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I. Genetic Variation and Evolution Genes within Populations Darwin: Evolution is descent with modification I. The Essence of Evolution II. Hardy-Weinberg III. Causes of Evolutionary Change IV. Natural Selection V. Forms of Selection VI. Adaptation VII.Constraints Evolution: Change through time 1. Species accumulate differences 2. Descendants differ from their ancestors 3. New species arise from existing ones Gene Pools, Alleles and Allele Frequency Gene pool: All the possible alleles for a particular gene (or all the genes) within a given population Allele Allele B b Number of 150 50 alleles Gene Pool 200 alleles Allele frequency 0.75 0.25 II. Hardy-Weinberg Principle Under certain conditions: allele and genotype frequencies in a population: will remain constant over time No evolution Æ equilibrium population How Many Natural Populations are in Genetic Equilibrium? So why use Hardy-Weinberg? 1) Provides a useful starting point for studying the mechanisms of Evolution by establishing a baseline to compare change 2) Helps identify forces of evolution. II. Hardy-Weinberg Principle p = dominant allele frequency q = recessive allele frequency Parental Population p+q=1 (p + q)2 = p2 + 2pq + q2 Offspring Genotype frequencies calculated from allele frequencies p2 + Homozygous Dominant 2pq Heterozygous q2 + Homozygous Recessive II. Hardy-Weinberg Principle Forces of Evolution Hardy-Weinberg Equilibrium applies if and only if: 1. no mutation 2. no gene flow 3. no genetic drift 4. random mating 5. no natural selection Forces of Evolution – Mutation Mutation: Grain Genomes are Rearranged Chromosome segments DNA Mutations Point mutations alter a single base. Base substitution, Insertion or Deletion Chromosome mutations Deletions – part of chromosome is lost Duplication – part of chromosome is copied Inversion – part of chromosome in reverse order Translocation – part of chromosome moved to new location Mutation Frequency How often? 1 in 100,000 cell divisions 1 in 50 million base pairs 1 in a million gametes Bacterial genome ≈ 5000 genes X 200 bacteria = 1,000,000 genes per 200 bacteria 1 mutation in every 1,000,000 genes 1 teaspoon of soil ≈ 1 billion bacteria 1 billion bacterial ÷ 200bacteria/1 mutation = 5 million mutations in 1 teaspoon of soil Forces of Evolution – Gene Flow Gene flow = migration between populations – – populations exchange genetic material can change allele frequencies by altering the gene pool Other examples of gene flow 1. Bacterial transformation from plasmid transfer 2. Virus and plasmid vectors in recombinant DNA technology 3. People Mutation - Summary 1. Mutation is the most basic force of evolution change in DNA sequence change in Chromosome organization 2. Ultimate source of new alleles 3. Mutation is fairly rare, but…rate of cell division and population size are important factors Forces of Evolution – Genetic Drift Æ Frequencies of particular alleles change by chance alone. A. Population bottlenecks Populations reduced to small # then recover Genetic bottleneck results in Loss of genetic variation Reduced capacity to evolve Founders Northern elephant seal 1920 ~ 20 animals 2007 ~ 130,000 animals Forces of Evolution – Non-Random Mating Mating of phenotypically similar individuals Positive Assortative mating Inbreeding Increases proportions of homozygotes Decreases phenotypic variation Forces of Evolution – Natural Selection Ancestral population Variation in offspring Negative Assortative mating Increases phenotypic variation Forces of Evolution – Natural Selection Survival of the fittest Natural Selection and Adaptation Other elements of natural selection 1. Two measures of fitness survival to reproductive maturity passing alleles on to the next generation 2. Interaction between phenotype and environment 3. Produces adaptive evolutionary change Peppered Moths 4. Allele frequencies change over time, not individuals 20 100 15 70 50 30 20 10 10 7 5 5 3 2 2 3 4 5 6 7 8 9 10 Birth weight in pounds Percent infant mortality Æ The second half of the twentieth century saw widespread implementation of pollution controls, thus trends appear to be reversing and light colored moths may again dominate. Forms of Selection: Stabilizing Selection Percent of births in population Peppered Moths and Industrialized Melanism Natural Selection leads to Adaptation Forms of Natural Selection Favors one extreme Favors average Favors both extremes Camouflage – one result of natural selection Flower mantid Leaf mantid in Costa Rica Constraints on Evolutionary Perfection Æ Evolution requires genetic variation Mutation is random Æ Selection can edit only existing variation Evolution tinkers with existing structures Æ Adaptations are compromises Seal flippers on land and water END Genes Within Populations Æ Chance and selection interact No prediction of future conditions Chapter 22: Evolutionevidence I. Artificial Selection II. Evidence of Natural Selection Fossil Record Comparative Anatomy Homologous structures Vestigial structures Embryology Genetic Analysis Biogeographical Evidence I. Artificial Selection Æ Darwin found evidence for his ideas in artificial selection Artificial selection: • organisms may be modified by controlled breeding • change drastically in short time periods! • examples… A) Plants ÆBrassica oleracea III. Speciation IV. Conclusions “Nothing in biology makes sense except in the light of evolution” --Theodosius Dobzhansky Variations due to artificial selection cabbage, cauliflower, broccoli, kale, Brussel sprouts Artificial Selection in animals II. Evidence of Evolution Change can happen over a very short geological period if the selection pressure is strong. Evolution = descent with modification Æ Macroeolution = large changes Æ Microevolution = small changes Both rely on same mechanisms Primarily natural selection Charles Darwin Observations of finches on the Galapagos Islands Darwin’s Finches: Evidence of Natural Selection Darwin’s: Evidence of Natural Selection “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species has been taken and modified for different ends.” - Darwin Large Ground Finch Cactus Finch Woodpecker Finch 14 species Beak shape variation •Seed crackers •Cactus eaters •Tool users •Vampire 1. More offspring born than survive 2. Populations accumulate differences 3. Descendants differ from their ancestors 4. New species arise from existing ones ÆEvolution is descent with modification II. Evidence - The Fossil Record - Making Fossils Fossils: Preserved “remains” of ancient organisms Æ Bury in sediment Æ Mineralization of organic material Æ Hardening of sediment II. The Fossil Record – Dating Fossils Dating fossils Relative dating – old layers under new Absolute dating – radioactive decay Half-Life examples U-238 Æ 4.5 billion years C-14 Æ 5730 years K-40 Æ 1.25 billion years Fossil Evidence of Evolution II. Evidence - Whale Evolution: A Record of Transition Modern toothed whales Rodhocetus kasrani reduced hind limbs could not walk; swam with up-down motion like modern whales Ambulocetus natans …walked on land …like sea lions …swam by flexing & paddling …like otters Pakicetus attocki Evolutionary change in body size and toe reduction of horses lived on land skull had whale characteristics Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A Brief History of Life Fossil Record: Diversity over the Last 600 Million Years 800 Number of families Billions of Years Ago 600 400 200 0 600 Cambrian (545-490) 500 400 Silurian (438-408) Ordovician (490-438) 300 200 0 100 Carboniferous Triassic Cretaceous (360-280) (248-213) (144-65) Devonian (408-360) Permian Jurassic (280-248) (213-144) Tertiary (65-2) Millions of years ago II. Evidence - Comparative Anatomy II. Evidence - Homologous structures Æ structures that may have different appearances and • Organisms can be grouped (classified) by unique anatomical characteristics • Similar anatomy is found in organisms with greatly divergent functions • Theme = Descent from a common ancestry functions, yet all derived from a common ancestor II. Evidence - Vestigial Structures Molecular Phylogeny DNA and the Hemoglobin Gene Biogeography & Convergent Evolution II. Evidence - Embryology & Comparative Development Comparative Development Reveals Descent from a Common Ancestor II. Evidence - Convergent Evolution III. The Nature of Species Biogeography - study of geographic distribution of species. 1. Interbreeding population 2. Potentially interbreeding population 3. Inherent willingness to interbreed 4. Production of viable, fertile offspring How does a new species appear? Convergent evolution - acquisition of the same biological trait in unrelated lineages. Biological Species Isolating Mechanisms - Geographic - Ecological - Behavioral - Temporal - Mechanical - Gamete fusion - Postzygotic Adaptive Radiation and Alpine Buttercup Speciation Snowfield Snowline fringe Stony fringe Sheltered Boggy New Zealand buttercups – 14 alpine species. Isolation and Speciation Lions and tigers are ecologically isolated IV. Conclusions Æ Support for Evolution comes from – Fossils, Order of Appearance, Relative and Absolute Dates – Anatomical Classification • Homologous Structures • Vestigial Structures – – – – IV. Conclusions Evolution – Both a Fact and a Theory • That evolution has occurred (and is currently occurring) is NOT in scientific debate now; it has been settled. • FACT – Biodiversity on Earth is a result of evolution from a common ancestor. • FACT – Evolution continues today. • THEORY – The intricacies of evolutionary change and mechanisms driving evolution. Embryology/Development Genetic Analysis/Molecular Phylogeny Convergence Speciation Æ based on isolating mechanisms END Evolution