evolution of populations

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  • Evolution of Populations

  • What is a Population?Populations evolve over many generations, individuals dontPopulations are groups of interbreeding individuals that live in the same place at the same timeIndividuals in a population compete for resources with each other

  • Genetic VariationIn genetic terms, evolution is any change in the amount of alleles in a populationDepends on:Gene pool- collection of genes among a populationRelative (or Allelic) frequency- % of a particular allele in a gene poolGenetic equilibrium - when frequency of alleles does not change from generation to generationGenetic drift -allelic frequencies change due to genetic variation

  • How does the first generation differ from the second generation?What complex pattern of heredity is shown in the illustration above?Why is the population of snapdragons in the above example considered to be in genetic equilibrium?Would the population of snapdragons above be considered to be evolving or not evolving?How would genetic drift affect a population in genetic equilibrium?What might introduce genetic variation to this population of snapdragons?

    Gene Pool

  • Genetic DriftSample of Original PopulationFounding Population AFounding Population BDescendantsIndividuals that carry a particular allele may leave more descendants than others, just by chanceOccurs in small populationsCauses one allele to become more common in a populationMay occur when a group of individuals forms a new habitat

  • Sources of Genetic VariationMutations can add new alleles to gene pool

    Migration of individuals into or out of populations

    Genetic Shuffling during production of gametes; crossing over

  • Other Sources of Genetic VariationNatural selection never acts directly on genes because it is the entire organism that survives and reproduces.If an individual dies or does not reproduce, it does not contribute its alleles to the populations gene pool.

    The # of phenotypes produced for a trait depends on how many genes control that trait.

  • Natural Selection on Single-Gene TraitsFrequency of Phenotype(%)100806040200Widows peakNo widows peakPhenotypeVariation in this gene leads to only two phenotypesSingle-Gene traits are controlled by a single gene with 2 alleles. EX. Widows peaks

  • Natural Selection on Polygenic TraitsPolygenic traits are controlled by two or more genes. EX. height Variation in this gene leads to a range of phenotypes

  • Natural Selection on Polygenic TraitsGreatest change in gene pools is due to natural selection.Three ways natural selection affects how phenotypes are distributed in a population:Stabilizing-average trait has the higher advantage. Directional- one of the extreme forms of a trait has the higher advantage. Disruptive- both extreme forms of trait have the higher advantage

  • Graph of Stabilizing SelectionKeyPercentage of PopulationBirth WeightSelection against both extremes keep curve narrow and in same place.Low mortality, high fitnessHigh mortality, low fitnessStabilizing SelectionWhen individuals near the center of the curve have higher fitness than those on either endEx. Weight of Human infants at birth

  • Graph of Directional SelectionDirectional SelectionFood becomes scarce.KeyLow mortality, high fitnessHigh mortality, low fitnessIndividuals at one end of curve have higher fitness than individuals in middle or other end of curveRange of phenotypes shifts as some individuals succeed while others do not survive or reproduce.Causes are limited resources

  • Graph of Disruptive SelectionDisruptive SelectionLargest and smallest seeds become more common.Number of Birds in PopulationBeak SizePopulation splits into two subgroups specializing in different seeds.Beak SizeNumber of Birds in PopulationKeyLow mortality, high fitnessHigh mortality, low fitnessWhen individuals at upper and lower ends of curve have higher fitness than individuals near middleSelection results in two different phenotypes

  • Evolution vs. Genetic EquilibriumPopulations evolve through mutations in individuals A population in genetic equilibrium does not evolve.


  • Hardy-Weinburg Principle:5 Ways to Maintain Genetic EquilibriumExplains how populations can remain stable if certain conditions are met:Only random matingPopulation must be very largeNo movement into or out of populationNo mutationsNo natural selection

  • AdaptationsCan arise in response to environmental pressuresTemperatureAntibiotic resistance in bacteriaPesticide resistanceMorphological changes in peppered mothsEvolution of pesticide resistance in response to selection

  • Bacterial Resistance to AntibioticsCertain species of bacteria are more fit and can survive antibiotic treatment

    These bacteria will develop a resistance to antibiotics

    Infections will require the development of new antibiotics or other forms of treatment Question: Overuse of antibiotics has led to antibiotic resistant strains of bacteria. Why might this happen in a short period of time? bacteria reproduce rapidly

  • Speciation is the evolution of new species

    Species - a group of organisms that look alike, interbreed and produce fertile offspring.

    Can occur quickly or slowly

    As new species evolve, populations become reproductively isolated from each otherTwo populations do not interbreed and produce fertile offspringQUESTION: A mule is a cross between a horse and a donkey yet is not a species. Why?mules cannot reproduceEvolution of Species

  • Types of reproductive isolation:Behavioral Isolation- two populations capable of interbreeding but reproductive behavior is different

    Geographic Isolation- Physical barriers can prevent interbreeding. (Ex. rivers, mountains)

    Temporal Isolation- two or more species reproduce at different times

    Chromosomal numbers change

    QUESTION: How are speciation and reproductive isolation related?Speciation occurs when populations are reproductively isolated; reproductively isolated populations have different gene pools and eventually form new speciesHow are new species formed?

  • REVIEWCan individuals evolve? Why or why not?What are the conditions needed for the Hardy-Weinburg Law to be accurate?What causes the greatest changes in the gene pool of a population?Give an example of:Stabilizing selectionDirectional selectionDisruptive selectionHow can you distinguish between a species in which there is a lot of variation and two separate species?How can geographic isolation occur? How does it affect the gene pool?