Lecture Notes - Phenotypic Evolution (1) PDF

BIOL 3506 Evolutionary Biology Lecture 13-14 Phenotypic Evolution Lecture 13-14  Peppered Moth Biston betularia (Linnaeus, 1758) The peppered moth occurs in two color phases (a) Both phases are displayed against an unpolluted, lichen-covered tree (b) Both phases are displayed aga...

BIOL 3506 Evolutionary Biology Lecture 13-14 Phenotypic Evolution Lecture 13-14  Peppered Moth Biston betularia (Linnaeus, 1758) The peppered moth occurs in two color phases (a) Both phases are displayed against an unpolluted, lichen-covered tree (b) Both phases are displayed against a dark tree, on which the lichen were killed by pollution  Peppered Moth Biston betularia In the mid 1800’s, air pollution in British cities covered trees with coal dust and soot (industrial revolution) In 1848, 5% of the population were dark colored moths while 95% were light colored In 1895, 98% were dark colored while 2% were light colored In 1995, 19% were dark colored while 81% were light colored ~150 years of rapid phenotypic changes What happened???? Bernard Kettlewell (1907-1979) and Industrial Melanism Kettlewell performed extensive field studies in Britain in the 1950s to test the hypothesis that bird predators were altering the frequencies of the color morphs based on the moths’ contrast to their backgrounds Bernard Kettlewell (1907-1979) and Industrial Melanism Kettlewell compared survival of dark and light moths in 2 environments (polluted and Nonpolluted)  Results: %Recaptured Woodland Melanics Pepper Urban 27.5 13.0 Rural 6.3 12.5 In brief… Coloration pattern of the Peppered moth has been under NATURAL SELECTION (predation). Release and capture in two areas indicated that peppered forms survived in greater numbers in unpolluted forests while melanic forms survived in greater numbers in polluted forests/urban systems This trait has shown evolutionary change as result of this selection There is a genetic component associated to this trait as it is transmitted across generations.  Peppered Moth Biston betularia The typical wildtype white phenotype is caused by a recessive allele that must be homozygous to be expressed  Peppered Moth Biston betularia The typical wildtype white phenotype is caused by a recessive allele that must be homozygous to be expressed The recessive allele predominates in wild population gene pools The melanic or black form is caused by a dominant allele that occurs spontaneously in nature Peppered moths rest on trees and depend on camouflage for protection  Peppered Moth Biston betularia In the mid 1950’s, air pollution controls were introduced in Britain When smoke pollution decreased in Britain, natural selection acted very quickly to favor survival of the wild type peppered morphs as bird predation eliminated melanic forms in progressively less polluted forests The frequency of the melanic form has declined ever since The industrial melanisms was happening in other moths Goal To understand how morphological, physiological and other biological characteristics/traits evolve under natural selection. Key subjects: Genetic architectures of quantitative traits Phenotypic variation and its components Natural selection over quantitative traits. Key questions: What are polygenic traits? How does natural selection act on polygenic traits? Do genes involved in phenotypic evolution have large or small effects on polygenic traits? What determines the evolutionary dynamics of a trait? Polygenic inheritance When single genes affect a character è discontinuous variation (categories) Most characters such as height or mass and even eye colour, show continuous variation May be due to an environmental influence such as diet OR it may be due to the interaction of several genes. The interaction of two genes  Comb shape in chickens These can come in four shapes which are controlled by two non-linked genes P and R Each gene has two alleles, one dominant (P and R) and one recessive (p and r) Comb shape in chickens Crossing pure breeding Rose type (ppRR) with Peas (PPrr) type gives Walnut (PpRr) as the F1 Comb shape in chickens An incestuous cross of the Walnut F1 produces all four types of comb in the F2 9 walnut 3 rose 3 pea 1 single The interaction of three or more genes More genes interacting = more variety in the offspring In the coat colour of mice the are several genes interacting to produce a range of different coat colours Three of these genes are A, B and C.  The interaction of three or more genes The A gene controls the production of a small yellow band near the end of each hair which gives rise Agouti (mousy) coat The recessive allele gives Non-agouti The B gene give the ground colour Black The recessive allele gives Brown colour The C gene controls the expression of the coat colour genes as a whole The recessive allele gives Albino.  The interaction of three or more genes Crossing pure breeding Wild Type (AABBCC) to Albino (aabbcc) gives Wild type offspring in the F1 (AaBbCc) The F2 produced from these gives 5 types of coat colour: Wild Type (agouti), Black, Chocolate, Cinnamon, Albino.  The interaction of three or more genes F2 ABC ABc AbC aBC Abc abC aBc abc ABC AABBCC AABBCc AABbCC AaBBCC AABbCc AaBbCC AaBBCc AaBbCc Ratio of polygenic traits departs from Agouti Agouti Agouti Agouti Agouti Agouti Agouti Agouti Mendelian 9:3:3:1 ABc AABBCc AABBcc AABbCc AaBBCc AABbcc AaBbCc AaBBcc AaBbcc Agouti Albino Agouti Agouti Albino Agouti Albino Albino The more genes that interact the greater the AbC AABbCC AABbCc AAbbCC AaBbCC AAbbCc AabbCC AaBbCc AabbCc range of phenotypes produced Agouti Agouti Cinnamon Agouti Cinnamon Cinnamon Agouti Cinnamon aBC AaBBCC AaBBCc AaBbCC aaBBCC AaBbCc aaBbCC aaBBCc aaBbCc This ultimately leads to a continuous Agouti Agouti Agouti Black Agouti Black Black Black distribution of traits where one blends into another. Abc AABbCc AABbcc AAbbCc AaBbCc AAbbcc AabbCc AaBbcc Aabbcc Agouti Albino Cinnamon Agouti Albino Cinnamon Albino Albino abC AaBbCC AaBbCc AabbCC aaBbCC AabbCc aabbCC aaBbCc aabbCc Agouti Agouti Cinnamon Black Cinnamon Chocolate Black Chocolate aBc AaBBCc AaBBcc AaBbCc aaBBCc AaBbcc aaBbCc aaBBcc aaBbcc Agouti Albino Agouti Black Albino Black Albino Albino abc AaBbCc AaBbcc AabbCc aaBbCc Aabbcc aabbCc aaBbcc aabbcc Agouti Albino Cinnamon Black Albino Chocolate Albino Albino A polygenic character typically shows: a continuous variation a normal (Gaussian) distribution several different genotypes may produce same phenotype. It may also be influenced by the environment E.g. Skin colour and UV light E.g. Height and diet Human height  Five Agents What drives the evolution of polygenic traits? of Evolutionary Change  Five Agents What drives the evolution of polygenic traits? of Evolutionary Change A population not in Hardy-Weinberg equilibrium is one in which allele frequencies are changing generation to generation due to one or more of the five evolutionary agents that are operating in the population How does natural selection affect single-gene and polygenic traits? Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.  Natural Selection § Many more individuals are born than survive (COMPETITION). § Individuals vary in traits directly related to their ability to survive and reproduce (VARIATION). § These advantageous traits are passed on to offspring (HERITABILITY). § This process is repeated generation after generation over long periods of time (ITERATION). Variation Differential survival Selection pressure & reproduction Heritability  Natural Selection is an Ecological Process and not an Evolutionary Process y re dit he No Selection occurs as a result of Evolutionary change is detected by the differential performance changes in gene frequencies at the of INDIVIDUALS level of the POPULATION  How does natural selection affect single-gene and polygenic traits? Natural Selection on Single-Gene Traits: The example of Lizard Color A lizard population is normally brown, but has mutations that produce red and black forms. Red lizards are more visible to predators, so they will be less likely to survive and reproduce. Therefore, the allele for red color will become rare.  How does natural selection affect single-gene and polygenic traits? Natural Selection on Single-Gene Traits: The example of Lizard Color Black might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster and escape predators. In turn, they may produce more offspring. The allele for black color will increase in relative frequency. How does natural selection affect single-gene and polygenic traits? Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. Polygenic traits have a range of phenotypes that often form a bell curve. The fitness of individuals may vary from one end of the curve to the other. How does natural selection affect single-gene and polygenic traits? Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. Polygenic traits have a range of phenotypes that often form a bell curve. The fitness of individuals may vary from one end of the curve to the other. Natural selection on polygenic traits can affect the distributions of phenotypes (shape of the curve) in three ways: directional selection, stabilizing selection, or disruptive selection. Natural Selection on Polygenic Traits  Statistical Methods Are Required for Analyzing Quantitative Characteristics The mean: the average The Variation and Standard Deviation Variance: the variability of a group of measurements Standard deviation: the square root of the variance.  Statistical Methods Are Required for Analyzing Quantitative Characteristics However, traits are not independent!!! so, trait distribution/curve may influence (or be influenced) by other traits Correlation Correlation: when two characteristics are correlated, a change in one characteristic is likely to be associated with a change in the other. Correlation Correlation coefficient: measures the strength of their association Correlation doesn’t imply a cause-and-effect relation. It simply means that a change in a variable is associated with a proportional change in the other variable. Trait 2 Trait 1  Phenotypic variation Genetic components Environmental factors VE# Additive VG Epistasis Other VE VG+ VE+ Phenotypic+variance+ Dominance Epigenetic Maternal VE Maternal VG Pleiotropy Environment  Phenotypic variation Genetic components Environmental factors VA+ VE# Additive VG Epistasis Other VE VA+ VD+++I+ VE+ Phenotypic+variance+ Dominance Epigenetic Maternal VG Maternal VE Pleiotropy Environment  Phenotypic variation : Vp Phenotypic variation Genetic variance: Vg Genetic components Environmental factors Va: additive genetic variance Vi: genic interaction variance Additive VG Epistasis Other VE VA+ Dominance Vd: dominance genetic variance Maternal VG Epigenetic Maternal VE Pleiotropy Environment  Phenotypic variation : Vp Phenotypic variation Genetic variance: Vg Genetic components Environmental Environmental variance: Ve factors Additive VG Epistasis Other VE VA+ Dominance Maternal VG Epigenetic Maternal VE Pleiotropy Environment  Phenotypic variation : Vp Phenotypic variation Genetic variance: Vg Environmental variance: Ve Genetic-Environmental Interaction Vge Genetic components Environmental factors VA+ Vp = Vg + Ve + Vge VE# Additive VG Epistasis Other VE VA+ VD+++I+ VE+ Vp = Va + Vd + Vi + Ve + Vge Phenotypic+variance+ Dominance Epigenetic Maternal VG Maternal VE Pleiotropy Environment not all genetic variance can be passed on to offspring, only the additive genetic variance. So, VP = VA + VD + VI + VE + VG X E VA = variance caused by additive effects of alleles at all relevant loci VD = variance caused by dominance deviations, i.e., non-additive interactions between alleles at a single locus VI = variance caused by epistatic deviations, i.e., non-additive interactions between alleles at different loci 55  Vp = Vg + Ve + Vge a. Which genotype has the higher phenotype depends on the environment in which rearing occurs b. "reaction norm" or "norm of reaction" = set of phenotypes produced by a given genotype across a range of environments (phenotypic plasticity)  Phenotypic variation à Heritability "A central question in biology is whether observed variation in a particular trait is due to environmental factors or biological factors — sometimes expressed as the nature–nurture debate.  Phenotypic variation à Heritability Heritability is a concept which summarizes how heritable a phenotype of interest is, in particular with reference to the resemblance of offspring and parents. Visscher, P. M., W. G. Hill, and N. R. Wray. 2008. Heritability in the genomics era — concepts and misconceptions. Nature Reviews Genetics 9:255-266.  Phenotypic variation à Heritability Heritability is a concept which summarizes how heritable a phenotype of interest is, in particular with reference to the resemblance of offspring and parents. Heritability: The proportion of the total phenotypic variation that is due to genetic difference. H2 = Vg/Vp Broad sense If H2 = 0, then none of the phenotypic variance is caused by genetic variance. If H2 = 1, then the phenotypic variance is 100% caused by genetic variance. Estimating heritability, h 2 Narrow sense heritability (1) Analyze related individuals: twins of different type, or parents and offspring (2) Measure the response of a population, in the next generation, to selection  Measuring h2: Parent-offspring regression § When there is genetic variation for a character there will be a resemblance between relatives. § Relatives will have more similar trait values to each other than to unrelated individuals. HERITABILITY OF BEAK DEPTH IN DARWINS’ FINCHES Measuring heritability from regression analysis HERITABILITIES FOR SOME TRAITS IN ANIMAL SPECIES h2 (%) IN: Falconer & Mackay. Introduction to Quantitative Genetics.1996. Longman.  Heritability of different human traits TRAIT HERITABILITY Fingerprint pattern >0.9 Height 0.7 IQ 0.7 Triglyceride 0.7 Autism, schizophrenia 0.3-0.6 Weight 0.5 Cholesterol level 0.45 Blood pressure 0.4 Handedness 0.3 Fertility 0.1 Estimating heritability, h 2 Narrow sense heritability (1) Analyze related individuals: twins of different type, or parents and offspring (2) Measure the response of a population, in the next generation, to selection The breeder's equation says adaptive phenotypic evolution consists of two parts: r = h2 s Narrow sense heritability r = response to selection = evolution from one generation to the next = change in the phenotypic mean of a population from one generation to the next 67  r = h2 s h2 = narrow-sense heritability = how much of the phenotypic variation in a population is caused by genetic effects that can be passed on from parents to their offspring additive genetic variance = -------------------------------------- total phenotypic variance 68  r = h2 s s = directional selection differential = difference in mean phenotype between the original whole population before selection and the mean of the individuals who actually breed to produce the next generation 69 Population Before Selection Consider the distribution of a phenotypic trait within a population MeanBefore 70 Population Before Selection MeanBefore Now imagine that a selective event Population After Selection occurs, e.g., a winter ice storm that kills most of the individuals in a bird population (Bumpus 1899). In this example, the larger birds survive. MeanAfter s = Mean After - MeanBefore 71 Population Before Selection MeanBefore This subset of Population After Selection "selected" individuals survives to breed MeanAfter 72 Population Before Selection MeanBefore Population After Selection MeanAfter The distribution of the trait in their offspring Next Generation might look like this Exactly where depends on inheritance because MeanNext Generation r = h2 s 73 Population Before Selection r = h2 s MeanBefore Population After Selection s = Mean After - MeanBefore r = Mean Next Generation - MeanAfter MeanBefore Next Generation h2 = r/s So this is one way to estimate the narrow-sense heritability. Usually called the realized heritability. MeanNext Generation 74 Univariate breeder's equation  This equation describes the response to directional selection of a single phenotypic trait, given its narrow- sense heritability and the intensity of selection: r = h2 s However, organisms comprise many phenotypic traits, and they may be correlated. Therefore, selection that affects one may affect another. 76 Univariate breeder's equation Multivariate breeder's equation