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Chapter 17 EVOLUTION OF POPULATIONS How does evolution function in genetic terms? Natural selection does not act on genes but on the whole organism Population = group of individuals of that same species that mate and produce offspring Share a gene pool (all the genes in a population) Genes and Variation Genotype and Phenotype – Through the use of the gene pool, we can evaluate the allele frequency, or how often a certain trait appears Genetic evolution involves a change in the allele frequency of a population Single trait genes (one gene controls) Polygenic trait (more than one gene controls) lead to changes in Sources of Genetic Variation Three sources: Mutations 2. Genetic Recombination - when does this occur? 3. Gene Transfer – bacteria 1. The number of phenotypes depends on how many genes control a trait. Single gene traits – trait only controlled by one gene lead to changes in phenotype frequencies Polygenic Traits – trait controlled more than one gene Lead to change in fitness and result in three types of changes Natural selection on polygenic traits Stabilizing selection: favors the intermediate Directional selection: favors one of the extremes Disruptive selection: favors individuals at both extremes, does not favor the intermediate Genetic Drift and Gene Flow Genetic drift: change in the gene pool of a population due to chance, 2 causes Bottleneck effect: where a natural disaster drastically reduces population size Founder effect: small colony on a new island Gene flow: a population loses or gains alleles when fertile individuals move out of into a population 100 greatest discoveries http://videos.howstuffworks.com/science- channel/29786-100-greatest-discoveries-naturalselection-video.htm Evolution vs Equilibrium Genetic equilibrium: no evolution means allele frequencies stay the Helps to predict frequencies of future alleles or to see if evolution is occurring Hardy-Weinberg Principle States that allele frequencies should remain constant unless one or more factors cause those frequencies to change Factors: 1. Nonrandom mating (sexual selection = individuals select mates based on traits) *Video 2. Small population (genetic drift will occur easily) 3. Immigration or Emigration (if individuals leave, frequencies change!) 4. Mutations (can introduce new alleles) 5. Natural Selection (different fitness levels) Hardy-Weinberg Makes predictions like punnett squares…but for populations not individuals Formula used instead of punnett square Hardy-Weinberg Equilibrium p = dominant allele q = recessive allele p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa Tay Sachs is a rare genetic disorder that is recessive and makes up about 4 percent of the humans population. What would be the genotype frequency in percentages? Blue eyes in humans is due to a recessive allele, which makes up about 28 % of the population. What would be the percentages of the homozygous dominant, heterozygous, and homozygous recessive conditions of the allele? In the human population a recessive genetic disorder called Down’s Syndrome is seen about 0.6 % of the time. What would be the allele frequencies for this disorder in percentages? What is a species? Species = A population or group of populations who can interbreed and produce fertile offspring Ex: similarity between two species of songbirds Speciation – the forming of a new species Reproductive Isolation If group members stop interbreeding, reproductive isolation has occurred = can no longer interbreed Three types.. Geographic isolation– different habit, sometimes separated by mountains or flooded area Behavioral isolation – differences in courtship Temporal isolation – differences in mating seasons http://dsc.discovery.com/videos/assignment- discovery-shorts-evolution-adaptation.html Molecular Evolution A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving together. Figure 17-18 How do we know species C is more closely related to species B than to species A? Evo-Devo Studies relationship of evolution and embryological development Small changes in Hox gene can produce large changes in adult animals