Transcript Evolution
Variation 9.1 Phenotypic variation caused by genetic differences and by the environment • Genetic variation is the foundation of evolution • Understanding the process of evolution requires understanding of the origin and transmutation of genetic variation • Phenotype, genotype, locus, allele • Phenotypic variation can be caused by genetic differences and by the environment 9.1 Phenotypic variation caused by genetic differences and by the environment 9.2 Multiple alleles underlie some genetic variation • In some cases, three or more alleles exist within a population • Papilio dardanus • Cepaea nemoralis • Homo sapiens 9.2 Multiple alleles underlie some genetic variation • Individuals may differ in phenotype due to environmental conditions • Identical twins • Learning • Maternal effects • Genetic and non-genetic sources of phenotypic variation may be disentangled through: crossing experiments, studies of heritability, common garden experiments 9.3 The frequency of three genotypes among females and males in one generation 9.4(1) A hypothetical example illustrating attainment of Hardy-Weinberg genotype frequencies 9.4(2) A hypothetical example illustrating attainment of Hardy-Weinberg genotype frequencies 9.4(3) A hypothetical example illustrating attainment of Hardy-Weinberg genotype frequencies 9.4(3) A hypothetical example illustrating attainment of Hardy-Weinberg genotype frequencies • Hardy-Weinberg principle is foundation on which most of the genetic theory of evolution rests • Genotypic frequencies attain their H-W values after a single generation of random mating • Not only genotype frequencies, but also allele frequencies, remain unchanged from generation to generation • Critical assumptions: random mating, large population, no gene flow, no mutation, no natural selection • Segregation distortion or meiotic drive 9.5 Hardy-Weinberg genotype frequencies as a function of allele frequencies at a locus with 2 alleles 9.6 Genotype frequencies in a the wild oat compared with those expected under Hardy-Weinberg • One form of nonrandom mating is inbreeding, i.e. the tendency to mate with relatives • Gene copies are identical by descent if they have descended, by replication, from a common ancestor, relative to other gene copies in the population 9.6 Genotype frequencies in a the wild oat compared with those expected under Hardy-Weinberg • One form of nonrandom mating is inbreeding, i.e. the tendency to mate with relatives • Gene copies are identical by descent if they have descended, by replication, from a common ancestor, relative to other gene copies in the population • H = H0(1-F) where H0 is the heterozygote frequency expected if the locus were in H-W equilibrium, and F is the inbreeding coefficient 9.6 Genotype frequencies in a the wild oat compared with those expected under Hardy-Weinberg Genotype frequencies at a locus with allele frequencies p=0.4 and q=0.6 when mating is random (F=0) and when the population is partially inbred (F=0.5) F = (H0-H)/H0 ; reflects reduction in heterozygosity compared to a panmictic population with equal allele frequencies 9.6 Genotype frequencies in wild oat compared with those expected under Hardy-Weinberg 9.10 Inbreeding depression in humans • Phenomenon is well known in human populations • The more closely related the parents, the higher the mortality rate among their offspring (data of offspring up to 21 years of age from marriages registered in 1903-1907 in Italian populations) 9.11 The golden lion tamarin is a small, highly endangered Brazilian monkey • Known problem in small, captive-bred populations • Golden lion tamarin (Leontopithecus rosalia); 500 ind in 140 zoos • Breeding scheme based on outbreeding 9.12 Population decline and increase in an inbred population of adders in Sweden • Isolated populations with less than 40 individuals • High levels of inbreeding caused small litter sizes and high juvenile mortality 9.13 Genetic variation in the enzyme phosphoglucomutase among 18 individual killifishes • To assess the evolutionairy potential of a population, one needs to quantify the degree of genetic variation • One therefore needs to know the fraction of polymorphic loci, the number of alleles per locus and their relative frequencies • Electrophoretic gel showing genetic variation in the enzyme phosphoglucomutase among 18 individual Killifishes (Fundulus zebrinus) 9.14 Nucleotide variation at the Adh locus in Drosophila melanogaster • Nucleotide variation at the Adh locus in Drosophila melanogaster • First study of genetic variation by means of complete DNA sequencing (Drosophila melanogaster) dates from 1983 • Nucleotide diversity per site (∏) reflects the proportion of nucleotide sites at which two gene copies randomly taken from a population, differ 9.15(1) The decay of linkage disequilibrium between two unlinked loci over three generations • Each gene is linked to certain other genes • Changes in allele frequencies at one locus may cause correlated changes at other, linked loci • Linkage disequilibrium refers to the association between certain alleles at different loci • Recombination between meiosis reduces the level of linkage disequilibrium 9.15(1) The decay of linkage disequilibrium between two unlinked loci over three generations • Each gene is linked to certain other genes • Changes in allele frequencies at one locus may cause correlated changes at other, linked loci • Linkage disequilibrium refers to the association between certain alleles at different loci • Recombination between meiosis reduces the level of linkage disequilibrium 9.15(1) The decay of linkage disequilibrium between two unlinked loci over three generations • Each gene is linked to certain other genes • Changes in allele frequencies at one locus may cause correlated changes at other, linked loci • Linkage disequilibrium refers to the association between certain alleles at different loci • Recombination between meiosis reduces the level of linkage disequilibrium 9.15(1) Variation in quantitative traits • Discrete genetic polymorphisms in phenotypic traits are much less common than slight differences among individuals (continuous, metric or quantitative variation) • Distributions of these traits often approach normality, and the genetic component of such variation is polygenic 9.17 The frequency distribution of the number of dermal ridges in a sample of 825 British men • Discrete genetic polymorphisms in phenotypic traits are much less common than slight differences among individuals (continuous, metric or quantitative variation) • Distributions of these traits often approach normality, and the genetic component of such variation is polygenic 9.18(1) An example of genotype environment interaction • A norm of reaction quantifies the variety of different phenotypic states that can be produced by a single genotype under different environmental conditions • Genotype x environment interaction on the number of bristles on the abdomen of male Drosophila pseudoobscura • Degree of phenotypic variation due to genetic differences between individuals depends on environmental conditions 9.18(1) An example of genotype environment interaction • A norm of reaction quantifies the variety of different phenotypic states that can be produced by a single genotype under different environmental conditions • Genotype x environment interaction on the number of bristles on the abdomen of male Drosophila pseudoobscura • Degree of phenotypic variation due to genetic differences between individuals depends on environmental conditions 9.18(1) An example of genotype environment interaction • A norm of reaction quantifies the variety of different phenotypic states that can be produced by a single genotype under different environmental conditions • Genotype x environment interaction on the number of bristles on the abdomen of male Drosophila pseudoobscura • Degree of phenotypic variation due to genetic differences between individuals depends on environmental conditions 9.19 Variation in a quantitative trait, such as body length • Analysis of quantitative variation is based on statistical analysis • Variance refers to distribution of values around mean • VP = VG + V E • h² = VG/(VG + VE) 9.20(1) The relationship between the phenotypes of offspring and parents • Heritabilities can be estimated through regression of offspring means on midparent means, or between other individuals with known relatedness • A regression coefficient of 1 indicates a very strong genetic basis • Often, heritability studies involve common garden experiments 9.20(1) The relationship between the phenotypes of offspring and parents • Heritabilities can be estimated through regression of offspring means on midparent means, or between other individuals with known relatedness • A regression coefficient of 1 indicates a very strong genetic basis • Often, heritability studies involve common garden experiments 9.21 Selection for movement in response to light in Drosophila pseudoobscura • Artificial selection can only act on the genetic component of phenotypic variation • It differs from natural selection in the way that reproductive success strongly depends on one (or a limited number of) trait(s) selected by the scientist • Selection for movement in response to light in Drosophila pseudoobscura 9.28 Gene flow causes populations to converge in allele frequencies • Only rarely populations are strictly isolated from each other; most often they show some degree of gene flow • Isolation-by-distance model • Gene flow (m) reflects the proportion of gene copies per generation that are derived from immigrants • Gene flow causes convergence in allele frequencies 9.30(1) Genetic differentiation among populations of the North American pitcher-plant mosquito • Variation in allele frequencies among populations can be quantified in different ways • FST = Vq/(q)(1-q) with FST varying between 0 (no variation among populations) and 1 (populations fixed for different alleles) • GST can be computed for a locus with more than 2 alleles • Genetic differentiation among populations of the North American pitcher-plant mosquito 9.30(1) Genetic differentiation among populations of the North American pitcher-plant mosquito D log p p i1 i 2 pi1 pi 2 • Genetic differentiation can also be expressed by Nei’s index of genetic distance (Nei 1987) • Expresses the probability that two gene copies from two populations comprise different allelic variants • Often visualized by means of phenograms (clustering algorithm) 9.30(2) Genetic differentiation among populations of the North American pitcher-plant mosquito 9.31 Geographic variation in mitochondrial DNA in MacGillivray’s warbler • DNA sequences may also provide information re. genealogic (phylogenetic) relationships between alleles • Allows reconstruction of the evolutionary history of species • Geographic variation in mitochondrial DNA in MacGillivray’s warbler 9.32(1) A division of the world’s human populations into eight classes of genetic similarity • Homo sapiens comprises one single biological species • Some studies discriminate between 3-60 races • Patterns of genetic variation strongly differ in relation to racial subdivision • Genetic similarity classes (enzymes and blood group loci) 9.32(2) The geographic distribution of skin color • Homo sapiens comprises one single biological species • Some studies discriminate between 3-60 races • Patterns of genetic variation strongly differ in relation to racial subdivision • Genetic similarity classes (enzymes and blood group loci)