Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Genetics for HumanEvolutionGenomeThe total genetic endowment of a speciesThe alleles you posses define your genotypeor hereditary makeup.(The way they are expressed physically is yourphenotype: the sum of observable traits)EVOLUTIONThis is the definition of evolution we will use:A change in the allele frequency of a population over timeWhere does the variation in populationscome from?1. Individuals vary as a result of sexual reproduction2. Populations vary through time due to a number of factors we’ll cover todayIn all but the sex cells, genes come in pairsAny particular gene may display variantscalled allelesThe ultimate source of alleles (variants) is mutation:a random change in the coding of a gene(that is, a change in the base pairs in codons)REVIEWVariation in individuals due to sexual reproductionin addition to mutation,other sources of variation in individualsmay also arise during sexual reproduction:Segregation (also called independent sorting)Linked genesCrossing over and the resultant RecombinationSegregation / independent sorting:During the production of gametes, when allelepairs split into haploid gametes during meiosis,Each allele in a pair sorts independently fromthe other pairs:You don’t just get your grandmother’s alleles, for example:you get some of your grandfather’sYOUPARENTSGRANDPARENTSLinked genesHowever, instead of sorting independently, sometimes a segment ofa chromosome will stay togetherCrossing over and RecombinationAnd sometimes during meiosis, pairedchromosomes may exchange material(crossing over). The result is a newmixture, a recombination.When one chromosome from the mother(with over 8 million possibilities) combines withone chromosome from the father (ditto), and this happens with all 23 chromosomesthat is 2 to the 23rd times 2 to the 23rd, or about 70 trillion possible zygotes!But evolution is a process that involveschange in POPULATIONS, not individuals,over timePopulation: a group of individuals that interbreedsWe can genetically characterize populationsby referring to the gene pool: the total genes of a population. One way to think about gene pool is to think ofallele or gene frequency. The process of evolution is genetic changein populations through time. Evolution is a change in allele frequency ina population over time.In theory, the gene pool or allele frequencyof a population could remain stable over time: no evolution This is expressed algebraically by the Hardy-Weinberg Equilibrium Principle:p2 + 2pq + q2 = 1This principle states that a population’s allelefrequency (gene pool) will remain the same fromone generation to the next IF:p qp pp pqq pq qqPunnett squareMating is entirely randomThe population is large enoughNo new variants are introduced into the gene poolAll individuals are equally successful at surviving and reproducingHardy-Weinberg Equilibrium Principle holds true IF:However, in life these conditions are rarely met!What are the sources of change inpopulations over time?At the population level, the following processes contribute to evolution (change in allele frequency over time). These are the mechanisms of genetic evolution:1. Mutation2. Natural selection3. Genetic drift4. Gene flow1. Mutations: random changes in DNA molecules.Mutations are frequent.Most are thought to be neutral.The only mutations that are passed on are those that take place in the sex cells!2. Natural selection: variation at the population level is already present. The environment exerts pressure that selects for some individuals and against others. So not all individuals are equally successful at surviving and reproducing (against the Hardy-WeinbergEquilibrium Principle) Whether there is change or not in the allele frequencyof the population depends on whether or not there is a change in the environmental forces throughtimeDirectional selection:Natural selection promotes change in theallele frequency of the populationStabilizing selection:Natural selection promotes stability becausethere is no environmental changeOscillating selection:Adaptive variation around a norm, in responseto environmental variationLater on we’ll talk about the rate of evolution,or how we picture evolution taking placeAccording to the Theory of Punctuated Equilibrium,long periods of stabilizing or oscillating selection are punctuated by bursts of change.We expect to see sudden new speciesThe traditional view of the rate wasGradualismin which changes gradually accumulateto the point that we call something a new species.We expect to see intermediate fossil forms3. Genetic drift: change in allele frequencyof a population by random fluctuations.Genetic drift occurs with small populationswhen they are isolated from the rest of the gene poolGene frequencies fluctuate from generation to generation: the smaller the population,the wider the fluctuations When a few members of a population splitoff from the original larger group (fission)you get what is called the Founder Effect:That small new group cannot contain allthe genetic diversity (gene pool) of the originallarger group.Fission: the population splits, and the new subpopulations will differ in allele frequency from each other and the parent population A new, small population has a founder effect50%50%17%83%67%33%Example: Old Order AmishFounder effect: migration from Europe toAmerica of a small religious group of about 200people originallyDo not allow outsiders to marry inToday a high incidence of Ellis-van Creveld (EvC) syndrome: dwarfism with polydactyly, strange fingernails, and heart abnormalities.Researchers charted the genealogies of all casesof this disease, and found they can all trace back to one couple of the original founders, Samuel King and wife. One of them evidently carried EvC.4. Gene flow: the introduction of new geneticmaterial (alleles) into a populationOccurs when two formerly separatedpopulations begin to interbreedHow does all this relate to evolution?The basis of evolution is the transmission oftraits, with variation, from one generation to the next. The study of this process is called genetics Natural selection takes place on the level of populations, not on individuals.Species: a population or group
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