THE EVOLUTION OF POPULATIONS CHAPTER 23 WHAT YOU MUST KNOW: • How mutation and sexual reproduction each produce genetic variation. • The conditions for Hardy-Weinberg equilibrium. • How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving. SMALLEST UNIT OF EVOLUTION Microevolution: change in the allele frequencies of a population over generations • Darwin did not know how organisms passed traits to offspring • 1866 - Mendel published his paper on genetics • Mendelian genetics supports Darwin’s theory Evolution is based on genetic variation SOURCES OF GENETIC VARIATION • Point mutations: changes in one base (eg. sickle cell) • Chromosomal mutations: delete, duplicate, disrupt, rearrange usually harmful • Sexual recombination: contributes to most of genetic variation in a population 1.Crossing Over (Meiosis – Prophase I) 2.Independent Assortment of Chromosomes (during meiosis) 3.Random Fertilization (sperm + egg) Population genetics: study of how populations change genetically over time Population: group of individuals that live in the same area and interbreed, producing fertile offspring • Gene pool: all of the alleles for all genes in all the members of the population • Diploid species: 2 alleles for a gene (homozygous/heterozygous) • Fixed allele: all members of a population only have 1 allele for a particular trait • The more fixed alleles a population has, the LOWER the species’ diversity HARDY-WEINBERG PRINCIPLE Hardy-Weinberg Principle: The allele and genotype frequencies of a population will remain constant from generation to generation …UNLESS they are acted upon by forces other than Mendelian segregation and recombination of alleles Equilibrium = allele and genotype frequencies remain constant CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM 1. No mutations. 2. Random mating. 3. No natural selection. 4. Extremely large population size. 5. No gene flow. If at least one of these conditions is NOT met, then the population is EVOLVING! Hardy-Weinberg Principle Allele Frequencies: • Gene with 2 alleles : p, q p = frequency of dominant allele (A) q = frequency of recessive allele (a) Note: p + q = 1 1 – p = q 1 – q = p Hardy-Weinberg Equation Genotypic Frequencies: • 3 genotypes (AA, Aa, aa) p2 + 2pq + q2 = 1 p2 = AA (homozygous dominant) 2pq = Aa (heterozygous) q2 = aa (homozygous recessive) ALLELE FREQUENCIES GENOTYPIC FREQUENCIES STRATEGIES FOR SOLVING H-W PROBLEMS: 1. If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles. 2. If you know phenotypes, then use “aa” to find q2, and then q. (p = 1-q) 3. Use p2 + 2pq + q2 to find genotype frequencies. 4. If p and q are not constant from generation to generation, then the POPULATION IS EVOLVING! HARDY-WEINBERG PRACTICE PROBLEM #1 The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths. AA = 1469 Aa = 138 aa = 5 Allele Frequencies: A = a = Genotypic Frequencies: AA = Aa = aa = HARDY-WEINBERG PRACTICE PROBLEM #2: PTC TASTERS • Taster = AA or Aa Nontaster = aa • Tasters = ____ Nontasters = ___ q2 = q = p + q = 1 p = 1 – q = p2 + 2pq + q2 = 1 CAUSES OF EVOLUTION CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM 1. No mutations. 2. Random mating. 3. No natural selection. 4. Extremely large population size. 5. No gene flow. If at least one of these conditions is NOT met, then the population is EVOLVING! Minor Causes of Evolution: #1 - Mutations • Rare, very small changes in allele frequencies #2 - Nonrandom mating • Affect genotypes, but not allele frequencies Major Causes of Evolution: • Natural selection, genetic drift, gene flow (#3-5) MAJOR CAUSES OF EVOLUTION #3 – Natural Selection • Individuals with variations better suited to environment pass more alleles to next generation MAJOR CAUSES OF EVOLUTION #4 – Genetic Drift • Small populations have greater chance of fluctuations in allele frequencies from one generation to another • Examples: • Founder Effect • Bottleneck Effect Genetic Drift FOUNDER EFFECT • A few individuals isolated from larger population • Certain alleles under/over represented Polydactyly in Amish population BOTTLENECK EFFECT • Sudden change in environment drastically reduces population size Northern elephant seals hunted nearly to extinction in California MAJOR CAUSES OF EVOLUTION #5 – Gene Flow • Movement of fertile individuals between populations • Gain/lose alleles • Reduce genetic differences between populations HOW DOES NATURAL SELECTION BRING ABOUT ADAPTIVE EVOLUTION? Fitness : the contribution an individual makes to the gene pool of the next generation Natural selection can alter frequency distribution of heritable traits in 3 ways: 1. Directional selection 2. Disruptive (diversifying) selection 3. Stabilizing selection Directional Selection: Disruptive Selection: Stabilizing Selection: eg. larger black bears eg. small beaks for eg. narrow range of survive extreme cold small seeds; large human birth weight better than small ones beaks for large seeds SEXUAL SELECTION • Form of natural selection – certain individuals more likely to obtain mates • Sexual dimorphism: difference between 2 sexes • Size, color, ornamentation, behavior SEXUAL SELECTION • Intrasexual – selection within same sex (eg. M compete with other M) • Intersexual – mate choice (eg. F choose showy M) PRESERVING GENETIC VARIATION • Diploidy: hide recessive alleles that are less favorable • Heterozygote advantage: greater fitness than homozygotes • eg. Sickle cell disease HHMI VIDEO: NATURAL SELECTION IN HUMANS RUNNING TIME: 14:03 MIN 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. SAMPLE PROBLEM Define the following examples as directional, disruptive, or stabilizing selection: a) Tiger cubs usually weigh 2-3 lbs. at birth b) Butterflies in 2 different colors each represent a species distasteful to birds c) Brightly colored birds mate more frequently than drab birds of same species d) Fossil evidence of horse size increasing over time.