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THE EVOLUTION OF POPULATIONS EVOLUTION AND VARIATION ¢ Microevolution- small scale evolution; change in allele frequencies in a population over generations. ¢ Discrete Characters- classified on an either-or basis ¢ Quantitative Characters- vary along a continuum ¢ Average Heterozygosity- (gene variability) the average percent of loci that are heterozygous. ¢ Nucleotide Variability- comparing DNA sequences of two individuals ¢ Geographic Variation- differences in genetic composition of separate populations. MUTATION ¢ Mutation- the ultimate source of new alleles ¢ Point mutations- a change in one base in a gene ¢ Neutral and Beneficial Mutations ¢ Mutations Rates Plants/Animals- 1/100,000 genes per generation Prokaryotes- fewer mutations, shorter generation span, more genetic variation Viruses- more mutations, shorter generation span, RNA genome with fewer repair mechanisms GENE POOLS AND ALLELE FREQUENCY ¢ Population- a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring. ¢ Gene pool- all of the alleles for all the loci in all individuals of the population. Fixed- only one allele exists for a particular locus and all individuals are homozygous for that allele HARDY-WEINBERG PRINCIPLE ¢ H-W Equilibrium describes a constant frequency of alleles within a gene pool. ¢ p2 + 2pq + q2 = 1 ¢ where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Alleles in the population Frequencies of alleles p = frequency of CR allele = 0.8 q = frequency of CW allele = 0.2 Gametes produced Each egg: Each sperm: 80% 20% chance chance 80% 20% chance chance HARDY-WEINBERG ASSUMPTIONS 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow *Departure from any of these conditions usually results in evolutionary change. PRACTICE HARDY-WEINBERG PROBLEM ¢ For a locus with two alleles (A and a) in a population at risk from an infections neurodegenerative disease, 16 people had genotype AA, 92 had genotype Aa, and 12 had genotype aa. Use the Hardy-Weinberg equation to determine whether this population appears to be evolving. MECHANISMS THAT ALTER ALLELE FREQUENCY Natural Selection Leads to adaptive radiation ¢ Genetic Drift- chance events that cause allele frequencies to fluctuate unpredictably from one genera8on to the next. ¢ ¢ Founder Effect- occurs when a few individuals become isolated from a larger popula8on. This smaller group may establish a new popula8on whose gene pool and allele frequencies differ from the source popula8on. Bottleneck Effect- a sudden change in the environment such as a fire or flood, may drastically reduce the size of a population Gene Flow- the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes. Fig. 23-9 Original population Bottlenecking event Surviving population EFFECTS OF GENETIC DRIFT 1. 2. 3. 4. Genetic drift is significant in small populations Genetic drift causes allele frequencies to change at random Genetic drift can lead to a loss of genetic variation within populations Genetic drift can cause harmful alleles to become fixed NATURAL SELECTION AND ADAPTIVE EVOLUTION ¢ Relative Fitness- the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals. ¢ Natural selection is the only evolutionary mechanism that continually leads to adaptive evolution. ¢ Occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting the frequency curve for the phenotypic character in one direction or another. Frequency of individuals DIRECTIONAL SELECTION Original population Phenotypes (fur color) Original population Evolved population (a) Directional selection DISRUPTIVE SELECTION ¢ Occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. Frequency of individuals Fig. 23-13b Original population Phenotypes (fur color) Evolved population (b) Disruptive selection STABILIZING SELECTION ¢ Acts against both extreme phenotypes and favors intermediate variants. Frequency of individuals Fig. 23-13c Original population Phenotypes (fur color) Evolved population (c) Stabilizing selection ¢ Sexual a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. ¢ Sexual Selection Dimorphism marked differences between the two sexes in secondary sexual characteristics, which are not directly associated with reproduction or survival. ¢ Intrasexual Selection within the same sex. Individuals of one sex compete directly for mates of the opposite sex. ¢ Intersexual Selection Selection “mate choice”- individuals of one sex (usually females) are choosy in selecting their mates from the other sex. PRESERVATION OF GENETIC VARIATION ¢ Diploidy Hides genetic variation from selection in the form of recessive alleles ¢ Balancing Selection Occurs when natural selection maintains two or more forms in a population. ¢ Heterozygote Advantage Individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes ¢ Frequency-Dependent The fitness of a phenotype declines if it becomes too common in the population ¢ Neutral Selection Variation Has no selective advantage or disadvantage WHY NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS ¢ Selection can only act on existing variations. ¢ Evolution is limited by historical constraints. ¢ Adaptations are often compromises ¢ Chance, natural selection, and the environment interact. EXIT SLIP ¢ Of all the mutations that occur in a population, why do only a small fraction become widespread among the population’s members? ¢ If a population stopped reproducing sexually (but still reproduced asexually), how would its genetic variation be affected over time? Explain.