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HWE Slides

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HWE Slides Biol 28600


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Notes from HWE slides
Introduction to Ecology and Evolution
Joshua Springer
Class Notes
Ecology, evolution, Biology
25 ?




Popular in Introduction to Ecology and Evolution

Popular in Biology

This 31 page Class Notes was uploaded by Sierra on Thursday February 11, 2016. The Class Notes belongs to Biol 28600 at Purdue University taught by Joshua Springer in Spring 2016. Since its upload, it has received 25 views. For similar materials see Introduction to Ecology and Evolution in Biology at Purdue University.


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Date Created: 02/11/16
Announcements • Pre-Test Grades will be uploaded over the next few days. Please be patient. • I will begin uploading clicker grades at the end of this week – 1 point for answering each question – Additional 1.5 points available for answering questions correctly • So…2.5 points every time I ask a question! • I will try to get Homework 1 online before Feb 2 Understanding the diversity we see in populations of species: • The central issue in population genetics is genetic variation – Its extent within populations – Why it exists – How it changes over the course of many generations • Emerged as a branch of genetics in the 1920s and 1930s – Its foundations are largely attributed to mathematicians GENES IN POPULA TIONS • Population genetics is a direct extension of Mendel’s laws of inheritance, molecular genetics, and the ideas of Darwin • The focus is shifted away from the individual and toward the population of which the individual is a member • All of the alleles of every gene in a population make up the gene pool – generationiduals that reproduce contribute to the gene pool of the next • Population geneticists study the genetic variation within the gene pool and how it changes from one generation to the next Some Genes Are Monomorphic, but Most Are Polymorphic • The term polymorphism refers to the observation that many traits display variation within a population • Polymorphism in the Hawaiian happy-face spider – All individuals are from the same species, Theridion grallator • But they differ in alleles that affect color and pattern Polymorphism in the Hawaiian happy-face spider iClicker (set to AA) Migration can be thought of as an evolutionary force when: -A) new individuals stay for a long time period -B) a group of new individuals form feeding groups with existing individuals -C) new individuals begin reproducing with existing individuals -D) if some individuals leave a population when new individuals arrive -E) I say it is. Population Genetics Is Concerned with Allele and Genotypic Frequencies • Two fundamental calculations are central to population genetics Number of copies of an allele  Allele frequency = in a population Total number of all alleles for that gene in a population Number of individuals with a  Genotype frequency = particular genotype in a population Total number of individuals in a population • Consider a population of 100 frogs Allele frequency – 64 dark green frogs with the genotype GG – 32 medium green frogs with the genotype Gg – 4 light green frogs with the genotype gg Number of copies of allele g in the population  Frequency of allele g = Total number of alleles G and Homozygotes g in the population Heterozygotes have two copies have only one of allele g (2)(4) + 32  Frequency of allele g = (2)(64) + (2)(32) + (2)(4) All individuals have two copies of each gene 40  Frequency of allele g = 200 = 0.2, or 20% • Consider a population of 100 frogs Genotype frequency – 64 dark green frogs with the genotype GG – 32 medium green frogs with the genotype Gg – 4 light green frogs with the genotype gg Number of individuals with  Frequency of genotype gg = genotype gg in the population Total number of individuals in the population 4 % of light  Frequency of genotype gg = 64 + 32 + 4 green frogs in the population 4  Frequency of genotype gg = 100 = 0.04, or 4% • For a given trait, the allele and genotype frequencies are always less than or equal to 1 – i.e., less than or equal to 100% • For monomorphic genes – The allele frequency for the single allele will be equal to 1.0 • For polymorphic genes – The frequencies of all alleles should add up to 1.0 – In our frog example • Frequency of G allele + frequency of g allele = 1 • Frequency of G allele = 1 – frequency of g allele = 1 – 0.2 = 0.8, or 80% iClicker • Which evolutionary force can counteract natural selection? – A) Genetic Drift – B) Migration (Gene Flow) iClicker • What is the source of genetic variation in any population? – A) Migration – B) Gene Flow – C) Recombination – D) Mutation – E) Evolution HARDY -WEINBERG EQUILIBRIUM • The Hardy-Weinberg equation was formulated in 1908 – relates allele and genotype frequencies in a population • The HW equation is also called an equilibrium – Under a given set of conditions (described on next slide) • The allele and genotype frequencies do not change over the course of many generations • The Hardy-Weinberg equation predicts an equilibrium-unchanging allele and genotype frequencies from generation to generation-if certain conditions exist in a population – 1. No new mutations – 2. No genetic drift. The population is so large allele frequencies do not change due to random sampling effects – 3. No migration – 4. No natural selection – 5. Random mating • The HW equation provides a quantitative relationship between the allele and genotype frequencies Hardy-Weinberg in Action • Consider our frog example with a polymorphic gene that exists in two alleles, G and g – The frequency of allele G is denoted by the variable p – The frequency of allele g is denoted by the variable q • p + q = 1 – For this gene, the Hardy-Weinberg equation states that 2 2 • (p + q) = 1 • p + 2pq + q = 1 Genotype Genotype Genotype frequency of GGfrequency of Gfrequency of gg • If p = 0.8 and q = 0.2, and if the population is in Hardy-Weinberg equilibrium, then 2 – frequency of GG = p = (0.8) = 0.64 – frequency of Gg = 2pq = 2(0.8)(0.2) = 0.32 – frequency of gg = q 2 = (0.2) = 0.04 • compare the Hardy-Weinberg equation with the Punnett square approach where alleles combine based on probability to create new offspring G g 0.8 0.2 GG Gg G (0.8)(0.8) (0.8)(0.2) 0.8 = 0.64 = 0.16 Gg gg g (0.8)(0.2) (0.2)(0.2) 0.2 = 0.16 = 0.04 GG genotype = 0.64 = 64% Gg genotype = 0.16 + 0.16 = 0.32 = 32% gg genotype = 0.04 = 4% The frequency of gametes carrying a particular allele is equal to the allele frequency for a population in Hardy-Weinberg equilibrium. Multiplying the allele frequencies gives the proportion of each allele combination in the population. • Hardy-Weinberg equilibrium can be used to predict the frequency of carriers (heterozygotes) for a recessive genetic disease • Cystic fibrosis affects 1 in 2500 individuals q = 1/2500 q = 0.0004 We take the square root and find q= 0.02 p = 1- q p = 1 - 0.02 = 0.98 frequency of heterozygous carriers is 2pq = 2 (0.98)(0.02) = 0.0392 or 3.92% This genotype predominates when This genotype the frequencies of predominates when Co1.0ight © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. the frequency of allele g and G are intermediate allele g is low 0.9 GG gg 0.8 0.7 This genotype 0.6 predominates when Gg the frequency of 0.5 allele g is high 0.4 Genotype frequency 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Frequency of allele g The relationship between allele frequencies and genotype frequencies according to the Hardy-Weinberg equilibrium • In reality, no population satisfies the Hardy-Weinberg equilibrium completely • However, in some large natural populations there is little migration and negligible natural selection – In these cases, the HW equilibrium is nearly approximated for certain genes • In addition, the HW equation can be extended to situations in which a gene exists in 3 or more alleles Haemochromatosis in people of Northern European descent • 1/200 humans of this ancestry have both recessive alleles • For Thursday calculate the frequency of heterozygous carriers. OVERVIEW OF MICROEVOLUTION • Genetic variation in natural populations changes over many generations • Microevolution describes changes in a population’s gene pool from generation to generation – Driven by: • Mutation • Random genetic drift • Migration • Natural Selection • Nonrandom mating NA TURAL SELECTION • In the 1850s, Charles Darwin and Alfred Russel Wallace independently proposed the theory of natural selection – According to this idea there is a struggle for existence – Those individuals that are most-adapted to their particular environment will survive and reproduce • Recently population geneticists have realized that natural selection can be related to mating efficiency and fertility – Not just to differential survival – Some individuals are just better at mating than others and are more fertile • A modern description of natural selection can relate molecular genetics to the phenotypes of individuals – 1. Within a population there is allelic variation arising from various factors such as mutations causing differences in DNA sequences • Distinct alleles may encode proteins of differing functions – 2. Some alleles may encode proteins that enhance an individual’s survival or reproductive capacity – 3. Individuals with beneficial alleles are more likely to survive and reproduce – 4. Over the course of many generations, allele frequencies of many different genes may change through natural selection • This significantly alters the characteristics of a species • The net result of natural selection is a population that is better adapted to its environment and/or more successful at reproduction In smaller populations, allele frequency fluctuates substantially from generation to generation Fixed alleles no longer fluctuate 1.0 Fixation of allele A N = 20 N = 20 A N = 1000 0.5 Frequency of In larger populations, allele frequency fluctuates much less N = 20 N = 20 N = 20 Loss of allele A 0 Generations A hypothetical simulation of random genetic drift Significant factors that affect the genetic diversity of populations • Bottleneck effect – In nature, a population can be reduced dramatically in size by Large, a natural disaster for example genetically diverse – Such a disaster randomly population eliminates individuals regardless of their genotype – The period of the bottleneck, Bottleneck: when the population size is Fewer individuals, less diversity very small, may be influenced by genetic drift Large, less  The African cheetah has lost genetically diverse nearly all of its genetic population variation  This is due to a bottleneck effect that occurred 10,000 to (a) Bottleneck effect 12,000 years ago • Founder effect – A small group of individuals separates from a larger population and establishes a colony in a new location – This has two important consequences • 1. The founding population is expected to have less genetic variation than the original population • 2. The founding population will have allelic frequencies that matter of chancedly from those of the original population, as a – Example: The Old Order Amish of Lancaster County, PA • Population of 8,000 is descended from just three couples that immigrated to the US in 1770 • dwarfism) is 7%lis-van Creveld syndrome (a recessive form of – This is much higher than in any other population • Mating and phenotypes – Assortative mating occurs when individuals do not mate randomly – Pmore likely to mate due to similar phenotypicduals are characteristics – Negative assortative mating occurs when individuals with dissimilar phenotypes mate preferentially • Mating and genotypes – Inbreeding is the mating between genetically-related individuals – Outbreeding is the mating between genetically-unrelated individuals – In the absence of other evolutionary forces, allele frequencies are not affected by in- or out-breeding • However, these patterns of mating do disrupt the balance of genotypes predicted by the HW equation


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