Presentation on theme: "Population Genetics and Speciation"— Presentation transcript:
1 Population Genetics and Speciation Chapter 16Population Genetics and Speciation
2 Section 1 Vocabulary Pretest Population GeneticsMicroevolutionGene PoolAllele FrequencyPhenotype FrequencyTotal genetic information in a populationPortion of gene copies of a given alleleStudy of the frequency and interaction of alleles and genes in populationsChange in the collective genetic material of a populationRatio of individuals with a given phenotype to the total population
3 Answer Key Population Genetics C Microevolution D Gene Pool A Allele Frequency BPhenotype Frequency E
4 Population GeneticsPopulation Genetics is the study of evolution from a genetic point of view (it is the study of microevolution)Microevolution—a change in the collective genetic material of a populationPopulation–members of the same species that can interbreed. It is the smallest unit in which evolution occurs.
5 Variation Populations show natural variety within a species. Many quantitative traits (height and weight etc.) follow a bell shaped curve.
6 Causes of VariationEnvironmental factors—amount of food, quality of food, etc.Genetic factorsMutations—random changes in genesRecombination—reshuffling of genesRandom pairing of gametes
7 Gene PoolGene pool —total genetic information available in a population
8 Allele and Phenotypic Frequency Allele frequency—expressed as a percent: it is determined by dividing the number of a certain allele by the total number of alleles of all types in the populationPhenotypic frequency—expressed as a percent: it is the number of individual with a particular phenotype divided by the total number of individuals in the population.
9 Hardy-Weinberg Genetic Equilibrium Developed by Wilhelm Weinberg (German physician) and Godfrey Hardy (British mathematician)Godfrey HardyWilhelm Weinberg
10 It is based on a “hypothetical population” that is not evolving. States that genetic frequencies in a population tend to remain the same from generation to generation unless acted on by outside influences.It is based on a “hypothetical population” that is not evolving.
11 Conditions of Hardy-Weinberg Equilibrium In Genetic Equilibrium:No net mutations occurPopulation size remains constantThe population is infinitely largeIndividuals mate randomlySelection does not occurThis flock of mallards probablyviolates some or all of theconditions necessary for theHardy-Weinberg geneticequilibrium
12 It is highly unlikely that all five of the conditions in the Hardy-Weinberg Model will happen in the real world.Therefore, Genetic Equilibrium is impossible in nature.It is a theoretical state that allows us to consider what forces could disrupt such balance (equilibrium).
13 Section 2 Vocabulary Pretest ImmigrationEmigrationGene FlowGenetic DriftSexual SelectionStabilizing SelectionDisruptive SelectionDirectional SelectionIndividuals move outIndividuals move inChoice of mates based on favorable traitsGenes move from one population to anotherAverage trait is selectedOne extreme trait is selectedTwo extreme traits are selectedAllele frequencies change randomly
14 Answer Key Immigration B Emigration A Gene Flow D Genetic Drift H Sexual Selection CStabilizing Selection EDisruptive Selection GDirectional Selection F
15 Disruption of Genetic Equilibrium Disruptions to the Hardy-Weinberg equilibrium can result in evolution.The five requirements for genetic equilibrium can be disrupted by the following outside forces:MutationGene FlowGenetic DriftNonrandom MatingNatural Selection
16 Requirement #1: No Net Mutations Occur Mutations occur constantly at very low rates under normal conditions.Exposure to mutagens (mutation-causing agents, i.e. radiation and chemicals) can increase mutations rates.Mutations produce new alleles for a traitThey can be harmful, harmless or helpfulHelpful mutations are a vital part of evolution.
17 Requirement #2: Population Size Remains Constant Individuals enter and leave populations constantly. Their “genes” move with them. This is called Gene Flow.Factors influencing gene flow include:Immigration—movement of individuals into a populationEmigration—movement of individuals out of a populationMigration and dispersal patterns can also influence the movement of individuals into new populationsBirth and Death Rates also remove or add genes from individuals to a population.
18 Requirement #3 Population is Infinitely Large In nature, population sizes are restricted rather than infinitely large.Genetic Drift can occur in small populations of organismsGenetic Drift—the random change in allele frequency in a populationSignificant changes can happen in small populations if even a single organism either fails to reproduce or reproduces too much.
19 If the frequency of an allele reaches zero in a population, then (assuming you started with two alleles), there is only one left.All individuals will be homozygous for that trait---creating no variations.This weakens a species.Ex: Northern Elephant SealHomozygous for every gene tested
20 Bottleneck EffectGenetic Drift can lead to a bottleneck effect in which variations are reduced overtime.
21 Requirement #4: Random Matings Organisms do not mate randomly in nature.Mate selection is influence by:Geographic proximity— choose mates nearby: can result in kinship matingAssortative Mating— choose mates with similar traits: reduces variationSexual Selection—choose mates based on favorable traits
22 Requirement #5: Selection Does Not Occur Natural Selection—organisms with favorable traits are more likely to survive and reproduce, passing on their favorable genes to the next generation.It is an ongoing process in nature and an important disruption to equilibrium.Three patterns of Natural Selection:
23 Stabilizing Selection: individuals with the average form of a trait have the highest fitness. Ex: Lizard body size
24 Ex: Shell Color of Limpets Disruptive Selection: individuals with either extreme variation of a trait have the highest fitness.Ex: Shell Color of Limpets
25 Ex: Nose and tongue lengths of anteaters Directional Selection: individuals with one extreme of a trait have the highest fitnessEx: Nose and tongue lengths of anteaters
26 Section 3 Vocabulary Pretest SpeciationGeographic IsolationAllopatric SpeciationReproductive IsolationSympatric SpeciationGradualismPunctuated EquilibriumA slow change in a species over millions of yearsBursts of rapid changeFormation of a new speciesPhysical separation of populationsInability to mate or produce offspringSpeciation resulting from geographic isolationSpeciation resulting from reproductive isolation
27 Answer Key Speciation C Geographic Isolation D Allopatric Speciation F Reproductive Isolation ESympatric Speciation GGradualism APunctuated Equilibrium B
28 Speciation Speciation—formation of a new species Species: a single kind of organism whose members are morphologically similar and can interbreed to produce fully fertile offspring.Two types of speciation: Allopatric Speciation and Sympatric Speciation
29 Allopatric Speciation Allopatric Speciation: species arise as a result of geographic isolation. (Allopatric = different homelands)Geographic Isolation—physical separation of members of a populationGene flow between the new subpopulations stops and the two begin to divergeEventually, they become incompatible for mating, creating new species.Debate exists as to whether or not allopatric species are different enough to be considered new species.
31 Sympatric SpeciationSympatric Speciation —occurs when two subpopulations become reproductively isolated within the same geographic area.Reproductive Isolation—the inability of members of the same species to mateCan be caused by disruptive selectionTwo types: prezygotic isolation and postzygotic isolation
36 Rates of Speciation Two Theories: Gradualism —slow change over millions of yearsPunctuated Equilibrium—short bursts of rapid change
37 Evidence exists that suggests that both have taken place over time.
38 The Hardy-Weinberg Equation Hardy and Weinberg went on to develop an equation that can be used to discover the probable genotype frequencies in a population and to track their changes from one generation to another.The equation is: p2+2pq+q2 = 1p= frequency of the dominant alleleq = frequency of the recessive alleleSee handout
39 Significance of the Hardy-Weinberg Equation Punnett Squares allow geneticists to predict the probability of offspring genotypes for particular traits based on the known genotypes of their two parentsThe Hardy-Weinberg equation essentially allowed geneticists to do the same thing for entire populations.
40 Before Hardy and Weinberg, it was thought that dominant alleles must, over time, wipe out recessive alleles (genophagy = “gene eating”)According to this wrong idea, dominant alleles always increase in frequency from generation to generation.Hardy and Weinberg demonstrated that dominant alleles can just as easily decrease in frequency.