Biology 101 Week 1 Notes
Biology 101 Week 1 Notes BIOL 102
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This 6 page Class Notes was uploaded by Denice Arnold on Thursday January 21, 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 25 views. For similar materials see Biological Principles II in Biology at University of Pennsylvania.
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Date Created: 01/21/16
Bio Chapter 21 Thursday, January 14, 2016 8:22 PM Chapter 21 Mechanisms of Evolution (Life: The Science of Biology p. 427 - 446) RelationshipBetween Fact and Theory Evolution- the change in genetic composition of populations over time Drives the origin and extinction of species Gives rise to diversification Evolutionarytheory - understanding of the mechanisms of evolutionary change Darwin and Wallace introduced the idea of natural selection Darwin went on a five-year voyage around the world Took note of differences between species in South America and Europe Temperate regions in South America had similar species to temperate regions in Europe Galapagos islands housed animals found nowhere else Both Darwin and Wallace independently developed ideas of natural selection, Darwin developed his first, so he is more closely associated with the concept Three major propositions for evolution: 1. Species undergo mutations that allow change over time Offspring resemble parents, but are not identical to each other or to either parent 2. Divergent species share common ancestors Descentwith modification 3. Survival and reproduction of individualsbased on variation of traits Natural selection Mendel's work on genetic inheritance influenced ideas about the mechanisms of evolution Modern synthesis of genetics and evolution Genetic and phenotypic variation Phenotypes- physical expression of genes Observable characteristics (ie. eye color) An example of a trait of would be "brown eyes" Genotypes - genetic composition Alleles - different forms of a gene Locus - particular site on a chromosome (plural is loci) Gene pool - sum of all alleles at all loci in a population Genotypes do not alone determine phenotypes Dominant alleles may cause phenotypes to be produced by more than one genotype (ie. AA and Aa may be phenotypically identical) Mechanisms of EvolutionaryChange Evolution in biology refers to change in populations over time Individuals do not evolve, populations do Four important mechanisms of evolution in addition to natural selection (mutation, gene flow, genetic drift, and nonrandom mating) 1. Mutation Source of genetic variation is mutation - any change in the nucleotide sequence in an organism's DNA Mutations arise from imperfections in DNA replication - occur randomly Natural selection is the force that acts on this random variation, which results in adaptation Mutations can also restore genetic variation that other evolutionary mechanisms have removed Even low mutations rates cause considerable genetic variation Ex: If probability of a point mutation is 10^-9 per base pair, DNA would average 3 new point mutations (3 x 10^-9 x 10^-9) and each diploid zygote would have about six new mutations A population of 7 billion people would be expected to carry about 42 billion new mutations Allele frequency- proportion of each allele in the gene pool Genotype frequency- proportion of each genotype among individualsin the population Artificial selection - purposeful selection of phenotypes This force is controlled by humans (agriculturalists, plant breeders, dog breeders, etc.) Adaptation - process by which a favored trait evolves through natural selection Alsorefers to the trait itself Positive selection - selection for beneficial changes Purifyingselection - selection against deleterious changes 2. Gene flow 2. Gene flow Migration of individualsand movement of gametes between populations This phenomenon can change allele frequencies in populations If arriving individualscan survive and reproduce in new location, they may add new alleles to the population's gene pool Ex: migration of Neanderthals in modern non-African human populations Traits such as red hair entered new populations 3. Geneticdrift Random changes in allele frequencies from one generation to the next Occurs in small populations May produce large changes in allele frequency over time Particularly potent when a population is reduced dramatically in size due to environmental events Populationbottleneck- only a small number of individualssurvive an environmental condition Surviving population has a new allele frequency (different ratio of red to yellow beans) Populations are likely to lose much genetic variation Genetic drift alsooccurs when few pioneering individualscolonize a new region Foundereffect 4. Nonrandommating Preferential mating of individualswith either the same (homozygous)or different (heterozygous) genotype Nonrandom mating systems that do not affect the reproductive success of individuals in a population do not result in evolutionary change Only change genotype frequencies (not allele frequencies) Sexual selection - individualsof one sex mate preferentially with particular individuals of the opposite sex May favor traits that enhance chances of reproduction even if they reduce its chance of survival Ex: females more likely to reproduce with males with a conspicious trait even though it may increase the chance that the male would be seen or heard by the predator MeasuringEvolutionaryChange Evolutionary change can be measured by allele and genotype frequencies Calculating allele frequency: The sum of NAA, Naa, and Naa is equal to N, the total number of individualsin the population Total number of copies (found in the denominator of p and q) is represented as 2N because each individualis diploid (has two copies) Note that for a population with two alleles at a given locus: p + q = 1 q = 1 - p If there was only one allele at a given locus, the frequency would be 1 Monomorphic population - allele is said to be fixed Geneticstructure - frequencies of different alleles at each locus and frequencies of the different genotypes in a population Although two populations may have the same allele frequencies for A and a (thus the same gene pool), if they are distributed differently among individuals,the genotype frequencies of the two populations differ Hardy-Weinbergequilibrium Model in which allele frequencies do not change across generations Genotypic frequencies can be predicted from allele frequencies Applies only to sexually reproducing organisms Five principal mechanisms of Hardy-Weinberg equilibrium (exact inverse of the five mechanisms of evolution) 1. No mutation Alleles present in population do not change and no new alleles are added to the gene pool 2. No selection among genotypes Different genotypes have equal probabilities of survival and reproduction 3. No gene flow No movement of gametes (or individuals)in or out of population 4. Population sizeis infinite The larger a population, the smaller will be the effect of genetic drift 5. Mating is random These idealized conditions are never met, but hypothetically, if they were met… Frequencies of alleles at a locus will remain constant from generation to generation No evolutionary change After one generation of random mating, genotypes will occur as follows: Natural selection acts directly on phenotypes Therefore, natural selection acts indirectly on genotypes Fitness - reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes Fitness is a function of the probability of those individuals surviving x the average number of offspring they produce over their lives Fitness is determined by relative rates of survival and reproduction Types of natural selection Stabilizingselection - favors average individuals Ex: human birth weight Babies who are lighter or heavier at birth than the population die at higher rates than babies whose weights are close to the mean Directionalselection - favors individuals that vary in one direction from the mean Over many generations, evolutionary trend is seen in the population When optimal phenotype is reached, stabilizing selection takes over When optimal phenotype is reached, stabilizing selection takes over Disruptiveselection - favors individualsthat vary in both directions from the mean Ex: two types of seeds most abundant (hard seeds and soft seeds) Birds with large bills that can crack hard seeds and birds with small bills that can crack soft seeds will be most likely to survive Gives rise to a bimodal bill-sizedistribution All three types of natural selection tend to reduce genetic variation within populations The Distributionand Maintenanceof GeneticVariation Neutral alleles - do not affect the fitness of an organism No better or worse than alternative alleles at the same locus Sexual recombination- crossing over and independent assortment of chromosomes during meiosis Short term disadvantages: Reduces the rate at which females pass genes onto their offspring Dividing offspring into separate sexes greatly reduces the overall reproductive rate Long term advantages: Generates endless variety of genotypic combinations Sexual recombination facilitates repair of damaged DNA Alsopermits the elimination of deleterious mutations Asexual organisms have no mechanism to eliminate deleterious mutations, which results in lower fitness Muller'sratchet - the accumulation of deleterious mutations Frequency-dependentselection - occurs when the fitness of a phenotype depends on its frequency in a population Clinal variation - pattern of gradual change in phenotype across a geographic gradient Constraintson Evolution Evolution can produce a wide variety of adaptive traits, but there are limitations If an allele does not exist in a given population, it cannot evolve, even if it would be highly favorable by natural selection All evolutionary innovations are modifications of previously existing structures Microevolutionarychanges - short-term changes in allele frequencies Macroevolutionarychanges - long-term changes in allele frequencies
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