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Bi 203-001 Week 1 notes

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by: Jaime Jackson

Bi 203-001 Week 1 notes BI 203-001

Marketplace > Portland State University > Biology > BI 203-001 > Bi 203 001 Week 1 notes
Jaime Jackson
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These notes are what we went over in the first week of class. They go over chapter 13 of the book Campbell Biology: Concepts.
Radhika Reddy
Class Notes
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"I had to miss class because of a doctors appointment and these notes were a LIFESAVER"
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This 4 page Class Notes was uploaded by Jaime Jackson on Thursday January 28, 2016. The Class Notes belongs to BI 203-001 at Portland State University taught by Radhika Reddy in Winter 2016. Since its upload, it has received 32 views. For similar materials see in Biology at Portland State University.


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Date Created: 01/28/16
Tuesday, March 29, 2016 Chapter 13: How Populations Evolve Objectives 1. Populations, Genes, and Evolution are related 2. Causes of Evolution 3. Mechanisms of Natural Selection Where do traits come from? In sexually reproducing organisms, half the genetic material originates from the mother and half from the father
 HowAre Populations, Genes, and Evolution Related? ▪ Evolution is not a property of individuals, but of populations (members of a species in a given area) ▪ Inheritance of genes provides the link between the lives of individual organisms and the evolution of populations Genes (genotype) and the environment interact to determine traits (phenotype) Coat color in hamsters illustrates the interaction between genotype and phenotype ▪ Dominant allele: enzyme for black pigment ▪ Recessive allele: enzyme for brown pigment The gene pool comprises all of the alleles in a population: the hamster example continued 25 hamsters have 50 alleles for coat color If 20 of those 50 alleles code for black coats, then frequency of the black allele = 20/50 = 0.40 = 40% NOTE: don’t confuse mendelian genetics (where punnett squares are predictive for individuals) with population genetics, where predictions rely on observed gene frequencies. Evolution is the change of allele frequencies within a population over time Evolving Population: allele frequencies change from one generation to the next Equilibrium population: allele frequencies do not change from generation to generation and the following conditions are met: 1. No mutations 2. No gene flow (immigration/emmigration) 3. The population must be very large 4. Mating must be random, with no tendency for certain genotypes to mate with specific other genotypes 5. No natural selection Hardy Weinberg The Hardy–Weinberglaw: 1. frequency of the alleles in the gene pool does not change over time 2. After one generation of random mating, frequencies for two alleles can be calculated as p + 2pq + q = 1 – p equals the frequency of allele A. – q is the frequency of allele a ▯ 1 Tuesday, March 29, 2016 Evolution is caused by violation of the rules for an equilibrium population Causes of evolution: 1. Mutations 2. Gene flow (immigration/emmigration) 3. Small populations 4. Non random mating, with tendency for certain genotypes to mate with specific other genotypes 5. Natural selection (environmental pressure) Mutations are the original source of genetic variability Inherited mutations are rare but important ▪ ▪ Mutations are spontaneous, not goal directed Gene flow between populations changes allele frequencies 
 Gene flow: movement of alleles from one population to another – Movement of individuals between populations is a common cause of gene flow – Alleles can move between populations even if organisms do not: Pollen (sperm) and seeds from flowering plants can move and distribute alleles The evolutionary effect of gene flow is to increase the genetic similarity of different populations of a species – If gene flow between populations of a species is blocked, the resulting genetic differences may grow so large that one of the populations becomes a new species Allele frequencies may change by chance in small populations Genetic drift:chance events change allele frequencies. occurs more in small populations, where chance may dictate which in the gene pool are passed on Examples of chance events which prevent inheritance: –Seeds that fall into a pond or parking lot –Flowers destroyed by wildfire before pollination –Any organism killed by a flood prior to reproducing –Genetic drift Population Bottleneck: population is drastically reduced by a natural catastrophe or overhunting In a population bottleneck, only a few individuals are available to contribute genes to the next generation, thus changing allele fequency – Elephant seals hunting in 1890s: population → 20 – After hunting ban, population → 30,000 – present-day northern elephant seals almost genetically identical Founder effect: Isolated populations founded by small number of individuals - Rare alleles can be amplified in populations derived from few founder organisms Non Random Mating: Mating within a population is almost never random Inbreeding (sexual reproduction among relatives) is more likely if species lack mobility ▪ and tend to stay near their birthplace 2 Tuesday, March 29, 2016 – Offspring inherit same alleles from both parents, increasing homozygosity and prevalence of genetic disorders. ▪ Assortative mating: mate choice bias. - Neither inbreeding nor assortative alone alter allele frequencies! - Rather the distribution of genotypes and, therefore, of phenotypes are changed. Natural selection: individuals with traits that help them survive and reproduce leave more offspring Reproductive fitness: likelihood for reproduction – Natural selection acts on phenotypes, the structures and behaviors displayed by the members of a population – The selection of phenotypes subsequently affects genotypes and alleles present in a population Mechanisms of Natural Selection Adaptations: characteristics improving reproductive fitness – Nonliving (abiotic) environmental factors: climate, the availability of water, and the concentration of minerals in the soil – Living (biotic) factors: other organisms – Competition: interaction among organisms competing for scarce resources. It is most intense among members of the same species – Predation encourages Co-evolution, in which species adapt in response to another species adaptations. Ex: Wolf predation selects against slow deer and swift deer select against slow wolves – Sexual selection is a type of selection that favors traits that help an organism acquire a mate Selection can influence populations in three ways 1.Directional selection favors individuals with an 
 extreme value of a trait and selects against both 
 average individuals and individuals at opposite 
 extremes 2. Stabilizing selection favors individuals with the 
 average value of a trait and selects against 
 individuals with extreme values 3. Disruptive selection favors individuals at both 
 extremes of a trait and selects against individuals 
 with intermediate values Directional selection shifts character traits in a specific direction ▪ If environmental conditions change in a consistent way, a species may respond by evolving in a consistent direction – For example, if the climate becomes colder, mammal species may evolve thicker fur ▯ ▯ 3 Tuesday, March 29, 2016 Stabilizing selection acts against individuals who deviate too far from the average ▪ Stabilizing selection commonly occurs when a trait is under opposing environmental pressures from two different sources ▪ For example, Aristelliger lizards of intermediate body size are favored over the extremes – The smallest lizards have difficulty defending territory – The largest lizards are more likely to be eaten by owls Disruptive selection adapts individuals within a population to different habitats ▪ Balanced polymorphism: population divides into two phenotypic groups over time ▪ Beak size in black-bellied seedcrackers – Birds with large, stout beaks are able to crack the hard seeds found in their habitat – Birds with small, pointy beaks are better adapted to process the soft seeds of the habitat – Birds with intermediate-size beaks have a lower survival rate than individuals with either large or small beaks Balanced polymorphism often occurs when environmental conditions favor heterozygotes Example: normal and sickle-cell hemoglobin alleles coexist in malaria-prone regions of Africa – Homozygotes for the sickle-cell allele suffer from severe anemia caused by clumping of the defective hemoglobin in their red blood cells, and often die – Heterozygotes have one allele for defective hemoglobin, which gives them increased resistance to malaria, and one allele for normal hemoglobin, which moderates their anemia, giving them a selective advantage over either type of homozygote 4


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