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BIO152 Summer Lectures 17&18 Evolutionary processes (Hardy-Weinberg principle) Evolutionary processes 1 Announcements Lab 5-Agents of evolution online –quiz at the beginning of lab Lab 3 & 4 bacteria results& discussion due Test marks and papers- Thursday No help sessions (lab or lecture) today Final exam! Tuesday, June 28th 9 – 12 (morning) Format-multiple choice and written Covers all labs, lectures, and readings Help session Monday June 27th 12-2pm 2 • Evolution = change in allele frequency over time • Natural selection acts on individuals • Evolutionary change occurs in populations Mechanisms for shifting allele frequencies in populations 1. Natural selection increases the freq of alleles which contribute to improved reproductive success 2. Gene flow (individuals move into or out of a population) changes the proportion of alleles 3. Mutation introduces new alleles-beneficial, detrimental, or no effect on reproductive survival 4. Genetic drift -random change in allele frequencies 3 Hardy-Weinberg principle Claim 1—about the frequency = 1 • Frequency of alleles for a trait p is the frequency of allele A1 q is the frequency of allele A2 p +q=1 • Frequency of genotypes for a trait A1A1, A1A2, A2A2 2 p + 2pq + q2 = 1 Hardy-Weinberg Fig 24-1 4 Hardy-Weinberg claim 2 When alleles are transmitted according to the rules of Mendelian inheritance, allele frequencies do not change over time. For evolution to happen, something must happen to change the allele frequencies HW = null hypothesis for evolution If allele frequencies remain the same, then evolution does not happen If allele frequencies are changing, then one of the processes (natural selection, drift, mutation, gene flow) is affecting the change and evolution is happening 5 Natural selection affects allele freq • Directional selection-allele freq change in one direction (higher or lower) Directional selection fig 24-2 6 Stabilizing selection Fig 24.3 Disruptive selection 24.4 7 Sexual selection z Individuals in a population differ in their ability to attract mates A special case of natural selection Another form of nonrandom mating Sexual selection cont Bateman-Rivers theory of sexual selection: Basic asymmetry of sex Pattern: sexual selection acts on males more strongly than females, traits affecting sexual selection are more elaborated (exaggerated) in males than females. 8 Sexual selection-process Mechanism Eggs are expensive, but sperm is cheap ►females usually invest more in their offspring So females are choosy & males compete More to follow! Thursday lecture z Lab 6 z Judson’s book z Text book section in chapters 12 & 24 z 9 Genetic drift Change in allele frequencies in a population due to chance Random, not adaptive change Most pronounced in small populations Can lead to loss of alleles Genetic drift-founder effect New population established, may effect allele freq depending on ‘sample’ founding new population E.g., inhabiting islands 10 Genetic drift-bottleneck effect Drastic drop in a population►change in allele frequencies E.g., disease, disasters Gene flow (migration) Movement of alleles from one population to another Tends to equalize allele frequencies between populations (Fig 24.11) Improved fitness? Depends… Increased genetic diversity may provide better solutions in the new population 11 Mutation Does not occur often enough (plus many are either bad or neutral), so not strong evolutionary force on its own Mutation rate per allele versus per genome—mutation introduces new alleles into every individual in every population in every generation Random/nonrandom mating Inbreeding = matings between relatives Relatives share alleles inherited from common ancesters 12 Nonrandom mating vs sexual selection Inbreeding affects all genes (in contrast to sexual selection affecting genes responsible for mate selection and competition) Inbreeding changes genotype freqency Sexual selection changes allele and genotype frequency 13