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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
Competition for survival and reproduction (1) NATURAL SELECTIONOn average, the fittestorganisms leavethe most offspring (2) EVOLUTION:The genetic makeup of the population changes over time, driven by natural selection (3) A Flowchart of Evolutionary Reasoning Formation of new genotypes leads to phenotypic variation Potential forrapid reproduction Relatively constantresources andpopulation over time Variability instructures and behaviors Struggle for existence Some variability is inherited; adaptations increase in future generations (observations) (conclusions) Survival of the fittest Adaptation
Natural Selection As the Mechanism for Evolution An increase in frequency of genotypes that confer a favorable advantage in a given environment.
Applying Your Knowledge • Adaptation • Evolution • Natural selection • The mechanism for evolution is • A progressive change in the characteristics of organisms is • 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.
BiogeographicalEvidence 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
Functional hindlimb in salamander Remnants of hindlimb seen in boa and whale Anatomical Evidence: Vestigial Structures
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
Microevolution Macroevolution Two Types of Evolution Change within a population or species Change to a new species
Microevolution led to an increase in dark-winged 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.
Remaining Genotypes X SS X S’S’ Malaria Eliminates SS SS’ SS’ X SS Anemia Eliminates S’S’ X S’S’ SS’ SS’ X SS X S’S’ SS’ SS’ Malaria as an Agent of Natural Selection
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
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
.7A .3a .49AA .21Aa .7A .3a .09aa .21Aa
Molecular Evolution Two Hypotheses for the Origin of Modern Humans
DNA Analyses Related to Human Origins Visit http://www.dnalc.org/ and choose Genetic OriginsMitochondrial Control RegionMedia and AnimationsSolving the Mystery of the Neanderthals Other Applications of DNA Analysis can be found at http://dnai.orgChoose Applications
Types of Selection • Stabilizing: eliminates extremes
Types of Selection • Disruptive: increases both extremes
Types of Selection • Directional: increases one extreme
Applying Your Knowledge • Stabilizing Selection • Disruptive Selection • 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
Diploid with chromosome set A andchromosome set B. Chromosomes duplicatebut do not separateTetraploid with two sets of A and B. Cross between diploid and tetraploid species Triploid with one each of chromosome sets A, B and D. Chromosomes duplicate but do not separate Hexaploid with three sets of A, B and D. Speciation by Polyploidy Modern Wheat
Applying Your Knowledge • Sympatric Speciation • Speciation by Polyploidy • Allopatric Speciation • Which process involves a sudden, large change in chromosome number? • Which process requires geographical separation? • 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
North American Desert Plants Cactus Euphorbs African Desert Plants Patterns of Evolution • Convergent • similar phenotypes arise in unrelated species as a result of environmental similarities
Predators and their Prey Flowering plants and their Pollinators Patterns of Evolution • Coevolution: species adjust together to maintain relationship