Slide 1How are Populations, Genes and Evolution Related?Slide 3Genes, Alleles, Genotype, and PhenotypeSlide 5Slide 6Allele FrequencySlide 8Slide 9So... we can predict five major causes of evolutionary changeSlide 11Slide 12Slide 13Slide 14Slide 15Slide 16Genetic Drift: Population BottleneckSlide 18Slide 19Key Points About EvolutionSlide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27There can be opposing selective pressuresSlide 29Slide 30Slide 31Slide 32Slide 33Chapter 15HOW POPULATIONS EVOLVE1How are Populations, Genes and Evolution Related?Lets review genes, alleles, genotype, and phenotype215.1315.1Genes, Alleles, Genotype, and Phenotype•Gene – a segment of DNA at a particular locus•The sequence of nucleotides in a gene encodes for the sequence of amino acids in a protein•Alleles – slightly different nucleotide sequences for a particular gene•Genotype – the specific alleles on an individuals chromosomes•Homozygous vs heterozygous•Phenotype- the physical traits of an organismGenes, Alleles, Genotype, and Phenotype4Fig 15.115.1515.1Population Genetics and the Study of Evolution•Population genetics – the frequency, distribution, and inheritance of alleles in populations•Gene pool – the sum of all the genes in a population•All of the alleles of all of the genes in all of the individuals of a population•Each gene can be considered to have its own gene pool•Allele frequency - the relative proportion of each allele in a gene pool615.1Population Genetics and the Study of EvolutionRemember: Evolution is the change of allele frequencies within a populationAllele Frequency7Allele frequency of B = 40%Allele frequency of b = 60%Fig 15.215.1815.1The Hardy-Weinberg PrincipleHypothetical model under which a population does not evolve•Equilibrium population – a hypothetical model about a population that doesn’t evolveSo why is a hypothetical model important???The hardy-Weinberg conditions are useful starting points for studying the mechanisms of evolutionThe Hardy-Weinberg Principle Depends on the Following Five Conditions1. No mutations2. No gene flow into or out of the population3. Large population4. All mating must be random5. No natural selectionIf ALL of these conditions hold true the population will NOT evolve915.1So... we can predict five major causes of evolutionary change 1. Mutation2. Gene flow (or a lack of gene flow)3. Small population size4. Nonrandom mating5. Natural selection1015.11115.2Mutations Are the Original Source of Genetic VariabilityMost mutations occur during DNA replication•Most mutations occur during cell division•The majority are corrected but some slip by•Unrepaired mutations in gametes may be passed on to an individuals offspringRemember that mutations are not goal oriented•Mutations may result in phenotypic changes that are good, bad, or in different12Fig 15.315.2Mutations occurspontaneouslyNot in response to environmental conditionsNatural selection works on variation already present1315.2Gene Flow Between Populations Changes Allele FrequenciesGene flow tends to increase genetic similarity •Individuals can move from one population to another and breed• Pollen(contains sperm) can be carried long distances by wind or pollinating insectsLack of Gene flow can cause differences and even the development of a new species1415.2Allele Frequencies May Drift in Small PopulationsChance can prevent organisms from reproducing•Seeds can fall into a pond•Flowers can be pummeled by hail•Organisms can be killed by a fire or flood•Etc...Genetic drift – is the process by which chance events can change allele frequenciesGenetic drift is the process by which chance events can change allele frequencies1515.21615.2Genetic drift has a larger effect on small populationsTwo causes of genetic drift include a population bottleneck and the founder effect•Population bottleneck- drastic reduction in a population due to a natural catastrophe or something like overhunting•Founder effect – when a population is founded by a small number of individualsGenetic Drift: Population Bottleneck17Fig 15.715.21815.2Nonrandom MatingMating within a population is almost never random•Inbreeding can occur because related individuals reside in the same area•Sexual selection•Individuals can prefer a certain “look”1915.2All Genotypes Are Not Equally BeneficialAny time an allele provides “a little superiority” natural selection favors the individuals who possess itSo lets summarize what we know so far:Key Points About Evolution1. Natural selection acts on variation that is already present2. Natural selection acts on individuals, but causes changes in the genetic traits of a population3. Evolution is the change in allele frequencies of a populationFitness of an organism is measured by reproductive success4. Evolution does not necessarily make organisms “better” 2015.22115.3So how exactly does natural selection work?Natural selection stems from unequal reproduction•Phenotypes that favor increased reproduction select for a particular individual•When an individual reproduces its gene are passed on to offspringThe selection of phenotypes, by natural selection, influences a population genotypes, driving evolution2215.3Natural Selection: some phenotypes reproduce more successfully than othersAdaptations – characteristics that help an individual survive and reproduceSuccessful phenotypes are those that have the best adaptations for their particular enviornment2315.3Natural Selection: some phenotypes reproduce more successfully than othersAn organisms environment imposes different types of selective pressures that can be agents of natural selection•Non living or abiotic factors- availability of water and resources such as minerals•Living or biotic factors- other organisms2415.3Agents for Natural Selection: CompetitionCompetition with other organisms for scarce resourcesCompetition is most intense amoung members of the same species2515.3Agents for Natural Selection: Predator and PreyCoevolution – when two species interact extensively each can exert a strong selective pressure on the other•Often found in predator-prey relationshipsWolves and deer:•Wolves select against slow or careless deer•Swift alert deer select against slow clumsy wolves2615.3Agents for Natural Selection: Sexual SelectionCan be driven by female preference or by sexual competition between males•Female guppies prefer brightly colored males•Male
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