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Evolution of Populations Question? • Is the unit of evolution the individual or the population? • Answer = while evolution effects individuals, it can only be tracked through time by looking at populations So what do we study? • We need to study populations, not individuals • We need a method to track the changes in populations over time • This is the area of Biology called ‘population genetics’ Population Genetics • The study of genetic variation in populations. • Represents the reconciliation of Mendelism and Darwinism. • Modern Synthesis uses population genetics as the means to track and study evolution • Looks at the genetic basis of variation and natural selection Population • A localized group of individuals of the same species. Species • A group of similar organisms. • A group of populations that could interbreed. Gene Pool • The total aggregate of genes in a population. • If evolution is occurring, then changes must occur in the gene pool of the population over time. Microevolution • Changes in the relative frequencies of alleles in the gene pool. Overview • A common misconception is that organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but only populations evolve • Individuals are selected; populations evolve! The Smallest Unit of Evolution • Genetic variations in populations contribute to evolution • Microevolution is a change in allele frequencies in a population over generations Is this finch evolving by natural selection? Concept: Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Genetic Variation • Variation in individual genotype leads to variation in individual phenotype • Not all phenotypic variation is heritable • Natural selection can only act on variation with a genetic component Nonheritable variation? Nonheritable variation? 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 • Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels • Average heterozygosity measures the average percent of loci that are heterozygous in a population • Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals Variation Between Populations • Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups Geographic variation in isolated mouse populations on Madeira Fig. 23-5 Porcupine herd MAP AREA Beaufort Sea Porcupine herd range Fortymile herd range Fortymile herd 1.0 0.8 0.6 Maine Cold (6°C) 0.4 Georgia Warm (21°C) 0.2 0 46 44 42 40 38 36 Latitude (°N) 34 32 30 A cline determined by temperature Mutation • Mutations are changes in the nucleotide sequence of DNA • Mutations cause new genes and alleles to arise • Only mutations in cells that produce gametes can be passed to offspring Point mutations • A point mutation is a change in one base in a gene • The effects of point mutations can vary: –Mutations in noncoding regions of DNA are often harmless –Mutations in a gene might not affect protein production because of redundancy in the genetic code Point mutations • The effects of point mutations can vary: –Mutations that result in a change in protein production are often harmful –Mutations that result in a change in protein production can sometimes increase the fit between organism and environment Mutations That Alter Gene Number or Sequence • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Duplication of large chromosome segments is usually harmful Mutations That Alter Gene Number or Sequence • Duplication of small pieces of DNA is sometimes less harmful and increases the genome size • Duplicated genes can take on new functions by further mutation Hardy-Weinburg Hardy-Weinberg Theorem • Developed in 1908. • Mathematical model of gene pool changes over time. • The frequency of an allele in a population can be calculated –For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 –The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles • By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies • The frequency of all alleles in a population will add up to 1 –For example, p + q = 1 Basic Equation •p + q = 1 • p = % dominant allele • q = % recessive allele Expanded Equation • p+q=1 • (p + q)2 = (1)2 • p2 + 2pq + q2 = 1 Genotypes • p2 = Homozygous Dominants 2pq = Heterozygous q2 = Homozygous Recessives • The Hardy-Weinberg principle describes a population that is not evolving • If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving • The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation • In a given population where gametes contribute to the next generation randomly, allele frequencies will not change • Mendelian inheritance preserves genetic variation in a population Fig. 23-6 Alleles in the population Frequencies of alleles p = frequency of CR allele = 0.8 q = frequency of CW allele = 0.2 Gametes produced Each egg: 80% chance 20% chance Each sperm: 80% chance Selecting alleles at random from a gene pool 20% chance • Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool • 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 • -where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over time • Natural populations can evolve at some loci, while being in HardyWeinberg equilibrium at other loci • The five conditions for nonevolving populations are rarely met in nature: –No mutations –Random mating –No natural selection –Extremely large population size –No gene flow Example Calculation • Let’s look at a population where: – A = red flowers – a = white flowers Starting Population • • • • N = 500 Red = 480 (320 AA+ 160 Aa) White = 20 Total alleles = 2 x 500 = 1000 Dominant Allele • A = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80% Recessive Allele • a = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20% A and a in HW equation • Cross: Aa X Aa • Result = AA + 2Aa + aa • Remember: A = p, a = q Substitute the values for A and a • p2 + 2pq + q2 = 1 (.8)2 + 2(.8)(.2) + (.2)2 = 1 .64 + .32 + .04 = 1 Dominant Allele • A = p2 + pq = .64 + .16 = .80 = 80% Recessive Allele • a = pq + q2 = .16 + .04 = .20 = 20% Importance of Hardy-Weinberg • Yardstick to measure rates of evolution. • Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur). • Way to calculate gene frequencies through time. Example • What is the frequency of the PKU allele? • PKU is expressed only if the individual is homozygous recessive (aa). Applying the Hardy-Weinberg Principle • We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: –The PKU gene mutation rate is low –Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele –Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions –The population is large –Migration has no effect as many other populations have similar allele frequencies • The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 q = 0.01 • The frequency of normal alleles is p = 1 – q = 1 – 0.01 = 0.99 • The frequency of carriers is 2pq = 2 x 0.99 x 0.01 = 0.0198 –or approximately 2% of the U.S. population PKU Frequency • PKU is found at the rate of 1/10,000 births. • PKU = aa = q2 q2 = .0001 q = .01 Dominant Allele • p+q=1 p = 1- q p = 1- .01 p = .99 Expanded Equation • p2 + 2pq + q2 = 1 (.99)2 + 2(.99x.01) + (.01)2 = 1 .9801 + .0198 + .0001 = 1 Final Results • Normals (AA) = 98.01% • Carriers (Aa) = 1.98% • PKU (aa) = .01% Result • Gene pool is in a state of equilibrium and has not changed because of sexual reproduction. • No Evolution has occurred. AP Problems Using HardyWeinberg • • • • Solve for q2 (% of total). Solve for q (equation). Solve for p (1- q). H-W is always on the national AP Bio exam (but no calculators are allowed). Remember Hardy-Weinberg Assumptions 1. Large Population 2. Isolation 3. No Net Mutations 4. Random Mating 5. No Natural Selection If H-W assumptions hold true: • The gene frequencies will not change over time. • Evolution will not occur. • But, how likely will natural populations hold to the H-W assumptions? Microevolution • Caused by violations of the 5 H-W assumptions. Causes of Microevolution 1. Genetic Drift 2. Gene Flow 3. Mutations 4. Nonrandom Mating 5. Natural Selection Genetic Drift • Changes in the gene pool of a small population by chance. • Types: –1. Bottleneck Effect –2. Founder's Effect • The smaller a sample, the greater the chance of deviation from a predicted result Genetic Drift • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next • Genetic drift tends to reduce genetic variation through losses of alleles Fig. 23-8-1 Genetic Drift CR CR CR CR CR CW CR CR CW CW CR CW CR CR CR CW CR CR CR CW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3 Fig. 23-8-2 Genetic Drift CR CR CR CR CW CW CR CW CR CW CR CR CW CW CW CW CR CR CR CW CR CW CR CR CR CR CR CR CR CW CR CW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3 CW CW CR CW CR CR CR CW Generation 2 p = 0.5 q = 0.5 Fig. 23-8-3 Genetic Drift CR CR CR CR CW CW CR CW CR CW CR CR CW CW CR CR CR CW CR CR CR CW CR CW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3 CW CW CR CW CR CR CR CR CR CR CW CW CR CR CR CW CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR CW Generation 2 p = 0.5 q = 0.5 CR CR CR CR Generation 3 p = 1.0 q = 0.0 Genetic Drift By Chance Bottleneck Effect Loss of most of the population by disasters. Bottleneck Effect • The bottleneck effect is a sudden reduction in population size due to a disaster or a change in the environment • Surviving population may have a different gene pool than the original population, it may no longer be reflective of the original population’s gene pool • If the population remains small, it may be further affected by genetic drift • Understanding the bottleneck effect can increase understanding of how human activity affects other species Case Study: Impact of Genetic Drift on the Greater Prairie Chicken • Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois • The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Fig. 23-10a Prebottleneck (Illinois, 1820) (a) Range of greater prairie chicken Postbottleneck (Illinois, 1993) Fig. 23-10b Location Population size Number Percentage of alleles of eggs per locus hatched Illinois 1000–25,000 5.2 93 <50 3.7 <50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 Minnesota, 1998 (no bottleneck) 4,000 5.3 85 1930–1960s 1993 (b) Prairie Chicken • Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck • The results showed a loss of alleles at several loci • Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90% Importance • Reduction of population size may reduce gene pool for evolution to work with. • Ex: Cheetahs Founder's Effect • The founder effect occurs when a few individuals become isolated from a larger population • Allele frequencies in the small founder population can be different from those in the larger parent population Founder's Effect • Genetic drift in a new colony that separates from a parent population. • Examples: • Old-Order Amish • A butterfly that gets blown to an island and lays her eggs Result of Bottleneck and Founder’s effects • Genetic variation reduced. • Some alleles increase in frequency while others are lost (as compared to the parent population). • Very common in islands and other groups that don't interbreed Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed Gene Flow • Gene flow consists of the movement of alleles among populations (in or out of a population) –Immigration –Emigration • Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Gene Flow • Gene flow tends to reduce differences between populations over time • Gene flow is more likely than mutation to alter allele frequencies directly Fig. 23-11 • Gene flow can decrease the fitness of a population • In Bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils • Windblown pollen moves these alleles between populations • The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment Fig. 23-12a 7 NON0 MINE 60 SOIL MINE SOIL 50 NONMINE SOIL Prevailing wind direction 40 30 20 10 0 20 0 20 0 20 40 60 Distance from mine edge (meters) 80 10 0 120 140 160 Fig. 23-12b Bent grass • Gene flow can increase the fitness of a population • Insecticides have been used to target mosquitoes that carry West Nile virus and malaria • Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes • The flow of insecticide resistance alleles into a population can cause an increase in fitness Result • Changes in gene frequencies within a population. • Immigration often brings new alleles into populations increasing genetic diversity. Mutations • Inherited changes in a gene. Result of mutations • May change gene frequencies (small population). • Source of new alleles for selection. • Often lost by genetic drift. Nonrandom Mating • Failure to choose mates at random from the population. • Inbreeding within the same “neighborhood”. • Assortative mating (like with like). • Results in increases in the number of homozygous loci. • Does not in itself alter the overall gene frequencies in the population. Sexual Mate selection • Sexual selection is natural selection for mating success • May not be adaptive to the environment, but increases reproduction success of the individual. Sexual Mate selection • This is a VERY important selection type for species. • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Fig. 23-15 Sexual dimorphism • Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex • Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates • Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival • How do female preferences evolve? • The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for Fig. 23-16a EXPERIMENT Female gray tree frog SC male gray tree frog LC male gray tree frog SC sperm Eggs LC sperm Offspring of Offspring of LC father SC father Fitness of these half-sibling offspring compared Fig. 23-16b RESULTS Fitness Measure 1995 1996 Larval growth NSD LC better Larval survival LC better NSD Time to metamorphosis LC better (shorter) LC better (shorter) NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males. Result • Sexual dimorphism. • Secondary sexual features for attracting mates. Comment • Females may drive sexual selection and dimorphism since they often "choose" the mate. Natural Selection • Differential success in survival and reproduction. • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Natural Selection • Only natural selection consistently results in adaptive evolution • Natural selection brings about adaptive evolution by acting on an organism’s phenotype • Result - Shifts in gene frequencies. Comment • As the Environment changes, so does Natural Selection and Gene Frequencies. Result • If the environment is "patchy", the population may have many different local populations. The Key Role of Natural Selection in Adaptive Evolution • Natural selection increases the frequencies of alleles that enhance survival and reproduction • Adaptive evolution occurs as the match between an organism and its environment increases Fig. 23-14a Color-changing ability in cuttlefish Fig. 23-14b Movable bones Movable jaw bones in snakes • Because the environment can change, adaptive evolution is a continuous process • Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment Genetic Basis of Variation 1. Discrete Characters – Mendelian traits with clear phenotypes. 2. Quantitative Characters – Multigene traits with overlapping phenotypes. Polymorphism • The existence of several contrasting forms of the species in a population. • Usually inherited as Discrete Characteristics. Examples of Polymorphism Garter Snakes Gaillardia Human Example • ABO Blood Groups • Morphs = A, B, AB, O Other examples Quantitative Characters • Allow continuous variation in the population. • Result – –Geographical Variation –Clines: a change along a geographical axis Yarrow and Altitude Sources of Genetic Variation • Mutations. • Recombination though sexual reproduction. –Crossing-over –Random fertilization The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population 1. Diploidy - preserves recessives as heterozygotes. 2. Balanced Polymorphisms preservation of diversity by natural selection. Diploidy • Diploidy maintains genetic variation in the form of hidden recessive alleles Heterozygote Advantage • Heterozygote Advantage - When the heterozygote or hybrid survives better (have a higher fitness) than the homozygotes. Also called Hybrid vigor. • Natural selection will tend to maintain two or more alleles at that locus Heterozygote Advantage • Can't bred "true“ and the diversity of the population is maintained. • Example of hybrid vigor – Sickle Cell Anemia • The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance Fig. 23-17 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Frequency-Dependent Selection • In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population • Selection can favor whichever phenotype is less common in a population Fig. 23-18a “Right-mouthed” “Left-mouthed” Fig. 23-18 “Right-mouthed” 1.0 “Left-mouthed” 0. 5 0 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year Comment • Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time. • Ex – Sickle Cell and Malaria, TaySachs and Tuberculosis Fitness - Darwinian • The relative contribution an individual makes to the gene pool of the next generation. Relative Fitness • Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Selection favors certain genotypes by acting on the phenotypes of certain organisms Relative Fitness • Contribution of one genotype to the next generation compared to other genotypes. • The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals • Reproductive success is generally more subtle and depends on many factors Rate of Selection • Differs between dominant and recessive alleles. • Selection pressure by the environment. Modes of Selection • Three modes of selection: –Directional selection favors individuals at one end of the phenotypic range –Disruptive selection favors individuals at both extremes of the phenotypic range –Stabilizing selection favors intermediate variants and acts against extreme phenotypes Stabilizing • Selection toward the average and against the extremes. • Ex: birth weight in humans Directional Selection • Selection toward one extreme. • Ex: running speeds in race animals. • Ex. Galapagos Finch beak size and food source. Diversifying • Selection toward both extremes and against the norm. • Ex: bill size in birds Comment • Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed. Fig. 23-13 Original population Original population Evolved population (a) Directional selection Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection Balancing Selection • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Neutral Variation • Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage • For example, –Variation in noncoding regions of DNA –Variation in proteins that have little effect on protein function or reproductive fitness Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact Fig. 23-19 Question • Does evolution result in perfect organisms? Answer - No 1. Historical Constraints 2. Compromises 3. Non-adaptive Evolution (chance) 4. Available variations – most come from using a current gene in a new way. Summary • Know the difference between a species and a population. • Know that the unit of evolution is the population and not the individual. Summary • Know the H-W equations and how to use them in calculations. • Know the H-W assumptions and what happens if each is violated. Summary • Identify various means to introduce genetic variation into populations. • Know the various types of natural selection. You should now be able to: 1. Explain why the majority of point mutations are harmless 2. Explain how sexual recombination generates genetic variability 3. Define the terms population, species, gene pool, relative fitness, and neutral variation 4. List the five conditions of HardyWeinberg equilibrium 5. Apply the Hardy-Weinberg equation to a population genetics problem 6. Explain why natural selection is the only mechanism that consistently produces adaptive change 7. Explain the role of population size in genetic drift 8. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection 9. List four reasons why natural selection cannot produce perfect organisms End of Chapter 23!