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Chapter 13 Genetic Variation in Populations

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1 Chapter 13 Genetic Variation in Populations
Figure CO: An albino whitetail fawn © Srcromer/

2 Overview Genetic variation arises from random mutations
Mutation rates vary Some loci are more likely to mutate than others In the short term, when microevolution operates, the frequency of different alleles in the population from prior mutations is more important than the creation of new mutations

3 Overview The ability of DNA to repair itself also limits the impact of new, potentially harmful, mutations Quantitative trait loci (QTL) that contain groups of genes often influence phenotypic characters that show continuous variation Genetic Drift and Gene Flow also impact the genome composition/gene pools of populations The genetic history of populations contributes to our understanding of patterns of species distributions

4 Mutation Mutation is occurs in all populations
New mutations that arise, if not neutral in effect, will rarely be better, and likely will be worse, than the alleles already present Changes in environmental conditions can elicit a genetic response based on the available genetic variation in a population (evolution)

5 Resistance to DDT in Common Houseflies
Rapid genetic changes occur in many insect populations exposed to pesticides such as dieldrin and DDT Both dieldrin and DDT are neurotoxins; DDT opens sodium channels and both DDT and dieldrin stimulate Ach synthesis and release Resistance develops when mutant neuron membrane proteins arise or the numbers of receptors change Figure 01_INS: Resistance to DDT in common houseflies © Frank B. Yuwono/ShutterStock, Inc. Adapted from Strickberger, M. W. Genetics, Third edition. Macmillan, 1985; based on data from Decker, G. E., and W. N. Bruce, Amer. J. Trop. Med. Hygiene 1 (1952):

6 Selection for Pesticide Resistance
Decreased Pesticide Uptake Targets

7 Resistance to DDT in Fruit Flies
Experimental populations were bred by hybridizing a resistant and a susceptible strain of D. melanogaster Resistant genes were found on three chromosomes (2, 3, and X) Resistance was cumulative; the more genes contributing to resistance, the greater the resistance Figure 02: Percent survival of fruit flies Adapted from Crow, J. F., Ann. Rev. Entomol., 2 (1957):

8 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

9 Peppered Moth Biston betularia
The typical wildtype white/speckled 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

10 Peppered Moth Biston betularia
In unpolluted areas, trees are covered in lichens and the light form of the moth is hard to see In the mid 1800’s, air pollution in British cities covered trees with coal dust and soot In Victorian era cities, the dark form became common and the light form rare

11 Peppered Moth Biston betularia
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

12 Peppered Moth Biston betularia
The melanic phenotype is due to underlying homozygous (BB) and heterozygous (Bb) dominant genotypes 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

13 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, such as tree bark, when they were at rest

14 Kettlewell’s Experiments
Aviary studies demonstrated that native birds did eat peppered moths Release and capture studies in two areas indicated that peppered forms survived in greater numbers in unpolluted forests while melanic forms survived in greater numbers in soot polluted forests Kettlewell also observed birds preying on the two colors morphs in nature

15 Kettlewell’s Experiments
Many other lepidopterists have confirmed Kettlewell’s conclusions Kettlewell continued to work with melanism throughout his career Kettlewell summarized his career’s work in The Evolution of Melanism: a study of recurring necessity; with special reference to industrial melanism in the Lepidoptera (1973)

16 Kettlewell’s Experiments
In Bernard Kettlewell’s famous experiment, he placed moths on dark and pale tree trunks and showed that background colors strongly influenced survival In the wild, however, moths take much more care about where they settle and rarely settle on large tree trunks Instead moths usually choose to rest in shady areas where branches join the trunk If the moth’s choice of site is adaptive, then moths in these positions should be taken less often by predators than those on open space tree trunks

17 Peppered Moths In an experiment in which dead moths were pinned to open tree trunks or the underside of branches birds consumed fewer of those on the undersides of branches

18 Peppered Moths Other moths also make very specific choices about where to rest The speckled moth usually perches head up with its forewings covering its body When given a choice of resting site these moths prefer birch trees

19 Peppered Moths Pietrewicz and Kamil (1977) tested whether these choices by moths were selectively advantageous They trained blue jays to respond to slides of moths by pecking a button for a food reward whenever they spotted a moth Results showed that blue jays spotted moths less often on birch trees and especially when a moth was oriented with its head up Thus, moth’s choices appear to reduce the risk of detection by visually oriented predators Try TechApps to hunt moths yourself!

20 Kettlewell’s Critics: Scientists
A minority of serious scientists have criticized various aspects of Kettlewell’s methodology in his industrial melanism studies But the scientific consensus remains that Kettlewell‘s work was valid and that Kettlewell had demonstrated microevolution in action

21 Kettlewell’s Critics: Others
British journalist Judith Hooper attacked Kettlewell in her book, but her book has been dismissed by scientists for lack of scientific understanding, prejudice and careless journalism Creationists have seized on the criticisms and claim that not only was Kettlewell’s work invalid but also that what he claimed did not demonstrate evolution since it did not document the origin of a new species

22 Mutation Rates Mutation rate is the probability that a gene will mutate when the cell divides A spontaneous mutation rate for E. coli = 1 in 109 replicated base pairs or 1 in 106 replicated genes 1 in a million is easy to acheive in bacterial colonies Mutagens increase the mutation rate by times to as high as 1 in 103 replicated genes

23 Mutation Rates Though most mutations are harmful, rare superior mutation rates are advantageous The mutation rates observed in nature allow populations to retain common adaptive phenotypes while accumulating new alleles which might contribute to the origin of new features in a few individuals A remarkable way in which some organisms accumulate mutations without experiencing their immediate effects is to bind their mutant gene products with heat shock proteins

24 Heat Shock Proteins (HSPs)
Heat shock proteins (HSP) are expressed in response to various biological stresses, including heat, high pressures, and toxic compounds HSPs are among the most abundant cellular proteins found under non-stress conditions

25 Heat Shock Proteins (HSPs)
HSPs include a family of proteins known as "chaperones," which are solely dedicated to helping other proteins fold and assume their proper functions Cells are efficient about getting the folding right because misfolded proteins can change the normal life of the cell In some cases change is good, in others deadly

26 Heat Shock Proteins in Protein Folding
As the ribosome moves along the molecule of messenger RNA, a chain of amino acids is built up to form a new protein molecule The chain is protected against unwanted interactions with other cytoplasmic molecules by heat-shock proteins and a chaperonin molecule until it has successfully completed its folding   Source: (

27 Heat Shock Proteins (HSPs)
Heat shock proteins may be disabled in new stressful environments dramatic change in temperature dramatic change in pH dramatic change in salinity etc. When the HSPs are disabled, mutant proteins may develop into new conformations These new shapes may permit new functions, which may allow natural selection to improve the mutant proteins over time

28 Neo-Darwinism and Genetic Polymorphism
New beneficial mutations seem to be very rare Neutral or deleterious but recessive mutations provide a reservoir of potential genetic variation Genetic variation in a population, where two or more alleles exist at a locus, is termed a genetic polymorphism

29 Genetic Variation in Nature
Morphology, Physiology, Behavior Size, color, shape of cell or body parts, etc. Respiration, digestion, excretion, etc. Nutrient acquisition, reproduction, migration, etc. Enzyme polymorphism Change in catalytic ability due to change in temperature, osmotic environment, pH, etc. DNA sequence polymorphism Changes in bases, codons, introns, exons, etc.

30 Neo-Darwinism and Genetic Polymorphism
In the fruit fly, Drosophila pseudoobscura, populations in different localities are polymorphic for a wide variety of gene arrangements Many linked alleles are protected inside inversion loops

31 Drosophila pseudoobscura Polymorphisms
The different inversion loops contain different linked alleles Those linked alleles control different metabolic phenotypes Since a fruit fly lives only about a month, different metabolic phenotypes can be selected over the course of a single season Figure 03B: Chromosomal inversions found at different months during the year in one locality, Mount San Jacinto, California Adapted from Dobzhansky, T., Heredity 1 (1947):

32 Drosophila pseudoobscura Polymorphisms
Those different phenotypes, whose linked genes are protected in the inversion loops, are also adapted to different climates in different locations The different locations differ in average temperatures, average rainfall, altitude, and other environmental factors Figure 03A: Third chromosome gene inversions in Drosophila pseudoobscura Adapted from Dobzhansky, T., Carnegie Inst. Wash. Publ., 554 (1944):

33 Genetic Polymorphism Genetic polymorphism provides a much greater source of genetic variation than do the relatively few new mutations that arise each generation Now let’s look again at the nature of genetic variation

34 Polygenic Inheritance
Polygenic inheritance, also known as quantitative or multifactorial inheritance, refers to inheritance of a phenotypic characteristic (trait) that is attributable to two or more genes, or to the interaction with the environment, or both Unlike monogenic traits, polygenic traits do not follow patterns of simple Mendelian inheritance (discontinuous traits) Instead, their phenotypes typically vary along a continuous gradient depicted by a bell-shaped curve (a normal distribution)

35 Figure 04: Heights of 1,000 Harvard College students aged 18 to 25
Continuous Variation Small heritable changes provide most of the variation on which natural selection acts Figure 04: Heights of 1,000 Harvard College students aged 18 to 25 Adapted from Castle, W. E. Genetics and Eugenics, Fourth edition. Harvard University Press, 1932.

36 Continuous Variation It is almost impossible for a single gene locus to control continuous variation

37 Population Genetics The study of genes and genotypes in a population
We want to know the extent of genetic variation, why it exists and how it changes over time This knowledge helps us to understand how genetic variation is related to phenotypic variation

38 Gene Pool The gene pool is all of the genes and different alleles in a population We study genetic variation within the gene pool and how genetic variation changes from one generation to the next Emphasis is often on variation in alleles between members of a population at certain loci of interest

39 Populations Group of individuals of the same species that can interbreed with one another Some species occupy a wide geographic range and are divided into discrete populations (demes)

40 Genes in Natural Populations Are Usually Polymorphic
Polymorphism – many phenotypic traits display variation within a population Due to 2 or more alleles at a locus that influence a phenotype Polymorphic gene/locus - 2 or more alleles Monomorphic gene/locus– predominantly a single allele [“fixed” locus] Single nucleotide polymorphism (SNPs) Smallest type of genetic change in a gene; a point mutation Most common – 90% of the variation in human gene sequences Large, healthy populations exhibit a high level of genetic diversity Polymorphisms are the raw material for evolution

41 Quantitative Trait Loci (QTL)
The term quantitative trait loci (QTL) is shorthand for all of the loci or genes (alleles) in a particular region of a chromosome that affect a quantitative aspect of the phenotype In sticklebacks, Gasterosteus aculeatus, there is variation in the pelvic girdle and in dermal plates Figure 06: Three spine sticklebacks Reproduced from Trends in Ecology & Evolution, 19(9), Foster, S. A, and Baker, J. A., Evolution in parallel…, pp. 456–459, copyright 2004, with permission from Elsevier. Photographs courtesy of Dr. W. A. Cresko, University of Oregon.

42 QTL Mapping Statistical analysis is required to demonstrate that different genes interact with one another and to determine whether they produce a significant effect on the phenotype QTL mapping identifies a particular region of the genome containing a gene that is associated with the trait being measured QTLs are shown as intervals across a chromosome where the probability of association is plotted for each marker used in the mapping experiment The QTL techniques were developed in the late 1980s and can be performed on inbred strains of any species QTL analysis allows scientists to quantify the contributions of both heredity and environment to polygenic traits

43 Population Genetics In our introductory course, we are not going to explore the molecular details of QTL analysis Instead, we will examine the foundation of population genetics, the Hardy-Weinberg Equilibrium

44 Population Genetics and Gene (Allele) Frequencies in Populations
The Neo-Darwinian theory Evolution is a population phenomenon Evolution is a change in gene (now allele) frequencies in a population because of various natural forces such as mutation, selection, migration, or genetic drift These changes in allele frequencies lead to differences among populations, species, and higher clades This population genetics view of evolution became known as Neo-Darwinian theory with its emphasis on the frequency of genes and alleles in populations

45 Allele Frequencies A population’s gene pool includes all the alleles for all the loci present in the population Diploid organisms have a maximum of two different alleles at each genetic locus Typically, a single individual therefore has only a small fraction of the alleles for a given locus that are present in the population as a whole

46 Allele and Genotype Frequencies
Related but distinct calculations Population 1 has 5 homozygous dominant and 5 heterozygous individuals Population 2 has 7 homozygous dominant, 1 heterozygous, and 2 homozygous recessive individuals Both populations have identical allele frequencies, 15 A : 5 a = 75% A and 25% a alleles

47 The Hardy-Weinberg Principle
Godfrey H. Hardy: English mathematician (1903) Wilhelm Weinberg: German physician (1908) W.E. Castle: American geneticist (1908) (left out) Working independently just a few years after the rediscovery of Mendelian genetics they concluded that: The original proportions of the allele frequencies in a population remain constant from generation to generation as long as five assumptions are met

48 The Hardy-Weinberg Principle
Five H-W Equilibrium assumptions: If: The population size is very large Random mating is occurring No mutation occurs No selection occurs No alleles transfer in or out of the population (no migration occurs) Then allele frequencies in the population will remain constant through future generations

49 Simplifying Assumptions for The Hardy-Weinberg Principle
1) diploid organisms 2) sexual reproducing organisms 3) generations are non-overlapping 4) all genotypes are equally viable If these simplifying assumptions are not met, it complicates the mathematics for the analyses Whether or not these assumptions are all met, biologists can use Mendelian ratios and Hardy-Weinberg analysis to measure rates of evolution

50 The Hardy-Weinberg Principle
p = frequency for first allele in the population’s gene pool q = frequency for second allele in the population’s gene pool Calculate allele frequencies with a binomial equation: p + q = 1 because there are only two alleles: p + q must always equal 1 (100% of the alleles) [Note: more alleles can be handled, e.g., with three alleles: p + q + r = 1]

51 The Hardy-Weinberg Principle
Calculate genotype frequencies with a binomial expansion (p+q)2 = p2 + 2pq + q2 = 1 p2 = individuals homozygous for first allele 2pq = individuals heterozygous for the alleles q2 = individuals homozygous for second allele because there are three phenotypic classes: p2 + 2pq + q2 must always equal 1

52 The Hardy-Weinberg Principle
p = 0.6 and q = 0.4 and therefore p + q = 1.0

53 Product Law of Probability
Product Law of Probability: The probability of two independent events occurring simultaneously is equal to the product of their separate probabilities Male gamete production is independent of female gamete production

54 The Hardy-Weinberg Principle
assume 100 viable offspring 36 BB and 48 Bb offspring have ( = ) 120 B alleles; 48 Bb and 16 bb have ( = ) 80 b alleles. Freq of B = 124/200 = 0.6 and freq. of b = 80/200 = 0.4. use the Hardy-Weinberg equation to predict frequencies in subsequent generations

55 The Hardy-Weinberg Equilibrium
If: The population size is very large Random mating is occurring No mutation occurs No selection occurs No alleles transfer in or out of the population (no migration) Then allele frequencies in the population will remain constant through generations

56 Another Example 49 red-flowered RR 42 pink-flowered Rr
9 white-flowered rr [100 diploid individuals carry 200 alleles] 49 RR and 42 Rr offspring have ( = ) 140 R alleles 42 Rr and 9 rr have ( = ) 60 r alleles Freq of R = 140/200 = 0.7 and freq. of r = 60/200 = 0.3 No allele freq. change in the F1

57 The Hardy-Weinberg Equilibrium
Relates allele and genotype frequencies under certain limiting conditions p2 + 2pq + q2 = 1 (the Hardy-Weinberg Equation) If we apply the equation to the flower color gene, then: p2 = the genotype frequency of RR 2pq = the genotype frequency of Rr q2 = the genotype frequency of rr If p = 0.7 and q = 0.3, then: Frequency of RR individuals = p2 = (0.7) 2 = 0.49 Frequency of Rr individuals = 2pq = (2)(0.7)(0.3) = 0.42 Frequency of rr individuals = q2 = (0.3) 2 = 0.09

58 The Hardy-Weinberg Equilibrium
[In H-W equilibrium, heterozygotes will never be greater than 50% of the population] Hardy–Weinberg proportions for two alleles: the horizontal axis shows the two allele frequencies p and q and the vertical axis shows the expected genotype frequencies Each line shows one of the three possible genotypes and their relative frequencies

59 The Hardy-Weinberg Principles Describes a Population at Genetic Equilibrium
Genetic equilibrium requires: The population size is very large Random mating is occurring No mutation occurs No selection occurs No alleles transfer in or out of the population (no migration occurs)

60 Five Agents 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

61 Agents of Evolutionary Change
Small Population Size: When a population is large, then allele frequencies are very unlikely to change due to random sampling error When a population is small, then, just by chance, some individuals fail to mate at all, not because they are unfit When a population is small, then, just by chance, some offspring fail to survive to reproduce, not because they are unfit When a population is small, gene frequencies may change due to these sorts of random effects – this is called genetic drift California condors

62 Genetic Drift Genetic drift: Random fluctuations in allele frequencies over time due to chance events important in small populations founder effect – a few individuals found a new population (with a small allelic pool) bottleneck effect – a drastic reduction in population, and gene pool size and complexity DNA studies indicate that polar bears have suffered repeated bottleneck events when the arctic climate warmed and also repeated hybridization causing gene introgression (HGT) from their sister group, the brown bears

63 Genetic Drift Figure 08: Numbers of brown (bw75) alleles in 107 populations of D. melanogaster In small populations, allele frequency changes that have no predictable constancy or direction from generation to generation Genetic drift is a consequence of random fluctuations in gene frequencies that arise in small populations In this experiment, 107 populations were established with p = q = 0.5 for brown (bw/recessive) and red (+/wildtype/ dominant) eye alleles, but at each generation, only 16 parents were drawn, at random, to produce the next generation By generation 19, brown eye had been eliminated from 30 populations and fixed at 100% in another 28 populations. (+/+) (bw/bw) Data from Buri, P., Evolution 10 (1956):

64 Table 13.1: Definitions of Genetic Drift and Comments on Them
Regardless of the definition, genetic drift tends to increase variation between populations, but in no particular direction, including not necessarily to increase the fitness of the population

65 Agents of Evolutionary Change
Random Mating is required for the Hardy-Weinberg Equilibrium The members of the population mate with each other without regard to their phenotypes and genotypes The members of the population are (relatively) equally likely to mate with any other member of the population of the opposite sex, i.e., have relatively equal access to all members of the population Humans mate without regard to ABO and Rh blood types This example shows that a particular gene can meet the H-W equilibrium criteria, even though the species as a whole does not Wind-pollinated plant species and many aquatic species who release their eggs and sperm into the water Animals who are members of large mobile schools, herds or flocks

66 Agents of Evolutionary Change
Nonrandom Mating: mating between specific genotypes shifts genotype frequencies Non-Random mating could be either like with like; also called Assortative Mating or opposites attracting each other; also called Non-Assortative Mating

67 Agents of Evolutionary Change
Classification of mating systems Monogamy, polygamy, polyandry (Darwin 1871) Monogamy, resource defense polygyny, harem defense polygyny, explosive mating assemblage, leks, female access polyandry, etc. (Emlen & Oring 1977) Promiscuous Self pollination/fertilization Asexual reproduction Apomixis: parthenogenesis in animals and apogamy in plants

68 Agents of Evolutionary Change
Nonrandom Mating: mating between specific genotypes shifts genotype frequencies Ladies, would you prefer to mate and produce offspring with one of these males over another? If so, you are practicing non-random mating based on phenotypic characteristics

69 Agents of Evolutionary Change
Nonrandom Mating: mating between specific genotypes shifts genotype and phenotype frequencies Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals Disassortative Mating: phenotypically different individuals mate; produces an excess of heterozygotes Assortative Mating Disassortative Mating Selfing Inbreeding Random Mating Obligate Outcrossing Less Genetic Diversity Hardy-Weinberg More Genetic Diversity More Homozygotes Equilibrium More Heterozygotes Clones/Clonal Lineages Many Genotypes More Uniformly Fit Individuals Individuals of Varying Fitness Less Potential to Adapt to Change More Potential to Adapt to Change

70 Inbreeding Mating between relatives or selfing in plants
Inevitable in smaller populations Occurs in nature because of proximity of relatives – example: natural stands of tree whose relatives are nearby because of limited seed dispersion If there is no natural selection, an inbreeding population will acquire an increase in the frequency of homozygotes without a change in allele frequency in the population

71 Coefficient of Inbreeding
Isonymy: having the same surname from both parents, an estimate of inbreeding in population records, e.g., birth certificates

72 Coefficient of Inbreeding
10 children Three of Darwin's 10 children died in childhood, while another three never had any children of their own, despite being married for years

73 Agents of Evolutionary Change
Mutation: Changes in a cell’s DNA Mutation is the ultimate source of genetic variation Since a new mutation transforms an allele into a different allele, it must also change allele frequencies Mutation rates are generally so low that they have little effect on Hardy-Weinberg proportions of common alleles in the short term, over a few hundred generations

74 Agents of Evolutionary Change
Variation from 1/10000 to 1/

75 Agents of Evolutionary Change
Mutation: Changes in a cell’s DNA Mutations are usually deleterious Stumpy Stumpy, born 2007, in UK, on a duck farm, who eventually lost the two extra legs to accidental traumas, and his companion, Alice

76 Agents of Evolutionary Change
Natural selection: environmental conditions determine which individuals in a population produce the most offspring Three conditions are required for natural selection to occur: Variation must exist among individuals in a population Variation among individuals must result in differences in the number of offspring surviving Variation must be genetically inherited We will return to natural selection in Chapter 15

77 Agents of Evolutionary Change
Gene Flow: A movement of alleles from one population to another Migration of individuals or gametes between populations Migration can be a powerful agent for evolutionary change Migration tends to homogenize allele frequencies between populations But migration is adding or removing alleles from the gene pool, so migration is going to change gene frequencies in the populations experiencing immigration or emigration

78 Agents of Evolutionary Change
Migration – the movement of breeding individuals into or out of isolated populations – results in evolutionary change because alleles move with the individuals. We call this movement gene flow. If enough migration occurs, the original isolates, with their inherent limited genetic variability, may fuse and form a new larger population with increased genetic variability.

79 Hardy-Weinberg Equation
Relates allele and genotype frequencies under certain conditions p2 + 2pq + q2 = 1 p = frequency of the dominant allele q = frequency of the recessive allele The genotype frequencies of a population are p2 is frequency of homozygous dominant genotype 2pq is frequency of heterozygous genotype q2 is frequency of homozygous recessive genotype

80 Sickle-Cell Anemia In sickle-cell anemia, hemoglobin (Hbs) has poor oxygen affinity Sequencing of the hemoglobin gene revealed one change from the normal amino acid sequence:

81 Sickle Cell Anemia Phenotypes Alleles Genotypes GAG (glutamate)
Normal globin gene (H) GAG (glutamate) Hh HH hh Alleles Genotypes GTG (valine) Sickle cell gene (h)

82 Sickle-Cell Anemia Evolution of populations is best understood
in terms of frequencies: 1. Phenotypes Alleles 3. Genotypes Phenotypes Gene Alleles Genotypes Normal hemoglobin hemoglobin H HH, Hh Sickle Cell h hh

83 Sickle-Cell Anemia Actual Phenotype Frequencies: Sickle Cell Anemia in the African American Population Phenotype # people Freq. Normal Homozygous Dominants & Carriers 29.94M 0.998 Sickle Cell Disease Recessive 0.06M 0.002 Total # African Americans 30M 1.000

84 Total # African Americans
Sickle-Cell Anemia Actual Genotype Frequencies: Sickle Cell Anemia in the African American Population Phenotype Genotype # People freq Dominant HH 27.4M 0.915 Hh 2.5M 0.083 Recessive hh 0.06M 0.002 Total # African Americans 30M

85 Sickle-Cell Anemia If p (0.998) + q (0.002) = 1
H-W Expected Frequencies: Sickle Cell Anemia in the African American Population Genotype H-W Expected Phenotype Frequency Actual Phenotype Allele Allele Freq HH 0.915 ↓ H 0.998 Hh 0.083 ↑ hh 0.002 ↑ h 0.002 If p (0.998) + q (0.002) = 1

86 Hardy–Weinberg Principle
According to the Hardy–Weinberg principle, in a population of randomly mating individuals, allele frequencies are conserved and in equilibrium unless external forces act on them What is going on with Sickle Cell Anemia in African-Americans? [~400 years is only 16 human generations or so] Figure 07: Hardy–Weinberg equilibrium Adapted from Falconer, D. S. and T. F. C Mackay. Introduction to Quantitative Genetics, Fourth edition. Longman, 1996.

87 Sickle-Cell Anemia Is the population size is very large? Yes! 30M
Is random mating is occurring? Is mutation significant (over generations since slave importation began)? Is selection occurring? Are alleles transferring in or out of the population (migration)? Yes! 30M Yes, as far as HgS is concerned No [estimated at 5x10-8] Yes; but against homozygotes Yes, from African populations with a higher incidence of HgS

88 Sickle-Cell Anemia’s Heterozygote Advantage
The recessive sickle-cell allele produces hemoglobin with reduced capacity to carry oxygen This mutation also confers malaria resistance in heterozygotes This heterozygote advantage leads to a larger proportion of the recessive allele than usual in areas where malaria is widespread These populations exhibit balanced polymorphism between the mutant and wild-type alleles

89 The Hardy-Weinberg Equilibrium
If a population meets all the assumptions of the Hardy-Weinberg Principle, then allele frequencies in the population will remain constant through future generations The population size is very large Random mating is occurring No mutation occurs No selection occurs No alleles transfer in or out of the population (no migration) A population in true Hardy-Weinberg equilibrium is rarely seen in nature!

90 Additional Sources of Variation in Populations
The extent to which a population departs from an optimal genetic constitution is called its genetic load, and is marked by the loss of some individuals through genetic death However, the presence of genetic variability in a population can be considered a sort of “insurance policy” for sexually reproducing species (and HGT is similar for asexual species)

91 Additional Sources of Variation in Populations
On the other hand, there is, potentially, a major cost to the less fit in the variable/polymorphic population In technologically advanced societies, we are actually increasing the genetic load with modern medicine and pharmacology: Diabetes mellitus Atherosclerosis Respiratory diseases Susceptibility to a variety of microbial pathogens Etc.

92 Populations, Allele Frequencies and the Gene Pool
A population defined as a group of sexually interbreeding or potentially interbreeding individuals A deme is a locally interbreeding subset of the population The deme is most often the focal point for evolutionary change Consider the California yarrow, Achillea borealis and A. lanulosa Figure B01B: A. borealis © Sergey Chushkin/ ShutterStock, Inc. Figure B01C: A. lanulosa © Brzostowska/ ShutterStock, Inc.

93 Demes Can Adapt to Local Conditions
Average height, timing and length of growing season, temperature and drought tolerance varied among the populations on an east-west transect Figure B01A: Locations of the populations and indication of genetic differences that have evolved Adapted from Clausen, J. D., D. Keck, and W. M. Hiesey, Carnegie Inst. Wash. Publ., No. 581, 1948: 1–219.

94 Demes Have Different Gene Pools
Individual yarrow plants were drawn from five different locales, shoots were cut and grown vegetatively into mature individuals These clones of individuals were then planted at different locations They showed considerable differences in fitness; some even dying in a different locale Figure B02: Responses of clones from representative of the common yarrow Adapted from Clausen, J. D., D. Keck, and W. M. Hiesey, Carnegie Inst. Wash. Publ. No. 581, 1–219.

95 Gene Flow / Gene Migration
At least three factors have an impact on the recipient population: the difference in gene frequencies between the two populations; the proportion of migrating genes incorporated into each generation and its gene pool; and the pattern of gene flow, whether occurring once or continually over time Gene flow can hinder local evolutionary changes by infusing genes from distant populations that are not so well adapted to local conditions Example: global blood group gene frequencies

96 Global Blood Group Gene Frequencies
The original colonists of North America were a small group of apparently O+ founders; Caucasian Type A individuals may have originated in Scandinavia; Type B individuals seem to have originated in Central Asia O A B Different ABO blood group phenotypes may have little to do with variation in O2 transport, but are correlated with other factors such as disease susceptibility, e.g., Type A individuals seem to be less resistant to smallpox

97 Selection, Variation and Increased Fitness
R. A. Fisher, one of the founders of population genetics noted that the greater the genetic variation upon which selection for fitness may act, the greater the expected improvement in fitness Variation itself is subject to selection, and so the propensity to vary (variability) is an important attribute of organisms

98 Phylogeography One tool for reconstructing the geographical history of a lineage uses knowledge of genetics to plot variation in allele frequencies on the distribution map of the demes or populations of a species Distribution of Mycobacterium tuberculosis strains in human populations

99 Phylogeography Another example: the spread of wheat
The pie charts record frequencies of two isozyme alleles, probably the tan representing a more cold and drought resilient form than the brown Figure 10: Asian common wheat Figure 09: Asian common wheat Adapted from Ghimire, S. K., Y. Akashi, C. Maitani, M. Nakanishi and K. Kato, Breeding Sci., 55 (2005): Adapted from Ghimire, S. K., Y. Akashi, C. Maitani, M. Nakanishi and K. Kato, Breeding Sci., 55 (2005): Another example: the spread of wheat Strains of the Asian common wheat, Triticum aestivu, reflect adaptations of a single species to a large number of environments

100 Why Doesn’t Natural Selection Eliminate All Genetic Variation in Populations?
Natural selection tends to reduce variability in populations by eliminating less fit alleles Mechanisms which counteract that elimination to preserve genetic variation include: The diploid condition preserves variation by “hiding” recessive alleles (Bb) Balanced polymorphisms (2 or more phenotypes are stable in the population) may result from: 1. heterozygote advantage: Aa superior to aa and AA 2. frequency-dependent selection 3. variation within the environment for a population

101 Frequency-Dependent Selection
Frequency-dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population In positive frequency-dependent selection, the fitness of a phenotype increases as it becomes more common In negative frequency-dependent selection, the fitness of a phenotype increases as it becomes rarer (this is an example of balancing selection) Frequency-dependent selection is usually the result of interactions between species (predation, parasitism, or competition) or between genotypes within species (usually competitive or symbiotic), and has been especially frequently discussed with relation to anti-predator adaptations negative frequency-dependent selection in fruit flies

102 Chapter 13 End


104 Genetic Polymorphism

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