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Evolution A Scientific Explanation for Similarities and Differences Between Species Evolution • Evolution = progressive change in characteristics of organisms as a result of changes in genetic composition • Two important aspects – Descent from a common ancestor – Adaptation to the environment • Adaptation = characteristic that makes it more likely that an organism will survive and reproduce in its environment A Flowchart of Evolutionary Reasoning Potential for rapid reproduction Competition for survival and reproduction (1) Struggle for existence Formation of new genotypes leads to phenotypic variation Relatively constant resources and population over time Variability in structures and behaviors NATURAL SELECTION On average, the fittest organisms leave the most offspring (2) Some variability is inherited; adaptations increase in future generations (observations) (conclusions) Survival of the fittest EVOLUTION: The genetic makeup of the population changes over time, driven by natural selection (3) Adaptation An increase in frequency of genotypes that confer a favorable advantage in a given environment. Natural Selection As the Mechanism for Evolution Applying Your Knowledge 1. Adaptation 2. Evolution 3. Natural selection A. The mechanism for evolution is B. A progressive change in the characteristics of organisms is C. A trait that makes a species survival more likely is called a(n) Evidence for Common Descent From the Fossil Record Progressive changes from simpler to more complex organisms can be seen in the fossil record. Biogeographical Evidence for Common Descent Different island species resemble each other. Biogeographical Evidence for Common Descent Island Populations resemble those on nearby land. The Galapagos finches resembled the grassquit found on the coast of Ecuador. Anatomical Evidence for Common Descent: Homologous Structures Flying Swimming Running Grasping Anatomical Evidence: Vestigial Structures Remnants of hindlimb seen in boa and whale Functional hindlimb in salamander Evidence for Common Descent from Biochemistry Evidence for Evolution from Biochemistry Similarities in sequence measured by ease of separating DNA strands by heat Evidence for Evolution: Genetics • Mutation generates diversity • Meiosis and Fertilization generate new combinations due to – Crossing Over – Alternate patterns of chromosome segregation – Unique genotype of fertilizing sperm combined with unique genotype of egg Two Types of Evolution Microevolution Macroevolution Change within a population or species Change to a new species Microevolution led to an increase in darkwinged Pepper Moths in industrial regions of Britain. Evolution As a Change in Genotypes Individuals carrying the S’ allele were more likely to survive when malaria is the selecting agent. Genotype SS SS’ S’S’ Phenotype No disease Susceptible to Malaria Sickle Cell Trait (mild symptoms) Resistant to Malaria Sickle Cell Anemia Die from anemia Malaria as an Agent of Natural Selection SS X S’S’ X SS’ Malaria Eliminates SS Remaining Genotypes SS’ SS X S’S’ X SS’ SS X SS’ S’S’ X Anemia Eliminates S’S’ SS’ SS’ Hardy-Weinberg Equilibrium • A condition where allele frequencies and genotypic frequencies remain constant from generation to generation • Changes from equilibrium values are used to determine if natural selection is occurring Hardy-Weinberg Equilibrium Allelic Frequencies p+q=1 p = frequency of dominant allele (eg. A) q = frequency of recessive allele (eg. a) Genotypic Frequencies p2 + 2pq + q2 = 1 p2= freq. of homozygous dominants (AA) q2= freq. of homozygous recessives (aa) 2pq = frequency of heterozygotes (Aa) Large population size Random mating No migration No mutation No selection Conditions Example Using Hardy-Weinberg Equilibrium • If the frequency of albinos in a population is 9%, what is the frequency of AA and Aa genotypes? • Let A = allele for normal skin pigmentation • Let a = allele for albinism Frequency of Albinos q2 0.09 q q2 0.3 p 1 q 0.7 .7A .3a .7A .49AA .21Aa .3a .21Aa .09aa Frequency of AA p 2 0.49 Frequency of Aa 2pq 0.42 Frequency of aa q2 0.09 Applying Hardy-Weinberg Equilibrium Values to RFLP Analysis Conditions of Hardy-Weinberg Equilibrium Condition Non-equilibrium Condition Large Population Size Genetic Drift: Changes in allele frequency due to small population sizes 1. Founder effect 2. Population Bottleneck Conditions of Hardy-Weinberg Equilibrium Condition Non-equilibrium Condition Random Mating Non-random mating: Alters genotypic but not allelic frequencies Conditions of Hardy-Weinberg Equilibrium Condition Non-equilibrium Condition No Migration Migration: Can add new alleles, remove alleles or change allele frequency Leads to Gene Flow between populations Conditions of Hardy-Weinberg Equilibrium Condition Non-equilibrium Condition No Mutation Mutation: Alters allele frequency, causes formation of new genotypes Conditions of Hardy-Weinberg Equilibrium Condition Non-equilibrium Condition No Selection Natural Selection: Increases frequency of genotypes with higher fitness Molecular Evolution Two Hypotheses for the Origin of Modern Humans DNA Analyses Related to Human Origins Visit http://www.dnalc.org/ and choose Genetic Origins Mitochondrial Control Region Media and Animations Solving the Mystery of the Neanderthals Other Applications of DNA Analysis can be found at http://dnai.org Choose Applications Types of Selection • Stabilizing: eliminates extremes Types of Selection • Disruptive: increases both extremes Types of Selection • Directional: increases one extreme Applying Your Knowledge 1. Stabilizing Selection 2. Disruptive Selection 3. Directional Selection Which type of selection has occurred if • The background is sandy with dark rocks and snails are found with either dark or light shell colors? • After spraying with malathion, more fruit flies are found to be resistant to this insecticide? Species Formation • Species = Group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups • Speciation depends on – isolation (lack of gene flow) – genetic divergence Mechanisms for Speciation • Allopatric Speciation – Occurs as a result of geographical isolation – Most common mechanism • Sympatric Speciation – Occurs in the same location – Can be due to ecological isolation – Can be due to Polyploidy • Occurs for plants that have a sudden change in numbers of chromosome sets Allopatric Speciation Single species (white mice); homogeneous habitat (a) Geographical barrier (impassable river); isolated populations (b) (c) (d) Genetic drift; genetic divergence; tan vs. white mice Barrier removed (river dries up); Mice mix but don’t interbreed. Summary of Allopatric Speciation • One group separates from the population. • Separate evolutionary pressures cause different genetic changes in both groups. (Is this (1) microevolution or (2) macroevolution?) • Sufficient genetic changes accumulate so that interbreeding cannot occur if groups are rejoined. (Is this (1) microevolution or (2) macroevolution?) Sympatric Speciation Single species (white mice); homogeneous habitat (a) Climate change; two habitats; isolated because don’t mix (b) (c) (d) Environmental pressure to adapt; genetic divergence; tan vs. white mice Sufficient divergence; now different species Speciation by Polyploidy Diploid with chromosome set A and chromosome set B. Chromosomes duplicate but do not separate Tetraploid with two sets of A and B. Cross between diploid and tetraploid species Triploid with one each of chromosome sets A, B and D. Modern Wheat Chromosomes duplicate but do not separate Hexaploid with three sets of A, B and D. Applying Your Knowledge 1. Sympatric Speciation 2. Speciation by Polyploidy 3. Allopatric Speciation A. Which process involves a sudden, large change in chromosome number? B. Which process requires geographical separation? C. Which process can occur as a result of small differences within the same local environment? Patterns of Evolution • Divergent – different phenotypes arise as related species encounter environmental differences Patterns of Evolution • Convergent – similar phenotypes arise in unrelated species as a result of environmental similarities North American Desert Plants Cactus African Desert Plants Euphorbs Patterns of Evolution • Coevolution: species adjust together to maintain relationship Predators and their Prey Flowering plants and their Pollinators