Charles Darwin 1809 - 1882 - Georgetown ISD



Chapter 15 - Mechanisms of evolution.Use of the word theory: Non scientific use: a theory is, at best, an educated guess. It implies that something is unproven or speculative. Scientific use: A scientific theory summarizes a group of hypotheses that have been supported with repeated testing and it becomes accepted as a valid explanation of a phenomenon. Charles DarwinIn 1831, Charles Darwin, a 22-year-old naturalist, accepted a position aboard the ship HMS Beagle that began a voyage around the world; it provided Darwin with many observations.Prior to Charles DarwinPre-Darwinian world-view was determined by theological beliefs.The earth is young.Each species was specially created and did not changeVariations are imperfections Observations are to substantiate the prevailing worldview.Aristotle and Judeo-Christian viewpoints – “scala natuae” Scientists were also usually theologiansLinneaus (1707-1778)Swedish taxonomist (taxonomy is the science of naming and classifying organisms.He developed a binomial system of nomenclature (two-part names for each species [e.g., Homo sapiens]). Like other taxonomists of his time, Linnaeus believed in the ideas ofa)special creation—each species had an?“ideal”?structure and function; andb)fixity of species—each species had a place in the?scala naturae, a sequential ladder of life.Lamarckfirst to state that descent with modification occurs Proposed inheritance of acquired characteristicshe believed that organisms become adapted to their environment.Lamarck mistakenly saw “a desire for perfection” as inherent in all living thingsShould be recognized for: claim of evolution, theorizing about great age of earth, emphasis on adaptationCharles LyellLyell was a geologist that popularized the idea that earth had been shaped by slow-acting forces that are still at work today. 2 ideas that Darwin took from Lyell’s work: The earth must be very old (much older than 6000 years)Many gradual changes over time will yield substantial changes.Charles Darwin 1809 - 1882HMS Beagle Sailed in 1831Darwin was ship’s naturalistSailed for 5 years and surveyed coast of South AmericaVoyage of the HMS BeaglePlants and animalsUniquely South AmericanMore closely related to each other than to organisms on other continentsFossils – uniquely SAGalapagos IslandsOrganisms lived nowhere else on earthClosely resembled organisms from the mainlandThree major propositions by Darwin species are not immutable –they change over time Divergent species share a common ancestor The mechanism that produces change is natural selectionNatural SelectionDarwin decided that adaptations develop over time; he sought a mechanism by which adaptations might arise.Natural selection was proposed by both Alfred Russel Wallace and Darwin as a driving mechanism of evolution caused by environmental selection of organisms most fit to reproduce, resulting in adaptationWhat is Natural Selection? Differential success in reproduction Occurs through interaction between environment and variability in individuals Product of natural selection is the adaptation of populations of organism to their environment Because the environment is always changing, there is no perfectly-adapted organism. There are three preconditions for natural selection. The members of a population have random but heritable variations. In a population, many more individuals are produced each generation than the environment can support. Some individuals have adaptive characteristics that enable them to survive and reproduce better. There are two consequences of natural selection.An increasing proportion of individuals in succeeding generations will have the adaptive characteristics. The result of natural selection is a population adapted to its local environment. Natural selection can only utilize variations that are randomly provided; therefore there is no directedness or anticipation of future needs. Extinction occurs when?previous adaptations are no?longer suitable to a changed anisms Struggle to ExistMalthus proposed that human populations outgrow food supply and death and famine were inevitable and Darwin applied this to all organisms; resources were not sufficient for all members to survive.Therefore, there is a constant struggle for existence; only certain members survive and reproduce.Variations1. In contrast to the previous worldview where imperfections were to be ignored, variations were essential in natural selection.2. Darwin suspected, but did not have today's evidence, that the occurrence of variation is completely random.3. New variations are as likely to be harmful as helpful.4. Variations that make adaptation possible are those that are passed on from generation to generation.5. Darwin could not state the cause of variations because genetics was not yet anisms differ in fitnessFITNESS is a measure of an organism’s reproductive successA polar bear can survive and reproduce in cold climates, it is not adapted to warmer climatesReproduction is important for evolution, only those who survive to reproduce pass on those advantageous genesArtificial SelectionDarwin used the power of artificial selection to reinforce natural selection Organisms Become Adapted An adaptation is a trait that helps an organism be more suited to its environment.?2. Unrelated organisms living in the same environment often display similar characteristics. (Convergent evolution)3. Because of differential reproduction, adaptive traits increase in each succeeding generation.Drawbacks of natural selectionCan only utilize variations that are randomly provided; there is no directedness or anticipation of future needsExtinctions occur when previous adaptations are no longer suitable to a changed environmentSome subtletiesPopulation is the smallest unit that can evolve—individuals DO not evolveHas to be a heritable characteristicSituationalExamplesEvolution of insecticide resistant insects Evolution of antibiotic resistant bacteriaEvidence of EvolutionHomologyAnatomical homologiesVestigial organsEmbryologyical homologiesMolecular homologiesAnalogous vs. HomologousAnalogous (convergent evolution)Occurs when two organisms live in such a similar (same) location that over time certain physical characteristics are favored and the two unrelated organisms end up looking very similar:Ex: Shark and DolphinEx: fusiform body shape in tuna, marlin, penguin, sealHomologous (divergent evolution)Occurs when 2 or more organisms share a common ancestor but live in such different environments that they have developed different structures used in different waysEX: forearm of many vertebratesVestigial structures:Human appendixPelvic bone in whalesEye sockets in blind salamander Fossils Back Up EvolutionTransitional forms – Fossils that link older forms of an organism to the current formDarwin predicted it and we have found many.Achaeopteryx – transition from reptile to bird. Had some reptile characteristics (tail with vertebrae, claws on wing, teeth) Also had bird characteristics feathers, wings, but pectoral girdle was not big enough to attach flying muscles to it. May have glided.Neil Shubin at Penn State found the Tiktaalik. It’s a transitional fossil showing an organism that bridged the gap from water to land. Front fins were under body (able to support weight) and had the classic vertebrate forearm structure of one large bone, two smaller bone and a bunch of tiny bonesMicroevolutionEvolution that occurs within a population = microevolutionPopulation -- all the members of a single speciesPopulation genetics – studies variations in gene poolsGene Pool and Allele ReviewGene pool – total of all the alleles in the populationAlleles – chromosome sections that code for specific proteins (traits). You have 2 for every trait. But there may be many more than 2 possibilities in the entire gene pool5 causes of Microevolutionsmall gene pool non random matingmutation migrationnatural selectionSmall PopulationA small population is more likely to have a change in allele frequency due to chanceThis is called genetic driftTwo situations in the which genetic drift occurs:Bottleneck effect – disaster which greatly reduces the population. Remaining survivors not representative of original population.Founder effect – when a few individuals from a large population colonize a new and isolated habitat.Non Random matingWhen sexually reproducing organisms select a mate based on some trait or behavior, evolution will take place as certain traits are selected for a passed on and others are not.Sexual selectionNatural selection for mating successCompetition amongst males for female mates Males have ritual displays and battles as well as to attract femalesIntrasexual selection is a direct competition for mates among individuals of the same species and genderSexual DimorphismDistinct physical differences between males and females of the same species (aside from reproductive organs)Ex: Male cardinals are red, females greyLion has mane, lioness doesn’tPeacock tail, pea hen no showy tailMutationMutation creates variationNew genes and new alleles originate only by mutationOnly mutations to sex cells can be passed onChanges the DNAChanges amino acid sequence, changes protein, changes fitness, usually makes an individual less fit, but occasionally, more fitGene FlowAny addition of genes from other populations is gene flow. This is what we would call “migration.” As we become more of a global community, the human race experiences more and more gene flow.Natural SelectionOrganisms in a population are not equal in our ability to survive and produce offspring.Thus some will pass their genes on and others will not. There are several types of natural selection:DiversifyingDirectional StabilizingTypes of selection: The effect of selection depending on what is “fit”Heterozygote advantage Heterozygotes can have a have a greater fitnessMaintain both alleles in populationSickle cell anemiaHeterozygotes are protected from the severest effects of malaria and do not develop sickle cell anemia Five fingered evolutionRemember the 5 things that cause microevolution using your hand:Pinky is small population size (genetic drift)Ring finger is non random matingMiddle finger is mutationIndex finger is gene flow (migration)Thumb is natural selectionHardy-Weinberg theoremDescribes allele frequencies in a non evolving population.They remain constant because no matter how many times we shuffle the alleles in a non evolving population, we always get the same ratiosWhy do we need Hardy Weinberg?Provides a base line or comparisonEvolution is a generation-to-generation change in a population’s frequencies of allelesSo if there is no change in allele frequencies, there is no evolutionSince allele frequencies are rarely equal, the dominants and recessives are represented by p and q. Frequency of alleles and genotypesFrequency of alleles:p + q = 1Suppose the p allele is for insulin production. q allele is for the mutated form where insulin is not produced. All the p and all the q added together is the entire human population. Frequency of genes:p2 + 2pq + q2 = 1freq of freq of freq ofRR Rr rr5 conditions of Hardy-WeinbergLarge population (pinky)Random mating (ring finger)No mutation (middle finger)No migration (index finger)No natural selection (thumb)If the allele frequencies change over generations, it must be due to one of these five things. HW equilibriumKeeping the same allele frequencies over generations is called Hardy-Weinberg equilibrium. A failure to maintain HW equilibrium indicates evolution has occurred.The pop. size is too small, there was non random mating, there was mutation, there was migration or there was natural selection. ................
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