Lecture 3 - Population Genetics 2 PDF

The Evolution of Population (Microevolution) Overview: The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve Genetic variations in p...

The Evolution of Population (Microevolution) Overview: The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve Genetic variations in populations Contribute to evolution Figure 23.1 Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution Is change in the genetic makeup of a population from generation to generation Figure 23.2  The Modern Synthesis Population genetics Is the study of how populations change genetically over time It is the study of the distributions and changes in allele frequency in a population, as it is subject to four main evolutionary processes: natural selection, genetic drift, mutation and recombination Reconciled Darwin’s and Mendel’s ideas The modern synthesis Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection Focuses on populations as units of evolution  Gene Pools and Allele Frequencies MAP A population ARE A Is a localized group of individuals that are Porcupine capable of interbreeding herd range NO TE RTH and producing fertile RR WE ITO ST RIE S offspring Fairb Species: a group of anks populations whose Fortymile individuals have the herd range ALAS YUK Whitehorse ON KA potential to interbreed & produce fertile offspring. Figure 23.3  Gene Pool AA aa Aa The gene pool Is the total aggregate of genes in a population at any one time Consists of all gene loci in all individuals of the population  The Hardy-Weinberg Theorem The Hardy-Weinberg theorem Describes a population that is not evolving States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work  Preservation of Allele Frequencies In a given population where gametes contribute to the next generation randomly, allele frequencies will not change For eg. every time a gamete is drawn from the Pool at random, the chance the gamete will bear an A allele is 0.8 & a allele is 0.2.  Hardy-Weinberg Equilibrium Hardy-Weinberg equilibrium Describes a population in which random mating occurs Describes a population where allele frequencies do not change A population in Hardy-Weinberg equilibrium Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 pq (80%) CR Eggs p2 64% 16% CRCR CRCW (20%) qp 16% 4% CW CRCW CWCW q2 If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 64% CR from 16% CR from 80% CR = 0.8 = p R R + R W C C homozygotes = C C homozygotes 4% CW from 16% CW from 20% CW = 0.2 = q + CRCW heterozygotes = CWCW homozygotes With random mating, these gametes will result in the same mix of plants in the next generation: Figure 23.5 64% CRCR, 32% CRCW and 4% CWCW plants  If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype p+q=1  Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem Describes a hypothetical population In real populations Allele and genotype frequencies do change over time  Population Genetics and Human Health We can use the Hardy-Weinberg equation To estimate the percentage of the human population carrying the allele for an inherited disease  Class Activity: PROBLEM #1. You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: A. The frequency of the "aa" genotype. B. The frequency of the "a" allele. C. frequency of the "A" allele. D. The frequencies of the genotypes "AA" and "Aa." E. The frequencies of the two possible phenotypes if "A" is completely dominant over "a."  Genera tion 1 CRCR CWCW genoty genoty pe pe Genera Mendelian inheritance tion 2 All CRCW (all pink flowers) Preserves genetic variation 50% 50% CR in a population gamete s CW gamete s Genera tion 3 50% 50% CR CW gamete gamete s s Genera tion 4 Alleles segregate, and subsequent generations also have three types Figure 23.4 of flowers in the same proportions  Activity: Problem # 2: Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Given this simple information, which is something that is very likely to be on an exam, calculate the following: a. The percentage of butterflies in the population that are heterozygous. b. The frequency of homozygous dominant individuals. Problem # 3: A very large population of randomly-mating laboratory mice contains 35% white mice. White coloring is caused by the double recessive genotype, "aa". Calculate allelic and genotypic frequencies for this population. Hardy Weinberg Practice using Chi square 1. a. In a certain population of newts, being poisonous (P) is dominant over not being poisonous (p). You count 200 newts, and 8 are not poisonous. What are the allele frequencies of the parent population? b. Fifty newts are washed downstream after a big storm and colonize a new pond. What do you expect the frequency and number of each genotype to be? c. You count the new population of newts and find 21 homozygous poisonous newts, 23 heterozygous poisonous newts, and 6 homozygous non-poisonous newts. (i) Is this what you expect? (Test using Chi square). (ii) If it is not, what are the new allele frequencies?  Practice on your own 2. a. Walking through the forest, you find a large population of toadstools. From your extensive knowledge of the kingdom fungi, you know that the allele for being spotted (S) is dominant over the allele for being plain (s). In this population of 1007, you find 14 toadstools that are not spotted. What are the allele frequencies? b. In a different forest, you find a somewhat smaller population of 548. Through genetic testing, you determine that there are 514 Spotted and 34 Plain toadstools. (i) Is this what you expected?(test using chi square analysis) (ii) If not, what are the allele frequencies of this population?  Conditions of Microevolution populations are rarely met The five conditions for non-evolving in nature A locus with 2 or more alleles will be in Hardy-Weinberg Equilibrium if these 5 conditions are met: Extremely large population size (many individuals in a pop) No gene flow ( no movement of individuals from pop. To pop.) No mutations (no biochemical change in DNA that produces new alleles) Random mating ( individuals mate at random) No natural selection ( differential genotypes have equal fitness)  CAUSES OF MICROEVOLUTION Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible Two processes, mutation and sexual recombination Produce the variation in gene pools that contributes to differences among individuals  Mutation Are changes in the nucleotide sequence of DNA Cause new genes and alleles to arise Figure 23.6  Point Mutations A point mutation Is a change in one base in a gene Can have a significant impact on phenotype Is usually harmless, but may have an adaptive impact Mutations That Alter Gene Number or Sequence Chromosomal mutations that affect many loci Are almost certain to be harmful May be neutral and even beneficial  Gene duplication Duplicates chromosome segments  Mutation Rates Mutation rates Tend to be low in animals and plants Average about one mutation in every 100,000 genes per generation Are more rapid in microorganisms  Sexual Recombination In sexually reproducing populations, sexual recombination Is far more important than mutation in producing the genetic differences that make adaptation possible Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow  Natural Selection Differential success in reproduction Results in certain alleles being passed to the next generation in greater proportions  Genetic Drift Statistically, the smaller a sample The greater the chance of deviation from a predicted result The disproportion of results in a small sample is known as sampling error. It is an important factor in the genetics of small populations. Genetic drift Describes how allele frequencies can fluctuate unpredictably from one generation to the next Tends to reduce genetic variation C RC R C RC R CW CW C RC R C RC R C RC W Only 5 of C RC W Only 2 of C RC R C RC R 10 plants 10 plants leave leave CW CW C RC R offspring CW CW offspring C RC R C RC R C RC R C RC W C RC W C RC R C RC R C RC R C RC W CW CW C RC R C RC R C RC R C RC W C RC W C RC W C RC R C RC R Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 0.5 p = 1.0 q (frequency of CW) = 0.3 q = 0.5 q = 0.0 Figure 23.7  The Bottleneck Effect In the bottleneck effect A sudden change in the environment may drastically reduce the size of a population The gene pool may no longer be reflective of the original population’s gene pool (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new Original Bottlenecking Surviving population and gold marbles are absent. population event population Figure 23.8 A  Understanding the bottleneck effect Can increase understanding of how human activity affects other species (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. Figure 23.8 B  The Founder Effect The founder effect Occurs when a few individuals become isolated from a larger population Can affect allele frequencies in a population  Gene Flow Gene flow Causes a population to gain or lose alleles Results from the movement of fertile individuals or gametes Tends to reduce differences between populations over time  Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Natural selection Accumulates and maintains favorable genotypes in a population  Genetic Variation Genetic variation Occurs in individuals in populations of all species Is not always heritable (a) Map butterflies (b) Map butterflies that that emerge in late summer: emerge in spring: black and white orange and brown Figure 23.9 A, B  Variation Within a Population Both discrete and quantitative characters Contribute to variation within a population  Discrete characters Can be classified on an either-or basis Quantitative characters Vary along a continuum within a population  POLYMORPHISM Phenotypic polymorphism Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable Genetic polymorphisms Are the heritable components of characters that occur along a continuum in a population eg: 1. garter snakes - 4 discrete patterns of coloration 2. biochemical character such as ABO blood group  MEASURING GENETIC VARIATION Population geneticists Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level Average heterozygosity Measures the average percent of loci that are heterozygous in a population  Variation Between Populations Most species exhibit geographic variation 1 2.4 3.14 5.18 7.15 6 Differences between gene pools of separate populations 8.11 9.12 10.16 13.17 19 XX or population subgroups Type of geographic variation is a Cline: a graded change in some trait along a geographic axis 1 2.19 3.8 4.16 5.14 6.7 9.10 11.12 13.17 15.18 XX Figure 23.10  Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Heights of yarrow plants grown in common garden Mean height (cm) EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of the resulting plants. RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at Atitude (m) which the seeds were collected, although the height differences were less than in the plants’ natural environments. Sierra Nevada Great Basin Range Plateau CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the Seed collection sites role of genetic as well as environmental differences. Figure 23.11  A Closer Look at Natural Selection From the range of variations available in a population Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations  Evolutionary Fitness The phrases “struggle for existence” and “survival of the fittest” Are commonly used to describe natural selection Can be misleading Reproductive success Is generally more subtle and depends on many factors Fitness Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus  Directional, Disruptive, and Stabilizing Selection Selection Favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection are Directional Disruptive (diversifying) Stabilizing Directional selection Favors individuals at one end of the phenotypic range eg. lizard tail (long), G. Fortis (Darwin’s Finch) beak size, giraffe (long neck), brachiopods (smoothness of shells), peppermoths Disruptive selection Favors individuals at both extremes of the phenotypic range eg. Squirrel tail length, peppermoths(color), hummingbirds(beak sizes) , West African Bird (2 beak size), osyter (color of shell), Rabbits(black vs white),mexican spade foot tadpoles (herbivore/omnivore), water snakes (color of skin) Stabilizing selection Favors intermediate variants and acts against extreme phenotypes eg: cactus spine density(medium), cat tail length(medium), human birth weight (3-4 kg), bird eggs (# of eggs in hatchlings) robin (4)  The three modes of selection Frequency of individuals Original population Original Evolved Phenotypes (fur color) population population (a) Directional selection shifts the overall (b) Disruptive selection favors variants (c) Stabilizing selection removes makeup of the population by favoring at both ends of the distribution. These extreme variants from the population variants at one extreme of the mice have colonized a patchy habitat and preserves intermediate types. If distribution. In this case, darker mice are made up of light and dark rocks, with the the environment consists of rocks of favored because they live among dark result that mice of an intermediate color are an intermediate color, both light and rocks and a darker fur color conceals them at a disadvantage. dark mice will be selected against. from predators. Fig 23.12 A–C  The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population  Diploidy Diploidy Maintains genetic variation in the form of hidden recessive alleles balanced polymorphism - 3 mechanisms  Balancing Selection Balancing selection Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Leads to a state called balanced polymorphism  Heterozygote Advantage Some individuals who are heterozygous at a particular locus Have greater fitness than homozygotes Natural selection Will tend to maintain two or more alleles at that locus The sickle-cell allele Causes mutations in hemoglobin but also confers malaria resistance Exemplifies the heterozygote advantage Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of 5.0–7.5% malaria caused by 7.5–10.0% Plasmodium falciparum 10.0–12.5% (a protozoan) >12.5% Figure 23.13  Frequency-Dependent Selection In frequency-dependent selection The fitness of any morph declines if it becomes too common in the population  An example of frequency-dependent selection On pecking a moth image the blue jay receives a food reward. If the bird Parental population sample does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity). Experimental group sample Phenotypic diversity 0.06 0.05 0.04 Frequency- 0.03 independent control 0.02 0 20 40 60 80 100 Generation number Plain background Patterned background Figure 23.14  Neutral Variation Neutral variation Is genetic variation that appears to confer no selective advantage  Sexual Selection Sexual selection Is natural selection for mating success Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics  Intrasexual Selection Intrasexual selection Is a direct competition among individuals of one sex for mates of the opposite sex  Intersexual selection Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex May depend on the showiness of the male’s appearance Figure 23.15  The Evolutionary Enigma of Sexual Reproduction Sexual reproduction Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Generation 4 Figure 23.16  If sexual reproduction is a handicap, why has it persisted? It produces genetic variation that may aid in disease resistance Why Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact (not all evolution is adaptive) Selection can only edit existing variations  Activity Form a group of 3 students. Your group has been asked to study two recently discovered fish species, both found in waters off a small island. The two species are very similar in general structure, but there are two key differences between them. In species A, males are brightly patterned with blue, red, and purple scales, whereas the females are drab, and males are much larger than females. In species B, males and females are the same size, and both are a dull brown color that blends in with the sandy bottom. You’ve decided to do an in-depth study of these two species, but to get your research grant, you must first develop some predictions about what you’ll find. What differences in behavior do you predict (list as many as you can think of)? How might the evolutionary histories of the two species differ? Discuss these questions, and submit a brief report of your answer.