during the drought of 1976-77 (Fig. 8.16)Two-trait analysis of antipredator defenses in garter snakes (Brodie 1992)The evolutionary response to directional selectionResponse to directional selection, R = h2SResponse to selection for increased tail-length in miceSelection response in Geospiza fortis, revisitedHeritability and natural selection on flower size in alpine skypilots (Candace Galen 1989, 1996)Is flower size in skypilots heritable?Estimating the heritability of flower size in alpline skypilots (Fig. 8.20)Do bumblebees select for larger flowers?The selection gradient on flower size in alpine skypilots (Fig. 8.21)Response to selection on flower size in alpine skypilotsSelection on flower size in alpine sky pilots – two questionsModes of selection (Fig. 8.23)Modes of selection and genetic varianceStabilizing selection on gall size in a gall-making fly (Weis and Abramson, 1986)Stabilizing selection on a gall-making fly (Fig. 8.24)Disruptive selection on beak size in the black-bellied seed cracker (Smith 1993) (Fig. 8.25)Misunderstanding and misusing quantitative genetics – 1PowerPoint PresentationMisunderstanding and misusing quantitative genetics – 2All of the difference in average plant height between these two genetically identical “populations” of Achillea is due to environmenal effects (Clausen, Keck and Heisey) (Fig. 8.26) Mather is in the foothills of the Sierra Nevada mountains Stanford is low altitude and near the Pacific coastPopulations of Achillea at different elevations are genetically different - but the direction of difference depends on the elevation of the “common garden” (Fig. 8.29)1Quantitative Inheritance - Pt.2Chapter 82Offspring-parent regression for height in humans (and why it’s called regression) (Fig. 8.11d)3Assumptions of offpring-parent regression as an estimate of heritability•The most important assumption being made in these analyses is that the only cause of resemblance between offspring and parents is shared genes•This assumption may be violated if parents and offspring share the same environment and if environment has strong effects on the trait4“Cross-fostering” and heritability of beak length in song sparrows (Fig. 8.12) - 15“Cross-fostering” and heritability of beak length in song sparrows (Fig. 8.12) - 26Estimating heritability from twin studies (Fig. 8.14)If heritability is high both monzygotic and dizygotic twins should resemble each other, but monzygotic twins should resemble each other more closely than dizygotic twins (because the former share all their genes, while the latter share only half their genes)If heritability is low, then neither type of twin should show close resemblance7The heritability (H2 ?) of “general cognitive ability” as measured in a study of Swedish twins is about 0.62 (Fig. 8.1c)8Estimating heritability from crosses between inbred lines:Corolla height in longflower tobacco (see Fig. 8.3)•F1 individuals all have same heterozygous genotype. Therefore F1 variance = VE•F2 individuals have variable genotypes (homozygotes and heterozygotes). Therefore, F2 variance = VG + VE•VG = (F2 variance) minus (F1 variance)9Measuring the strength of directional selection (Fig. 8.15)Selection for increased tail length in mice10Selection differential and selection gradient•The directional selection differential, S, is the difference between the mean phenotype of the selected parents (t* in the previous slide), and the mean phenotype of the entire population from which the parents were selected (t “bar” in the previous slide). It allows us to predict the evolutionary response of a population to selection.•The selection gradient is the relationship between relative fitness and the phenotypic value. It shows how strongly phenotypic variation affects fitness.11Two-trait analysis of selection on Geospiza fortis on Daphne Major during the drought of 1976-77 (Fig. 8.16)Beak widthFitness12Two-trait analysis of antipredator defenses in garter snakes (Brodie 1992)For striped snakes, the best survival strategy is straight-line escape.For unstriped or spotted snakes, the best survival strategy is to reverse direction many times13The evolutionary response to directional selection•Evolutionary response (in generation t + 1) to a directional selection episode (in generation t), R = h2S•R is the change in the mean phenotype of the population over one (or more) generation(s)•Note: if h2 = 0, the population will not evolve14Response to directional selection, R = h2S15Response to selection for increased tail-length in mice•Di Masso et al. (1991) selected for longer tails in mice for 18 consecutive generations.•Average tail length increased by about 10%–This is a rather modest selection response–It suggests that the heritability of tail length in this population of mice was low, or that the intensity of selection, S, was low, or both.–A selection response, R, indicates that a trait is heritable, h2 = R/S, and that there is additive genetic variance for the trait (in this case tail length)–Closer analysis showed that long-tailed mice had more vertebrae in their tails (28 vs. 26-27 in controls)–Therefore, what was actually heritable (had additive genetic variance) was number of tail vertebrae16Selection response in Geospiza fortis, revisitedFrom the figure at left, R = 9.7 - 8.9 = 0.8 mmAverage beak depth of the survivors of the drought was ~ 10.1 mm: S = 10.1 - 8.9 = 1.1 mmTherefore, the realized heritability of beak length is:h2 = R/S = 0.8/1.1 = 0.7317Heritability and natural selection on flower size in alpine skypilots