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Biology 1201A
Jennifer Waugh

THEORY OF EVOLUTION LECTURE 10 Theory An untested idea or opinion; speculation - Explanation of a set of natural phenomena based on proven/ testable hypothesis/ observations (restricted correct scientific definition) Early Evolutionary Thought Classification + Characterizing Aristotle – Scala Natura - Hierarchy of things compared to the Gods (ladder). Carl Von Linne – Linnean System - Wanted to name everything; Linnean nomenclature Le Comte de Buffon – Vestigal Traits - Discovered some organisms had vestigial traits - Maybe useful to ancestors but no longer relevant Transformation Jean Baptiste Lamarck – Evolved species to best-fit environment. - No longer accepted theory -> tested but proven wrong Geological Georges Cuvier – “Catastrophism”/ fixity of species - Catastrophism: massive events changing everything but other than that everything remains constant Charles Lyell – “Uniformitarianism” – naturally changing - Earth has been changing gradually over time due to natural agents Darwinian Evolution Charles Darwin – Evolution by natural selection - Species change gradually b/c of interactions between individuals’ traits and their environment NATURAL SELECTION LECTURE 11 Natural Selection Differential survival and reproduction of individuals in a population due to current environmental influences: Evolution by natural selection is observable: • Antibiotic resistance in bacteria • Pesticide resistance in insects • Heavy metal tolerance in plants • Beak size in Darwin’s finches Fitness The degree to which an individual contributes offspring (genes) to future generations - In this definition, genes means alleles - Parent A has a W of 2, parent B has a W of 4 - Relative fitness (w) is the fitness of the individual relative to others in the (highest fitness of population gets a w of 1) - Then all other parents get rankings compared to that highest w of 1 - In the example parent B has a w of 1, and parent A has half the fitness of that (0.5) Another example: Squirrels: (BB) = 4, (Bb) = 6, (bb) = 2 - (bb) have a relative fitness of w=0.33 - 2 is one third of 6 (the max absolute fitness) Adaptations Traits that increase the probability that an individual with that trait will survive or reproduce in a particular environment - An adaptation is a trait that is associated with fitness - They are things that help and make individuals reproduce and survive in their environment such as thorns on a rose - Could also be behavioral traits, such as schools of fish (they have a lower chance of being eaten when there is so many of them) - Selection pressures are things that influence fitness (crabs were a selection pressure on the shell thickness) Constraints on adaptation • Available variation for selection to act upon • Changing environments over time • Conflict between selection pressures (trade-offs in fitness) Natural Selection – link to genes • ‘Beach mice’ are light colored; ‘mainland mice’ are darker • Blending with native soil color reduces predation • Light color a result of a single amino acid change in Mc1r • Frequency of ‘light’ allele correlates with degree of lightness in populations (Micro)Evolution Small-scale changes in genetic make-up of a population - This is the definition of evolution (micro) - Population level process that causes changes in mean (most frequent) phenotype - In the example, over time if the predators eat all the blue and green flowers, or some other factor occurs that leaves mostly yellow flowers, our end population will look much different than that of our beginning one HARDY-WEINBERG PRINCIPLE LECTURE 12 Hardy-Weinberg Principle Background on pig population & color inheritance: - In the Mendelian pigs experiment we tried to get pure bred pigs (pure bred brown, and pure bred black) - Artificial selection = selective breeding of animals or plants to ensure that desirable traits appear in successive generations - From the experiment we found out that black (B) is a dominant trait, and brown (W) is recessive - We also found out that phenotype ratios of offspring for a particular cross are predictable (with clean ratios 1:0, 1:1, 3:1) Calculating proportions (frequencies) Proportion = # of items of interest # of items in total A large population exists in which homozygotes and heterozygotes have the same fitness: allele frequencies are: f(B) = 0.3, f(W) = 0.7. What happens to allele frequencies overtime? - The frequencies hovered around what they were to begin with; there weren’t any big changes - Nothing is changing because none of the pigs are experiencing evolution; since the fitness is the same they all have the same rate of reproduction - The allele component of the next generation is the same as the preceding one because alleles are just passed down to offspring Conclusions - Knowing whether an allele is dominant or recessive does not tell you if it is going to be common or rare (and the same vice versa) - Phenotype ratios for populations aren't "nice" (clean ratios) - Just because an allele becomes common in a population, it doesn’t mean it becomes dominant Punnett Squares Allele frequencies give probabilities of gamete ‘genotypes’ and expected offspring genotype frequencies based on probability - p = probability of getting an A1 egg and an A1 sperm - 2(p x q) = getting heterozygotes - q = probability of getting a homozygous recessive Genotype frequencies of offspring: 2 2 P =p A A1 1 2(p x q) = 2pq A1 2 q =q2 A 22 Investigating the HW Principle Are genotype frequencies predictable? - Initial allele frequencies: - f(B) = 0.4, let p = f(B), f(W) = 0.6, let q = f(W) - Predicted genotype frequencies: - f(BB) = p = 0.4 = 0.16 - f(BW) = 2p2 = 2(024 x 0.6) = 0.48 - f(WW) = q = 0.6 = 0.36 - After ~ 360 months - f(BB) = 0.14, f(BW) = 0.45, f(WW) = 0.41 - Compare the observed and the predicted values and use a chi-square test to see if the outcomes are within probability and if we accept or reject our null hypothesis CONCLUSION Following two statements define the Hardy-Weinberg principles: 1) Allele frequencies don't change over time (its an equilibrium) 2) Genotype frequencies (and therefore phenotype frequencies) in a population can be predicted from allele frequencies - p and q are allele frequencies, remember p2, 2pq, q2 The following 5 criteria are required to predict genotype frequencies from allele frequencies: 1) In this simulation, no phenotype had a higher fitness, so when we say that we can predict frequencies there must be NO selection 2) Random mating took place, there was no external factors acting on the pigs 3) The only alleles in the gene pool were the ones from the beginning, no mutations occurred 4) The population was closed (no immigration or emigration), this is known as having no "gene flow" (new inputs of alleles into the gene pool) 5) No random events, no genetic drift, this comes from assuming a large population Testing for departures from equilibrium Example 1: population of 1000 mice, 422 brown (BB), 455 tan (Bb), 123 albinos (bb). Does this population demonstrate the Hardy-Weinberg Principle? - There are 844 B alleles from the homozygous BB, and 455 alleles from the heterozygous Bb (total = 1299) - There are 246 b alleles from the homozygous b, and 455 b alleles from the heterozygous Bb (total = 701) - So the proportion of alleles that are B = 1299/2000, which equals 0.6495 Example 2: Population: 1000 mice, 300 (BB), 550 (Bb) 150 (bb). H-W Principle? - Figure out the allele frequency by dividing the amount of alleles by the total amount of alleles - Take those frequencies and figure out the genotype frequencies by using p- squared, 2pq, and q-squared - Use genotype frequencies and multiply them by the total number of individuals in the population to get your expected numbers on genotypic individuals - Use your expected numbers (numbers you solved for) and your observed numbers (original numbers given) and use a chi-squared test to determine if they are at H-W equilibrium Example 3: rare disease (recessive phenotype). Frequency = 0.001. in London, how many of the 350,000 people would carry (not show) this allele? - Homozygous recessive = death but we want the people that are Dd (carriers) - Frequency of the disease = people that are (dd) = 0.001 - f(dd) = p2, f(DD) = q2, f(Dd) = 2pq, f(dd) = p2 = 0.001, p = square root of 0.001 = p = 0.03162 - To get the allele frequency work backwards from genotype frequency - only possible if the population is at H-W equilibrium - Subtract 0.03162 from 1 to get q, -> 2pq -> # of carriers. - q = 1 - p - q = 1 - 0.03162 - q = 0.96838 - we want f(Dd), which = 2pq - Dd = 2(0.03162)(0.96838) - Dd = 0.06124 - Multiply frequency of q (which we just solved for) by # of individuals - Carriers = 0.06124 x 350 000 - Carriers = 21 434 EVOLUTIONARY MECHANISMS LECTURE 13 Mechanisms of Evolution: - Mutation, selection, sampling drift, gene flow, non-random mating Where does genetic variation come from? - Mutations = deletion, duplication, translocation, inversion - Chromosomal mutations = polyploidy, aneuploidy, trisomy, etc.. Shuffling of genes and alleles - Random mating, fertilization, crossing over, recombination - Open population (new alleles entering), independent assortment (do maternal and paternal chromosomes mix from metaphase 1?) Mutations Spontaneous and heritable change in DNA: rare, random errors, deleterious mutations, and advantageous mutations => Ultimate source of genetic variation. - Usually the phenotypic of a deleterious mutation is unfavorable, and decreases the fitness - Advantageous mutations can have favorable phenotypic effects, and increase the fitness - Mutation = RANDOM, doesn’t matter what the selection favoritism is. Also is the greatest source of genetic variation and the fuel for natural selection How is so much variation possible if mutations are rare and usually harmful? - Homeotic genes regulate the expression of other genes - Mutations in regulatory genes for development can generate new body shapes - Misconception is that mutations have a negative effect, this is untrue they are only the ones with phenotypic differences - Many mutations go unnoticed and can be helpful, or neutral - Homeotic genes are like tropic hormones; they regulate expression of other genes Selection Shouldn’t selection reduce genetic variation? Advantageous mutations should ‘fix’ in a population (fixation) => genetic variation should be temporary but: - A mutation that fixes in a population means that it becomes the only allele present - Selection should weed out all the inferior alleles and leave the strongest ones, but this isn’t the case - Selection is not the only thing that can affect genetic variation Modes of selection 1: Directional - Fitness increases with phenotype value - Mean phenotype changes over time, variance reduced (but may be unchanged) - One extreme favored, the other extreme becomes non-favored. Mode of selection 2: Stabilizing - Intermediate phenotype has highest fitness - Mean phenotype maintained, variance reduced - In stabilizing selection, there is selection against the extreme phenotypes Human Birth Weight - Historically, large/ small babies had reduced survival relative to 7-8 lb. babies - Modern medicine has since reduced the variation in fitness Mode of selection 3: Disruptive - Extreme phenotypes have highest fitness - Does not alter mean phenotype, variance increases (perhaps multi-modal distribution) - In this situation it is good to be the extreme phenotype (opposite of stabilizing) - Over time you could expect intermediate phenotype to disappear and end up with a bi-modal selection (two bumps on the graph (extremes)) Bluegill sunfish male strategies - Large males defend territories, attracting females; small males sneak in and steal - Both large/ small males = high reproductive success than medium males Heterozygote advantage: higher fitness than either homozygote (sickle-cell/ malaria) - In this example it is good to be heterozygous for sickle-cell anemia because being a carrier for it confers for resistance to malaria - Homozygous normal isn't good because then they have no defense against malaria, and homozygous sickle-cell is unfavorable because they die from that disease - Hence heterozygote (carriers) is most favorable, and will help maintain genetic variation because they pass on both alleles Sampling Drift Change in allele frequencies due to the effect of chance in small samples - Unpredictable changes in the short term - Should cause fixation in the long term - Sam
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