Biology 102, Week 1 Notes
Biology 102, Week 1 Notes BIOL 102,
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This 6 page Class Notes was uploaded by Cambria Revsine on Sunday February 7, 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 17 views. For similar materials see Biological Principles II in Biology at University of Pennsylvania.
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Date Created: 02/07/16
Biology 102—Week 1—Chapter 21 Lecture 1: All species lines have evolved to this point because of adaptations that make organisms more fit for survival: Adaptation: changes to more successful traits in a population over time o Camouflage, mimicry, makeup of cells etc. Diversity and unity: balance between differences that set species apart, and the concept that all species, and smaller subgroups of species, share certain similarities o Why similar organisms tend to live closer than nonsimilar organisms Natural Selection discovered by Charles Darwin during his Beagle voyage from Britain to Galapagos Islands and around the world (18311836) o Same ideas were discovered by Alfred Russel Wallace, Wallace wanted to publish his ideas which persuaded Darwin to finally publish On the Origin of Species (1859) Species evolve over time Species share common ancestors and branched off gradually (Darwin developed early phylogenetic tree concept) Species change because of natural selection *Theory of Evolution* Evolution explains reasoning for adaptation and diversity, and natural selection is the primary cause of evolution Lecture 2: Evolution: change in allele frequency of a population over generations o Individuals develop, populations evolve Alleles: different forms of a gene o At each genetic locus of an individual, there are two alleles, one from the mother and one from the father o In a population, there can be many alleles for a given gene, but allele frequency always sums to 1.0 Evolutionary Biology: DarwinWallace (1858) Gregor Mendel: Mendelian inheritance (early 1900s) o Darwin and Mendel did not know about genes Four Evolutionary Forces Forces that alter allele frequency: Selection: fitness differences among individuals in a population mean those that are good at survival and/or reproduction are selected for, causes the evolution of adaptive features Genetic Drift: random change in allele frequencies by chance in a population because populations are finite, most prominent in small populations Migration (gene flow): change in allele frequency from individuals joining or leaving a population Mutation: Spontaneous change of the base pairs of an individual’s DNA, it can change the overall allele frequency of a population but often very slowly o the ultimate source of allele variation that is acted on by the other three sources Calculating Allele and Genotype Frequencies: 1. Begin with number of individuals in a population with AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive) genotype 2. Find frequencies of both alleles: 2N +AA Aa Frequency of allele A: p= 2N 2N aa Aa Frequency of allele a: q= 2N *N= number of individuals, Nxx number of individuals with specific genotype *2N= number of individuals x2, aka number of alleles in the population *p + q = 1 3. Find frequencies of the three genotypes: N AA Frequency of genotype AA: N N Frequency of genotype Aa: Aa N N aa Frequency of genotype aa: N 4. If you are given the genotype frequencies, to find allele frequencies: Frequency of p: Frequency of AA + ½ frequency of Aa Frequency of q: Frequency of aa + ½ frequency of aa 5. If you are given the allele frequencies, to find the genotype frequencies: 2 Frequency of AA: p Frequency of Aa: 2pq Frequency of aa: q2 Hardy Weinberg equilibrium: hypothetical situation where no evolutionary forces are acting on a population and mating is random “no evolution”; genotypic and allelic frequencies stay the same 1.) Selection Produces Adaptations: 1. Variation for a trait (from mutation/ allele differences) 2. Different fitness rates (survival and/or reproduction) depending on trait value 3. Transmission of trait value to next generation (must be heritable) 4. Change in allele frequency, higher proportion of the population now has the advantageous trait Natural Selection Example: The peppered moth, Biston betularia, in 19 and 20 century Britain Two forms were studied o typical form (typica), lightcolored o melanic variant (carbonaria), darkcolored, first noticed at 1% in 1848 Manchester (highly polluted area), rose to 98% of population in 1895 Carbonaria were found to be located in correlation with polluted areas acroth the country, which declined after the induction of antipollution laws in mid20 c. Scientists tested the hypothesis that Carbonaria’s dark color gave them a selective advantage in highly polluted areas and viceversa with typica by tracking rates of bird predation both in a controlled experiment and using markreleaserecapture methods in nature; this hypothesis was proven true Proved that bird predation was the natural selective cause of the increase of Carbonaria Lecture 3: Genotype + Environment = Phenotype (Both nature and nurture) *Selection acts based on phenotype (differences in how traits are expressed in real life), not genotype *However, evolution only occurs due to genetic variation, not affected by environmental variation Patterns of Selection: Most traits are not simple onelocus genes, but are affected by environment and many different gene loci quantitative traits Exhibit a bellshaped curve In quantitative traits, changes in allele frequency of multiple genes is the basis of their evolution Three main patterns of selection (trait value along the xaxis, fitness of trait yaxis): If the trait value is most fit at intermediate value (i.e. medium height), average trait value of population will condense to the middle, away from outliers If the trait value is most fit at one extreme (i.e. slim beaks), average trait value will shift to one end, away from the end that isn’t as fit anymore If the trait value is most fit at either end of value scale (i.e. light or dark coloring), average trait value will split to both extremes, in some cases can divide a population/ species into two variants Sexual selection: access to mating depends on traits Intrasexual selection: traits that make individuals better at competing with others of the same sex for mates Intersexual selection: traits that make individuals more attractive to members of the opposite sex *Usually in a situation of sexual reproduction where one sex (usually females) expends more energy than the other on gametes (eggs and sperm), so the males usually compete for females * Often a tradeoff for males between survival and mating (showiness can be disadvantageous in survival) Artificial Selection: controlled selection over breeding by humans on plants and animals i.e. dog domestication from wolves; chickens selected for size, meatiness 2.) Genetic Drift: Chance events outside of phenotype/genotype can affect survival and reproduction o random injuries, weather So, allele frequency changes randomly in each generation in all finite populations The smaller the population, the greater the effect of randomness –genetic drift The probability an allele will go to fixation (frequency = 1) equals its starting frequency Bottleneck Effect: A population might start out with near equal allele frequencies, but due to a chance disaster, only a small percentage of the population survives that has much different allele frequencies for traits from the original ones Cheetahs have gone through bottleneck effect; their population has greatly reduced in the last decade resulting in 0% heterogeneous gene loci for the 52 surveyed More consequences of genetic drift: Harmful alleles might increase in frequency, and rare advantageous alleles might be lost Bottleneck effect might increase prevalence of rare genetic diseases o Ellisvan Creveld (EVC) syndrome in Amish due to founder effect (like bottleneck effect, when a small subset of a population leaves the pop.) 3.) Migration (gene flow): From individuals and gametes moving between populations Can add new alleles a population/ change existing frequencies
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