Biol 2 Week 2 notes
Biol 2 Week 2 notes BIOL 102
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This 5 page Class Notes was uploaded by at19 on Tuesday January 26, 2016. The Class Notes belongs to BIOL 102 at University of Pennsylvania taught by Dr. Sniegowski in Spring 2016. Since its upload, it has received 14 views. For similar materials see Biological Principles II in Biology at University of Pennsylvania.
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Date Created: 01/26/16
Evolutionary forces II Wednesday, January 20, 2016 1:02 PM Hardy Weinberg o No selection occurring: Finesses equal, allele frequencies stable if no other evolutionary forces operating on population. Genotype frequencies as shown if mating is random o Selection occurring: individuals bearing the A allele have higher fitness: A allele increases in frequencies going from p = 0.5 to 0.57 aa survives half as well Taking into account the reduction in the frequencies of the aa genotype as a result of its lower survival rate, we calculate new allele frequencies p and q. we then calculate HW frequencies for the next gen based on these new p and q values: they are shown in line 3 of the second table Reminder: nature vs nurture o Genotype + environment = phenotype Genotype = genetic makeup of organism Phenotype = observable characteristics of an organism, depends on genes and environment Heritable trait = is at least partly determined by genes o Only way to get evolutionary change if due to heritability Selection o If a pop only had environmental change (and no genetic variation), could be dif phenotypes but there would be no evolution bc no genetic diversity o EVOLUTION ONLY OCCURS WITH HERITABLE GENETICS Evolution by natural selection: quantitative traits o Natural selection cannot act directly on genotypes, but acts on phenotypes o There are a variety of patterns of selection, that depend on the relationship between phenotype and fitness o Changes in allele frequency at multiple gene loci are the basis for the evolution of quantitative trains Patterns of selection: o directional Fitness and trait value: fitness is higher with low trait values Directional selection for small trait value so distribution of bell curve of trait value will shift left In this case Only if trait is heritable o Stabilizing Intermediate trait value has most fitness Bell curve will shrink Ex: birth weight in humans (smallest mortality occurs at median birth weight, aka intermediate trait value) If graph were flat for birth weight, it shows there's selection vs how it's shaped like a bell now Goes on in nature a lot --> not like directional bc there's a limit for directional o Disruptive If individuals at both extremes have high fitness (low trait value and high trait value) Variation in pop is increased, and a bimodal pattern may result Ex: small billed birds and lard billed birds (intermediates don’t survive) Sexual selection: access to mating depends on traits o Females produce bigger gametes…access to females is often limiting in reproductive success so males wind up in a bind o Intra-sexual selection: trait makes bearer able to compete with members of the same sex for access to mates o Inter-sexual selection: trait makes bearer more attractive to opposite sex Usually males being showy o Fitness = survival and reproduction Artificial selection: human mediated selection, important in plant and animal breeding o Ex: dog breeding Still wolves but we created their own species name o Artificial vs natural Natural takes place spontaneously in nature Artificial--some human surveys the crop and picks and chooses o Ex: agriculture o Ex: turkeys --> artificial directional selection Genetic drift o Random change in allele frequencies that occurs because pop are finite. It is most pronounced in small pop o Chance events can affect whether an individual survives or reproduces regardless of its phenotype or genotype Chace injuries, storms Even very well adapted organisms are affected o Chance events can also effect whether an allele is passed on or not Sometimes Aa genotype will only produce a gametes o At population level as a result of these chance events, allele frequencies change randomly from one gen to the next (= gen drift) True in all real (finite) populations, especially in small pops If it were infinite, would not occur o Ex: flipping a coin and expected 50/50 if you flip it twice vs 1 mil times o Principles: Trajectory of allele frequency under gen drift is random (drunkard's walk): no two pops show the same trajectory of allele frequency) The effect of drift is stronger in smaller populations Drift ultimately causes loss of gen variability in pop: one allele at random eventually goes to fixation (freq = 1) under a pure drift process (goes more rapidly in small populations) The prob that an allele goes to fixation under genetic drift is equal to its starting frequency (ie 50% if it starts at p = 0.5) o Bottlenecks in pop size can accelerate genetic drift Chance environmental (founder event) event reduces the pop size which accelerates drift process Ex: cheetahs Most genetically uniform vertebrae o Further consequences Harful alleles may increase in freq by drift and rare advantageous alleles may be lost by drift If pop are reduced to a small size, genetic drift can increase prevalence of rare genetic diseases Ex: Amish and Ellis-van Creveld syndrome Migration (gene flow) o Change in allele freq that occurs when alleles move between pop. Migration is important in limiting divergence of pop within a species Result of migration of indiv movement of gametes between pop o Gene flow can add new alleles to a pop or change the freq of existing alleles Mutation o Sponateous change in makeup of genome which can cause changes in allele freq in pop o Slow, but ultimate source of allelic varation that is acted upon by other three forces o Caused by chace, heritable variation in the DNA molecule o Happens in an individual genome Result of damage, ie smoke Most are either harmful or neutral o Changes allele frequency o Most populations have standing variation --> all arose out of mutatoin o Mutation is weak because mutation rates are low (one in a million) but continuously pumps in variation Allele frequencies in populations result from interactions between forces o Only evolutionary force that fashions adaptation is NATURAL SELECTION o Mutation --> selection balance Deleterious mutations constantly introduced by mutation --> thus doesn't get to frequency = 1 (bc deleterious) Weeded out by selection Mutation/selection balance Natural selection purges out harmul alleles o Migration -- selection balance Selection leads to adaptation to local environment Migration brings in alleles that are not locally adapted o Drift --> selection balance Selection acts most effectively in very large populations Drift is most powerful in very small populations If pop is intermediate --> progress of allele under selection is not consistent Natural selection cannot fashion perfection o Non-adaptive evolution Mutation, gene flow, genetic drift compromise this o All individuals will not be perfectly fit to their local environment Environments constantly changing; there may be a lag in adaptation Natural selection cannot fashion perfect organisms o Evolution is limited by physical constraints o Adaptations are often compromises (trade-offs) Energy given to survival cannot be used for reproduction --> limited amount of energy Ie sexual selection dances o Evolution is limited by historical constraints Neanderthal DNA o Modern humans are 0-4% neanderthal o Neanderthals and modern humans coexisted in Europe and the middle east and interbred o How are the alleles maintained for 30k years? Some are involved in immunity (toll-like receoptors (TLRs))