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EMPIRICAL EVIDENCE FOR EVOLUTION BY NATURAL SELECTION Evolution of resistant strains of bacteria. Mycobacterium tuberculosis causes tuberculosis (TB) in humans and other primates. In the middle of the 20th century, new antibiotics nearly eradicated TB Late 1980's: new resurgence of TB in developing nations Some strains were resistant to antibiotics to which they had been sensitive before. How did this happen? "Felt need?" "Natural Selection?" The answer came from an AIDS patient. .. o HIV+ individual diagnosed with TB o treated with rifampin and isoniazid (normally effective against M. tuberculosis) o Infection cleared; chest radiographs normal o Two months later, patient readmitted with TB symptoms o Treated with same antibiotics, but died 10 days later o Culture and sensitivity test on lung discharge revealed M. tuberculosis, but these were resistant to rifampin. o Was it Natural Selection? Test Darwin's Four Tenets: a. Did variation exist in the bacterial population? YES (both sensitive and resistant strains were present in the patient) b. Was the variation heritable? YES (Rifampin's mode of action is to bind specificallyi to RNA polymerase of M. tuberculosis , interfering with transcription of all genes. The RNA polymerase produced by a single point mutation in the RNA polymerase gene (rpoB) changed ONE amino acid of the enzyme (leucine--coded by TCG) to another (serine-- coded by TTG). This rendered the RNA polymerase unrecognizable to the rifampin. The mutation was heritable; all progeny of the mutant, resistant strain had the same point mutation. c. Did natural selection occur? YES. (M. tuberculosis with the point mutation left more offspring than those that lacked the mutation.) d. Did a non-random subset of the original population remain after selection? YES The populations before and after rifampin administration were significantly different, with resistant bacteria replacing rifampin-sensitive individuals. Once again, Darwin's theory is not refuted. Another Example: Evolution of flower size in flowers, such as Polemonium viscosum (Alpine Skypilot)
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How does pollinator preference and the influence of nectar thieving ants result in a change in the flower population (in terms of size, morphology, and fragrance)? Read about this HERE! Is Darwin's idea about Natural Selection testable? What do you think? Can you think of other ways to put the idea to rigorous testing? EVIDENCE FOR EVOLUTION: HOMOLOGY What observable evidence exists to support the idea that life on earth has evolved its diversity through a long process of DESCENT WITH MODIFICATION? One of the most powerful forms of evidence is HOMOLOGY In biology, a HOMOLOGY is a characteristic shared by two species (or other taxa) that is similar because of common ancestry . morphological homology
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