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The Evolution of Population Figure 23.2 Average beak depth (mm) Natural selection acts on individuals within a population, population evolve!!! 10 9 8 0 1978 1976 (similar to the (after prior 3 years) drought) • Microevolution is a change in allele frequencies in a population over generations • Three mechanisms cause allele frequency change: – Natural selection – Genetic drift – Gene flow • Only natural selection causes adaptive evolution © 2011 Pearson Education, Inc. Driving Force of Evolution: Genetic Variation • Mutation, mutation, mutation!! • Variation in heritable traits is a prerequisite for evolution – Result of variations in DNA sequence – Cause variations in phenotype – Natural selection acts on phenotype © 2011 Pearson Education, Inc. Variation Between Populations 1 2.4 8.11 9.12 3.14 5.18 10.16 13.17 6 7.15 19 XX 1 2.19 3.8 4.16 5.14 9.10 11.12 13.17 15.18 6.7 XX Cline: a graded change in a trait along a Ldh-Bb allele frequency geographic axis as a result of natural selection © 2011 Pearson Education, Inc. Latitude (ºN) Maine Cold (6°C) Georgia Warm (21ºC) Formation of New Alleles • Mutation can cause a change in an allele • Only mutations in cells that produce gametes can be passed to offspring © 2011 Pearson Education, Inc. Three mechanisms for shuffling alleles • Point mutation • Chromosomal mutation • Sexual reproduction – Crossing over – Independent assortment – fertilization Rapid Reproduction • The average rate of eukaryotic mutation is about one in every 100,000 genes per generation • Mutations rates are often lower in prokaryotes and higher in viruses © 2011 Pearson Education, Inc. The Hardy-Weinberg equation can be used to test whether a population is evolving key terminology: • Population: a localized group of individuals capable of interbreeding and producing fertile offspring • Gene pool: all the alleles for all loci in a population – A locus is fixed if all individuals in a population are homozygous for the same allele © 2011 Pearson Education, Inc. MAP AREA CANADA ALASKA Figure 23.6 Beaufort Sea Porcupine herd range Porcupine herd Fortymile herd range Fortymile herd Hardy-Weinberg Equation Allelic Frequency: the percentage of the frequency of an allele at a locus in a population -The frequency of all alleles in a population will add up to 1 -For example, p + q = 1 Practice Problem: consider a population of wildflowers that is incompletely dominant for color: 320 red flowers (CRCR) 160 pink flowers (CRCW) 20 white flowers (CWCW) • Calculate the number of copies of each allele: – CR (320 2) 160 800 – CW (20 2) 160 200 • To calculate the frequency of each allele: – p freq CR 800 / (800 200) 0.8 – q freq CW 200 / (800 200) 0.2 • The sum of alleles is always 1 – 0.8 0.2 1 © 2011 Pearson Education, Inc. 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 Alleles in the population Gametes produced Frequencies of alleles p = frequency of CR allele = 0.8 Each egg: Each sperm: q = frequency of CW allele = 0.2 20% 80% chance chance 20% 80% chance chance • Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool • Consider, for example, the same population of 500 wildflowers and 100 alleles where – p freq CR 0.8 – q freq CW 0.2 • The frequency of genotypes can be calculated – CRCR p2 (0.8)2 0.64 – CRCW 2pq 2(0.8)(0.2) 0.32 – CWCW q2 (0.2)2 0.04 • The frequency of genotypes can be confirmed using a Punnett square © 2011 Pearson Education, Inc. Figure 23.8b Sperm CR (80%) CW (20%) CR (80%) 64% (p2) CRCR Eggs CW 16% (pq) CRCW 4% (q2) CWCW 16% (qp) CRCW (20%) 64% CRCR, 32% CRCW, and 4% CWCW Gametes of this generation: 64% CR (from CRCR plants) R + 16% C R W (from C C plants) = 80% CR = 0.8 = p 4% CW (from CWCW plants) W + 16% C R W (from C C plants) = 20% CW = 0.2 = q Genotypes in the next generation: 64% CRCR, 32% CRCW, and 4% CWCW plants Remember that the Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation © 2011 Pearson Education, Inc. • Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci © 2011 Pearson Education, Inc. Mechanisms that alters allele frequencies: © 2011 Pearson Education, Inc. Figure 23.9-1 Genetic drift CRCR CRCR CRCW CWCW CRCR CRCW CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Figure 23.9-2 Genetic drift CRCR CRCR CRCW CWCW 5 plants leave offspring CRCR CWCW CRCW CRCR CWCW CRCR CRCW CRCW CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCW CRCR CRCW Generation 2 p = 0.5 q = 0.5 Figure 23.9-3 Genetic drift CRCR CRCR CRCW CWCW 5 plants leave offspring CRCR CWCW CRCW CRCR CWCW CRCR CRCW CRCW CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCW 2 plants leave offspring CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW Generation 2 p = 0.5 q = 0.5 CRCR CRCR CRCR CRCR Generation 3 p = 1.0 q = 0.0 The Founder Effect © 2011 Pearson Education, Inc. Figure 23.10-1 The Bottleneck Effect Original population Figure 23.10-2 The Bottleneck Effect Original population Bottlenecking event Figure 23.10-3 The Bottleneck Effect Original population Bottlenecking event Surviving population 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 © 2011 Pearson Education, Inc. Figure 23.11 Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken (a) Location Illinois 1930–1960s 1993 Population size Percentage Number of alleles of eggs per locus hatched 1,000–25,000 <50 5.2 3.7 93 <50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 (b) 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 © 2011 Pearson Education, Inc. Gene Flow 60 Survival rate (%) 50 Population in which the surviving females eventually bred Central Eastern Central population NORTH SEA Eastern population Vlieland, the Netherlands 40 2 km 30 20 10 0 Females born in central population Females born in eastern population Parus major • Gene flow can increase the fitness of a population • Consider, for example, the spread of alleles for resistance to insecticides – 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 © 2011 Pearson Education, Inc. Natural selection is the only mechanism that consistently causes adaptive evolution Evolution by natural selection involves both change and “sorting” -New genetic variations arise by chance -Beneficial alleles are “sorted” and favored by natural selection © 2011 Pearson Education, Inc. Original population Evolved population (a) Directional selection Frequency of individuals Directional, Disruptive, and Stabilizing Selection Original population Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection Figure 23.15 Sexual Selection Preserving genetic variation in a population: • Diploidy maintains genetic variation – Heterozygotes carry recessive alleles hidden from the effects of selection • Balancing selection: natural selection maintains stable frequencies of two or more phenotypic forms in a population – Heterozygote advantage – Frequency-dependent selection © 2011 Pearson Education, Inc. Figure 23.17 Key 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% Figure 23.18 “Left-mouthed” P. microlepis Frequency of “left-mouthed” individuals 1.0 “Right-mouthed” P. microlepis 0.5 0 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year Why Natural Selection Cannot Fashion Perfect Organisms 1. 2. 3. 4. Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact © 2011 Pearson Education, Inc.