ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Twenty Outline INTRODUCTION Most Organisms Have Different Genetic Compositions Evolution Is Dependent on Variation in Environment Macroevolution Evolution of new species from old species Changes occurring over long periods of time Microevolution Evolutionary changes within species Natural selection for certain characteristics Characteristics favor increased reproductive success Adaptation is the result of natural selection Evolution is a progressive series of adaptive changes brought about by natural selection, which when accumulated, result in the creation of new species GENE FREQUENCIES IN NATURE Genetic Variation Is the Raw Material for Selection Over 75 genetically variable genes in blood groups Great deal of variation at enzyme-specifying loci tbl 20.1 Measure protein migration via electrophoresis 5% of enzyme loci in humans are heterozygous Polymorphic Loci Locus with more variation than can be explained by mutation Modern study based on techniques like electrophoresis Insect and plants polymorphic at over half of loci POPULATION GENETICS Study of the Properties of Genes Within Populations Explains behavior of alleles in populations Evolution results from changes in allele frequency The Hardy-Weinberg Principle Genetic variation in populations puzzled Darwin and contemporaries Selection should always favor an optimal form Basis of Hardy-Weinberg equilibrium Large population, random mating and absence of other forces Original proportions of genotype remain constant over time Dominant alleles do not replace recessive alleles Genotypes of population in equilibrium Mathematical basis: binomial expansion of algebraic equation Frequency = specific case/total number of individuals (p + q)2 = p2 + 2pq + q2 Frequency of A allele = p Frequency of a allele = q p + q = 1 Example: coat color in cats Initial counts: black (BB or Bb) = 64; white (bb) = 16 Frequency of bb: q2 = 0.16 Frequency of b: q = 0.4 Since 1 = p + q ; frequency of B: p = 0.6 Frequency of Bb: 2pq = 2 x 0.4 x 0.6 = 0.48 Frequency of BB: p2 = 0.6 x 0.6 = 0.36 Genetic reassortment during sexual reproduction fig 20.1 Random mating, alleles B and b randomly mixed Individual chance to get B allele = 0.6 Individual chance to get b allele = 0.4 Chance for BB: 0.6 x 0.6 = 0.36 Chance for bb: 0.4 x 0.4 = 0.16 Chance for Bb: 2 x 0.6 x 0.4 = 0.48 Example: cystic fibrosis in North Americans of Caucasian descent Frequency of allele: 22 per 1000 = 0.022 = q Proportion affected: 0.00048 = 1 per 2000 Dominant allele frequency: p = 1 - 0.022 = 0.978 Calculate carriers: 2pq = 0.043 = 43 per 1000 WHY DO ALLELE FREQUENCIES CHANGE? Hardy-Weinberg Predicts Consistency Large, randomly mating population Used as baseline to measure changes Expressed as heterozygosity: likelihood of individual being heterozygous at locus Factors that affect equilibrium Mutation Migration Genetic drift Nonrandom mating Selection Only one that produces evolutionary change Only one dependent on nature of environment Mutation Change from one allele to another Alters proportion of alleles in population Generally low rate with slow accumulation of mutations Migration Movement of individuals from one population to another Immigration into population Emigration out of population Subtle movements of drifting immature stages or gametes fig 20.2 Even low levels tend to homogenize allele frequency in populations Gene pool: all alleles present in given population Gene flow: movement of genes between populations Via migration Via hybridization between adjacent populations Genetic Drift Changes in allele frequency in small population Appears to be random, drifting event Small, isolated populations become very different May be major factor in human evolution Founder principle Few individuals begin new, isolated population Source population rare allele may be significant in new population Important factor in oceanic island evolution fig 20.3 Bottle neck effect Populations greatly reduced in size Surviving individuals represent random genetic sample of original population Example: current cheetah population Nonrandom Mating Mating of certain individuals more common than expected Inbreeding: mating with relatives Increases proportion of homozygote individuals Promotes occurrence of double recessive combinations Increases likelihood of genetic disorders fig 20.4 Rare in US, more common in Japan Outcrossing: mating with nonrelatives Plants breed with individuals other than self Have higher proportion of heterozygotes fig 20.5 Selection Artificial selection Breeder selects characteristics Example: pigeons fig 20.6 Natural selection Environment selects characteristics Conditions in nature favor reproduction of most fit Proportions of genes of future populations affected Selection acts directly on phenotype Determined by interaction of genotype and environment Link between alleles and characteristics is variable Limits of selection Alternative alleles may interact with other genes Example: chicken clutch size Example: speed of thoroughbred horses Selection acts only on phenotypes Only expressed characters interact with environment Does not operate on rare recessive alleles Selection against undesirable traits difficult Eugenics not advocated by geneticists SELECTION IN ACTION Successful Operation of Selection Individuals best suited to environment leave the most progeny Fate of any individual not predictable Long term fate of population predictable via statistics Forms of Selection Complicated by interactions between genes fig 13.17 Greatest effect on genes that contribute most to phenotype Directional selection Eliminates one extreme from array of phenotypes fig 20.7a Decreases frequency of promoting extreme Example: Drosophila fig 20.8 Stabilizing selection Eliminates both extremes from array of phenotypes fig 20.7b Increases frequency of intermediate, already the most common Prevents change away from middle range Example: human infant birth weight fig 20.9 Disruptive selection Eliminates intermediate type fig 20.7c Partitions population into homozygous groups Example: color patterns of African butterfly Which Force Is the Most Important in Evolution? All five forces cause genetic variation in populations Individual alleles make varying contributions to fitness Difficult to ascertain precise role of individual allele Only selection produces adaptive evolutionary change Other four are random in direction and essentially neutral