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In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how
Ch 22/23 Warm-up List different pieces of evidence for evolution (Review) What are the ways that sexual reproduction produces genetic diversity? What is thing you are grateful for today? Ch 23 Warm-up In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb) a) The allele frequencies of this population are: B allele: _ b allele: _ b) The genotype frequencies are: BB: _ Bb: _ bb: _ Use the above info to determine the genotype frequencies of the next generation: B (p): _ b (q): _ BB (p2): _ Bb (2pq): _ bb (q2): _ 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 Crossing Over (Meiosis – Prophase I) Independent Assortment of Chromosomes (during meiosis) 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: alleles for a gene (homozygous/heterozygous) • Fixed allele: all members of a population only have 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 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? Natural selection can alter frequency distribution of heritable traits in ways: 1.Directional selection 2.Disruptive (diversifying) selection 3.Stabilizing selection Directional Selection: eg larger black bears survive extreme cold better than small ones Disruptive Selection: eg small beaks for small seeds; large beaks for large seeds Stabilizing Selection: eg narrow range of human birth weight Sexual selection • Form of natural selection – certain individuals more likely to obtain mates • Sexual dimorphism: difference between 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: Diploidy hide recessive alleles that are less favorable • Heterozygote advantage: advantage greater fitness than homozygotes • eg Sickle cell disease HHMI Video: Natural Selection in Humans Running Time: 14:03 Natural selection cannot fashion perfect organisms Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises 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 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 ... 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. .. 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... 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