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Figure 21.CO: Water buffalo grazing © Robert Adrian Hillman/ShutterStock, Inc. Variation and Selection • Modern Synthesis (Darwin + Mendel) – Darwin: evolution is heritable adaptation to environment • No knowledge of heredity – Mendel: genes are passed down as units from one generation to the next • Genetics thought static, no understanding of the origin of variation or impact on evolution – Modern synthesis: evolution is the change in allele frequency within a population over time Variation and Selection • Modern Synthesis (Darwin + Mendel) – Mutation introduces variation – Subject to selection if expressed as phenotype and affects reproductive success – Resulting in adaptive changes in phenotype – Eventually resulting in organismal diversity • Questions Provoked by the Modern Synthesis – What are the allele frequencies in populations? – What causes allele frequencies to change? – Can alternate alleles be neutral with respect to selection? The effect of selective breeding on a domestic plant Figure 21.01a: Teosinte Figure 21.01b: Maize Populations and Allele Frequencies • Deme – Local group of an interbreeding population • A subpopulation – Geographic features can affect size of group • Impassable river, landslip • Allele frequency – Proportion of the different alleles of the genes in a population • Gene pool – Sum total of alleles in the reproductive gametes of a population Hardy-Weinberg Equilibrium = p2 + 2pq + q2 = 1 The frequency of a recessive phenotype in a population is q2. Then p = 1 – q, and the expected frequencies of all genotypes can be calculated Figure 21.03: Genotypic frequencies generated under random mating for two alleles Assumptions of Hardy-Weinberg Equilibrium • Random mating - no mate choice, inbreeding – Large population size (reduces inbreeding) • No mutation (no new variation) • No migration ( no transfer between populations) • No selection (no single allele has any advantage) • No genetic drift (no random change in frequency) Departures from H-W equilibrium indicate that one or more of these factors has affected genotype frequency Figure 21.04: Genotypic frequencies at Hardy-Weinberg equilibrium (Adapted from Wallace.) Figure 21.05: Genotypic frequencies generated under conditions of random mating when there are three alleles Alleles of different genes will reach equilibrium (random association) in a population due to independent assortment of the alleles of the two genes. Linkage disequilibrium occurs when two genes are linked and do not assort independently. Equilibrium is reached more slowly as genes are more tightly linked. Figure 21.06: The proportion of linkage disequilibrium that remains in various generations (Adapted from Strickberger.) Association Mapping and the HapMap Project • Gene mapping – Linkage analysis of individuals with known pedigree – Identifies genes affecting diseases or syndromes • Association mapping – Linkage analysis at the population level • More complete analysis of allele past history • Finer scale mapping of the allele • Detection of rare alleles, or with small affects • HAPMAP – Consortium from six countries – Mapping variation of humans from different regions Inbreeding • Related individuals of • Consequences of similar genotype breed inbreeding with each other – Nonrandom mating • Causes of inbreeding – Self-fertilization • most plants capable of this cross fertilize using pollinators – Limited population size interferes with HardyWeinberg equilibrium • Does not affect allele frequency – Decreases heterozygosity – Causes inbreeding depression • Rare harmful recessives become homozygous at higher frequency Inbreeding coefficients at generations 1 to 15 for four different systems of inbreeding Degrees of heterozygosity (Adapted from Strickberger.) Mutation Rates, Genetic Drift • As A mutates to a, a will increase in frequency. • Typical observed mutation rates of 5 x 10-5 • Forward mutation rate exceeds reverse mutation rate • New recessive mutations are masked by dominant alleles, not subject to selection – change in frequency only by genetic drift • Drift = random changes in allele frequencies that lack consistent direction • Genetic drift – Random sampling of the gene pool of each generation – Leads to random variation in allele frequency within population, but in no predictable direction • Rate of fixation, 1/2N, N = population size Selection and Fitness: Selection measured as s, the extent to which fitness is reduced When s = 1, phenotype is lethal Number of Generations Required to Change Allele Frequencies Change in allele frequency Number of Generations Required under Different Selection Coefficients – a Dominant Allele From To s=1 lethal 0.99 0.80 0.50 0.20 0.10 0.01 0.001 0.75 4 7 17 35 350 3496 0.75 0.50 1 2 5 11 110 1099 0.50 0.25 1 2 5 11 110 1099 0.25 0.10 1 2 5 11 110 1099 0.10 0.01 3 5 12 24 240 2398 0.01 0.001 3 5 12 24 231 2312 0.001 0.0001 3 5 12 23 230 2303 1 Number of Generations Required to Change Allele Frequencies Change in allele frequency Number of Generations Required under Different Selection Coefficients – a Recessive Allele From To s=1 lethal 0.99 0.90 0.90 0.75 0.75 0.50 0.50 0.25 0.25 0.80 0.50 0.20 0.10 0.001 3 5 13 25 250 2 3 7 13 132 2 3 9 18 176 2 4 6 15 31 310 0.10 6 9 14 35 71 710 0.10 0.01 90 115 185 462 924 9240 0.01 0.001 900 1128 1805 4512 9023 90231 0.001 0.0001 9000 11515 18005 45011 90023 900230 1 Recessive alleles present at low frequencies are likely to be numerous in most populations • Thousands of generations may be necessary to eliminate a moderately disadvantageous allele • Environmental change likely to occur before allele eliminated • Recessive alleles will accumulate at low frequencies in population Figure 22.07: Relationship between birth weight and survival in females (Adapted from Karn and Penrose.) Figure 22.08: Modes and effects of selection Some traits show intermediate inheritance that is controlled by more than one gene: This is called quantitative inheritance and is controlled by polygenes Polygenes in progeny of a dihybrid self Figure 3-15 step 6 Outcome of a cross with three quantitative genes: Grain color in wheat Modern genetic usage: ABC = A1A2A3 abc = a1a2a3 (From Genetics Third Edition by Monroe W. Strickberger. Copyright © 1985 by Monroe W. Strickberger. Reprinted by permission of Prentice Hall, Inc., Upper Saddle River, NJ.) Modern Genetic Designations: a1 /a1; a2 /a2; a3 /a3 A1 /A1; A2 /A2; A3 /A3 Migration • Gene flow – A source other than mutation for “new” alleles, genetic variation – Overcomes effects of inbreeding • Gene flow prevents genetic divergence of populations – New mutations in one population eventually are passed to another • When gene flow is blocked populations eventually diverge genetically due to the randomness of mutation Bottlenecks and the Founder Effect • Small, isolated population – Exposed to genetic drift, increased homozygosity from inbreeding, and changes in adaptive landscape • Radical change in selection pressure – Little interspecific competition – Few predators, if any – Very different environment • Evolution: unpredictable directions Speciation and adaptive radiation result from: genetic drift due to founder effects inhibited gene flow from geographic isolation novel selective environments Figure 22.14: Colonization pattern showing founder events (Adapted from Carson.) The Adaptive Landscape • Adaptive Peak – Situations of highest fitness for a specific environment • Population adaptive peaks can vary from individual genomic peaks – Altruism, parasitism • Adaptive Landscapes – Not all peaks are equal – Fitness values, peaks and valleys Evolution may take any of several potential paths An adaptive landscape with different selective peaks, or potential optima Multiple potential paths may lead to the same peak Multiple potential paths may originate from a single origin