Biology 152 Week 3 Notes
Biology 152 Week 3 Notes Bio 152
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This 9 page Class Notes was uploaded by Lexi Kazen on Friday September 18, 2015. The Class Notes belongs to Bio 152 at University of Wisconsin - Madison taught by Dr. McCulloh in Summer 2015. Since its upload, it has received 16 views. For similar materials see Intro biology 2 in Biology at University of Wisconsin - Madison.
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Date Created: 09/18/15
o The distribution of continuous traits looks like a bell curve with phenotype as the xaxis and frequency as the yaxis 0 Continuous traits vary along a continuum because they are encoded by multiple genes 0 Type of selection for continuous traits 0 Selection favors an extreme phenotype phenotype either on the high end or low end of the bell curve 0 Shift in allele frequencies in direction favored by selection 0 Reduced genetic diversity at certain aees get rid of maladapted genotypes Only at SPECIFIC LOCI not the entire genome diggigigiign 0 Examples Drug resistance in HIV Sexual selection for a trait 0 Sexual Directional Selection Selection favors an extreme 39DliIFEGIiQIWaI V SEIIECIIDII I phenotype and direction pushes curve in that direction Example of this is peacock feather Before Size selection oSma feathers not favored so male peacocks with small feathers less likely to mate sma trait disappears over V time shifting curve for peacock feather phenotype to the right MEET more variance reproductive success in males pickier females stronger directional selection O 0 O O 0 O O O O O O O 0 Type of selection for continuous traits Selection favors intermediate trait values Example humans have between 2 and 8 ngers having 5 ngers is most favored so genetic diversity is reduced at the extreme loci Basically the bell curve just gets thinner since genetic diversity is being reduced Type of selection for continuous traits This type of selection is the opposite of stabilizing selection as it favors the extreme traits Example humans have between 2 and 8 ngers and it s optimal to have either 2 ngers or 8 ngers rather than 5 genetic variety increases as equal numbers of people have each of the extreme traits Occurs for both continuous AND discrete traits NOTthe same thing as stabilizing selection Acts to maintain genetic variation in a population Example selection for traits in different environments or at different times uctuating selection selection for the heterozygote form of the gene Heterozygote advantage Want to maintain both alleles dominant and recessive in a population Sickle cell anemia with malaria present is an example of balancing selection since the heterozygous form is resistant to malaria When malaria is NOT present directional Frequency Phenotype Enga height Stabilizing 7 selection I Disr uptiv g seriecti n selection favors the homozygous dominant form not having sickle cell anemia What is speciation 0 Formation of a new species oWhat is a species 0 There are multiple concepts that describe the speci c de nition of a species 0 Genetic Models of Speciation I How do genetic drift natural selection mutations etc play a role in the process of speciation DobzhanskyMullerlncompatibilities 0 Only applies to sexual species 0 Links micro and macroevolution How population genetic processes lead to speciation 1 Reproductive isolation bc of genetic divergence of a species 2 Mutations begin to accumulate in each isolated group 3 These mutations over time become xed by natural selection andor genetic drift 4 The mutations don t allow the two populations to come back together and form a hybrid crossing between divergent populations leads to infertility or death in the hybrid offspring 0 Species Concepts next lecture 0 Horizontal gene transfer transfer of genetic material ie DNA to another organism that is not its offspring o This mechanism is common in bacteria and is often how bacteria develop drug resistance so quickly 0 This topic will be discussed further in lectures about prokaryotic diversity Mutations changes in the genetic code 0 There are many kinds of mutations 0 At nucleotide level point mutations Nucleotide substitutions insertions and deletions Caused by 0 DNA replication error during mitosis or meiosis Error in repair of sites damaged by mutagens ie UV light 0 At gene level Gene insertions and deletions Transposons Transposable elements 0 Make up a large portion of a eukaryote s genome Exon shuf ing 0 Different exons either within a gene or between non allelic genes are mixed end up with new protein Gene duplications followed by differentiation 0 This is the source of new genes 0 End up with quotgene familyquot ie hemoglobin opsin genes etc 0 At chromosome level Chromosome duplications inversions deletions fusions 0 At genome level Polyploidy The presence of extra sets of chromosomes due to accidents during cell division Autopolyploidization 0 An individual with more than 2 sets of chromosomes derived from its ancestral species Allopolyploidization o a individual with multiple sets of chromosomes in which these genomes were derived from 2 or more species 0 most mutations are 39neutral meaning they have no effect on the tness of an individual 0 this is because most of the genome is nonfunctional o unfavorable recessive mutations last longer in a population than dominant ones because only homozygous recessive individuals can be selected out heterozygous individuals mask the mutation selection for favorable mutations leads to ADAPTATION 0 only a small number of mutations are favorable 0 mutations only get passed down to the next generation if they occur in the germ line ie sperm or egg 0 every individuals has about 100 new mutations as compared to their parents 0 the male germ line is more prone to mutations because its process of meiosis is ongoing through a male s life leading to an exponential accumulation of mutations as a man ages males are more likely to pass on genetic diseases especially as they age 0 the female germ line only grows through meiosis once so less possibilities for mutations to occur 0 maledriven evolution 0 Lamarck revisited 0 We now know that Lamarck was correct sometimes 0 Epigenetics alterations in gene expression but where the DNA itself remains unchanged Epigenetic marks are made across the genome at each generation and determine which genes are turned 39on and off 0 These marks are typically removed from generation to generation but occasionally end up getting passed on 0 Could be environmentally induced but we re not sure Epigenetic differences could explain phenotypic differences in identical twinsclones ie cancer obesity hairfur color 0 Some types of epigenetic modi cations 0 Chromatin modi cation epigenetic alteration of chromatin structure affecting gene expression Methylation l active genes are less methylated than inactive genes ie agouti gene in mice fat yellow mouse vs skinny brown mouse the mice are identical twins but look completely different histone modi cations chromatin remodeling o RNAmediated modi cation posttranscriptional RNA modi cations Example RNA interference RNAi based mechanism 0 Since epigenetics is a newer area of study we don t know much about epigenetic modi cation in humans 0 We have a fairly decent understanding of it in plants 0 No new alleles are formed from this process only new genotypes o Shuf ing of the combination of alleles along a chromosome Random mating shuf e the gametes shuf ing of allelic combinations to form new genotypes Recombination results in new combinations of DNA within chromosomes 0 Crossing over leads to offspring having a different combo of alleles than their parents Separation of homologous chromosome pairs allows random and independent shuf ing of haploid chromosomes gametes during random mating Increased genotypic variation Can get a combination of favorable mutations more quickly as compared to asexual reproduction Remove unfavorable mutations more quickly meiosis i I m crawling mill5mm New alleles mutations New genotypes meiosis o Crossing over 0 Independent assortment Sex generates genotypic variation 0 Get combo of favorable mutations more quickly 0 Remove unfavorable mutations more quickly 0 Asexual species go extinct more quickly than sexual species If sex is so great why are some species asexual o Reduces population growth by 12 because males can t reproduce 0 Loss of tness relative to clonal populations 0 Many clonal organisms grow and divide and are in a sense immortal genotypically speaking 0 Sex Death in other words only half of your genome is passed on to the next generation and the other half of your genome dies with you since no two people have the same exact genome As already has been said multiple times you need genetic andor epigenetic variation in order for selection to occur Darwin s contribution population speciation as a result of natural selection 0 Too many offspring are produced limited resources competition 0 Better adapted individuals survive 0 Survival of the ttest fittest leave the most offspring Some phenotypes are better suited for a certain environment Mutations can be favored or disfavored by the environment 0 HIV needs to keep the host alive long enough that it can get passed onto the next host High transmission rate high virulence Genetic strain that grows the fastest will win 0 Can jump from host to host often Low transmission rate low virulence If you kill the host you die too because there s not a great chance you ll get to another host in a short time o The condom campaign came about to lower the transmission rate of HIV Lowering the transmission rate lowers the virulence so if less people have HIV the disease becomes less deadly o In the 80 s when condoms weren t widely popular many people developed HIV so it was easy for HIV viruses to jump from host to host this allowed selection to favor a more virulent strain More virulent more deadly The direction of selection can change with a change in the environment With selection what matters is the relative tness of the different genotypes Genetic drift can interfere with natural selection Recessive alleles are more difficult to remove from the environment than dominant ones because the recessive allele can hide in the heterozygous form Positive selection natural selection favors an allele or heritable trait Negative selection natural selection disfavors an allele or heritable trait 0 Discrete trait is controlled by only one or a few genes ie yellow vs green peas 0 Continuous trait is controlled by multiple genes ie beak shape body size etc