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Variation
9.1 Phenotypic variation caused by genetic differences and by the environment
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Genetic variation is the foundation of evolution
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Understanding the process of evolution requires understanding of the
origin and transmutation of genetic variation
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Phenotype, genotype, locus, allele
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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
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In some cases, three or more alleles exist within a population
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Papilio dardanus
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Cepaea nemoralis
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Homo sapiens
9.2 Multiple alleles underlie some genetic variation
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Individuals may differ in phenotype due to environmental conditions
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Identical twins
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Learning
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Maternal effects
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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
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Hardy-Weinberg principle is foundation on which most of the genetic
theory of evolution rests
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Genotypic frequencies attain their H-W values after a single generation of
random mating
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Not only genotype frequencies, but also allele frequencies, remain
unchanged from generation to generation
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Critical assumptions: random mating, large population, no gene flow, no
mutation, no natural selection
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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
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One form of nonrandom mating is inbreeding, i.e. the tendency to mate
with relatives
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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
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One form of nonrandom mating is inbreeding, i.e. the tendency to mate
with relatives
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Gene copies are identical by descent if they have descended, by
replication, from a common ancestor, relative to other gene copies in the
population
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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
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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)
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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
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D   log 
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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)