BIOL 152 1st Edition Lecture 3 Outline of Last Lecture - Intro to Evolution Outline of Current Lecture Evolution of Populations- Microevolution = change in genetic makeup of a population from generation to generation- In the past the blending hypothesis was still widely accepted - Mendel’s hypothesis not published, so the blending hypothesis was still popularReview some definitionsSpecies - a group of populations whose individuals have the potential to interbreed and produce fertile offspring in naturePopulation - localized group of individuals belonging to the same speciesNatural selection acts on individuals, but populations evolve!!Describing variationPolymorphism - More than one readily noticeable morph in a populationAverage heterozygosity - is a fraction of individuals in a population that are heterozygous for a particular locusGeographic clines - is a gradual change of a trait across a geographic area (between populations)o The song sparrow, Melospiza melodia31 subspecies or races of these song sparrows Geographic cline affects body size, plumage, and song characteristics of birds o Genetic versus Environmental FactorsNot all variation is due to genetic differences, environment also has an effect on an individual’s phenotypes Selection upon VariationFitness - the genetic contribution of an individual to the next generation, relative to the contribution of others- To be successful an individual must survive to reproductive age and successfully reproduceThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.‘Survival of fittest’ is misleadingWhy Natural Selection cannot Fashion ‘perfect’ Organisms1. Can only act on existing variation 2. Limited by historical constraints 3. Adaptations are compromises4. Chance, natural selection, and environment interactHow does natural selection change populations?- Certain phenotypes ‘selected for’ or ‘selected against’- Selection alters the frequency of alleles:o When only one extreme of alleles survive - Directionalo Centered around a mean, where only the intermediates survive - Stabilizingo When the intermediates survive and the extreme alleles can’t successfully reproduce - DisruptiveSexual SelectionIndividuals in a population (typically males) differ in their ability to attract mates- Non-random mating- Sexual dimorphism- Intrasexual – direct competition of males for female- Intersexual – the female chooses the mate How is variation maintained?- The diploid status of chromosomes allow recessive alleles to persist in population- Heterozygote advantage - individuals who are heterozygous at a gene sometimes have higher fitnessSources of VariationMutations - only mechanism by which new alleles originate - if mutations occur in the gametes, it will be passed to the offspring- Mutation rate varies between organismsSexual Reproduction - offspring with different combinations of alleles than parents (shuffling of existing alleles)- Recombination- Independent Assortment Fertilization Other Effects on VariationGenetic Drift - when allele frequencies fluctuate randomly between generations. Over time, it typically diminishes in variation. Especially true for small populationsGenetic Drift - a biological form of Sampling Error - Sampling Error refers to chance events -- random departures from expectations based on underlying probabilities -- - Chance events are more likely in small samples than in large samplesThought experiment: - One student flips a coin 6 times; expectation is 3 heads, 3 tails; .5 H, .5T.- One student flips a coin 60,000 times; same expectation of .5H, .5T.- If you had to bet on which student had results closer to expected probability, which one would you bet on.2 types of Genetic Drift- Bottleneck Effect- Founder EffectConsequence - the new population is not represented of the original Gene flow - is the allele movement between populations; over time gene flow reduces differences between populationsWhy is genetic variation important?- Populations with heritable variation, i.e. genetic diversity, have the capacity to adapt to the changing environment. - Populations with little or no genetic diversity have little or no capacity to adapt.What are 4 processes that change the allele frequencies in a population?1. Mutation 2. Gene Flow – brings new alleles or changes existing alleles3. Genetic Drift – affects small populations (frequencies change randomly) 4. SelectionAnalyzing change in allele frequenciesCommon, powerful approach to studying how natural selection affects populations: - Develop mathematical models - Collect data from real populations to test the predictions of the modelModel developed by Hardy and Weinberg (1908) - Predict future allele frequencies at a single locus when none of the four evolutionary processes are operating (it is a null model)Hardy-Weinberg Principle - Frequencies of alleles and genotypes in a population remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work Mathematically track the change in allele frequencies between generations- We now focus on outcome of all mating events in a population, rather than Mendel’s mating between 2 parents- All gametes are considered as one gene poolo Two gametes can be chosen at random from gene pool to represent an individualo We can calculate the genotypes in the offspring by considering what would happen with repeated sampling from the gene poolDefining the allele frequenciesp: frequency of allele CR q: frequency of allele CW The frequency of both alleles = 1 p + q = 1If p=0.8, then q=0.2, What is frequency of CRCR, CRCW, CWCW?The frequency of CRCR genotype is p2 The frequency of CRCW genotype is 2pqThe frequency of CWCW genotype is q2p2 + 2pq + q2 = 1Used to predict the allele frequencies in subsequent populations, and assess whether allele frequencies are changing (i.e., evolution is occuring) p2 = 0.8*0.8 = 0.642pq = 2(0.8*0.2) = 2(0.16) = 0.32q2 = 0.2*0.2 = 0.040.64+0.16 = 0.80 CR 0.4+0.16 = 0.20 CW A change in allele frequencies = evolutionThe Hardy-Weinberg Principlep2 + 2pq + q2 = 1If frequencies of A1 and A2 are given as p and q, then the frequencies of each genotype will be p2, 2pq, and q2 for every successive generation- ONLY WHEN certain conditions apply…Hardy-Weinberg Assumptions1. No natural selection 2. No gene flow 3. Random