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Population Genetics and Natural Selection 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Darwin • 1835 Charles Darwin visited the Galapagos Islands and became convinced various populations evolved from ancestral form. • 1838 After reading an essay by Thomas Malthus, he theorized some individuals would have a competitive advantage conferred by favorable characteristics. 2 Darwin’s Theory of Natural Selection • Organisms begat like organisms. • Chance variation between individuals. – Some are heritable. • More offspring are produced each generation than can survive. • Some individuals, because of physical or behavioral traits, have a higher chance of surviving than others in the same population. 3 Gregor Mendel • Augustinian Monk – Studied garden pea (Pisum sativum). – Discovered characteristics pass from parent to offspring in form of discrete packets called genes. • Exist in alternate forms - alleles. • Some prevent expression of others. 4 Variation Within Populations • Variation in Plant Populations – Many plant species differ dramatically in form from one elevation to another. • Clausen et al. found evidence of adaptation by ecotypes to local environmental conditions in Potentilla glandulosa. – Distinctive ecotypes. 5 Variation Within Populations • Variation in Plant Populations – Phenotypic differences (growth and flower production) within clones grown at the 3 elevations are the result of environmental differences • Phenotypic plasticity 6 7 Variation in Animal Populations • Genetic Variation in Alpine Fish – Movement of cold adapted aquatic species into the headwaters of glacial valleys that lace the Alps created clusters of geographically isolated populations. • Douglas and Brunner used microsatellite DNA to conclude Coregonus populations are highly diverse and exhibit a high level of differentiation. 8 Hardy Weinberg • Hardy Weinberg principle states that in a population mating at random in the absence of evolutionary forces, allele frequencies will remain constant. p2+2pq+q2 = 1.0 9 Calculating Gene Frequencies • SS (81%) SA (18%) AA (1%) – Frequency of S allele ? • SS + 1/2SA = .81 + ½(.18) = .90 – (.90)2 + 2(.9x.1) + (.10)2 = 1.0 10 Conditions Necessary for Hardy Weinberg • • • • • Random Mating No Mutations Large Population Size No Immigration Equitable Fitness Between All Genotypes – Likely, at least one of these will not be met and allele frequencies will change. • Potential for evolutionary change in natural populations is very great. 11 Natural Selection • Some individuals in a population, because of their phenotypic characteristics, will have higher survival and produce more offspring. – Fitness is the measure of an individual’s contribution of offspring, or genes, to future generations. – Natural selection can favor, disfavor, or conserve the genetic make-up of a population. 12 Stabilizing Selection • Stabilizing selection acts to impede changes in a population by acting against extreme phenotypes and favoring average phenotypes. 13 Directional Selection • Directional selection leads to changes in phenotypes by favoring an extreme phenotype over other phenotypes in the population. 14 Disruptive Selection • Disruptive selection creates bimodal distributions by favoring two or more extreme phenotypes over the average phenotype in a population. 15 Evolution by Natural Selection • Natural selection, which changes genotypic and phenotypic frequencies in populations, can result in adaptation to the environment. – Depends on heritability of trait. h2 = VG / VP • VG : Genetic variance • VP: Phenotypic variance 16 Adaptive Change in Colonizing Lizards • Losos et al. – Genus Anolis • Great diversity includes large amount of variation in size and body proportions. – Length of hind limbs appears to reflect selection for effective use of vegetation. » Diameter of perching surfaces. 17 Rapid Adaptation by Soapberry Bugs • Carroll and Boyd – Soapberry Bug (Jadera haematoloma) feeds on seeds from family Sapindaceae. • Slender beaks to pierce fruit walls. – Distance from outside fruit wall to seeds varies widely - beak length should be under selection. » Found close relationship between fruit radius and beak length. 18 Change Due To Chance • Random processes such as genetic drift can change gene frequencies in populations, especially in small populations. • Major concern of habitat fragmentation is reducing habitat availability to the point where genetic drift will reduce genetic diversity within natural populations. 19 Evidence of Genetic Drift in Chihuahua Spruce • Picea chihuahuana now restricted to peaks of Sierra Madre Occidental in N. Mexico. – Ledig et al. examined populations to determine if the species has lost genetic diversity as a consequence of reduced population size. • Found significant positive correlation between population size and genetic diversity of study populations. 20 Genetic Variation In Island Populations • In general, genetic variation is lower in isolated and generally smaller, island populations. – Reduced genetic variation indicates a lower potential for a population to evolve. 21 Genetic Diversity and Butterfly Extinctions • Frankham and Ralls point out inbreeding may be a contributor to higher extinction rates in small populations. – Reduced fecundity, depressed juvenile survival, shortened life-span. • Saccheri et al. conducted genetic studies on populations of Glanville fritillary butterflies (Melitacea cinxia). – Populations with highest levels of inbreeding had highest probabilities of extinction. 22 Evolution and Agriculture • “Artificial selection” is used to describe selective breeding of domesticated organisms to produce or maintain desirable traits. • “Genetic engineering” is the introduction or deletion of genes in domesticated organisms. – These organisms are termed “genetically modified organisms” or GMOs. 23 Unintended Evolutionary Consequences • The use of chemicals in agriculture can have evolutionary consequences – Plant and animal pests may evolve resistance to the chemicals used to control them • Resistance among pests have been shown to be quick and widespread – Vila-Aiub et al. showed how Johnsongrass quickly evolved resistance to herbicides in Argentina 24 25