Evolutionary Analysis Chapter 1 End of chapter questions and answers graded A+ already passed

Evolutionary Analysis Chapter 1 End of chapter questions and answers graded A+ already passed The pope said " (HIV) cannot be overcome by the distribution of condoms. On the contrary, they increase it" Is the popes first statement correct? How about his second statement? How do we know? - correct answer The Pope's first statement is that Africa's problem with HIV/AIDS "cannot be overcome by the distribution of condoms." The statement is true in that no single strategy will prevent the transmittal of HIV/AIDS. For instance, condoms provide some protection from sexually transmission of HIV/AIDS, but they would not prevent an infected mother from passing the virus to her babies via breast milk. The Pope's second statement is that the distribution of condoms actually increases the problem of HIV/AIDS transmission. We know this second statement could be argued against from numerous data collected in South Africa and elsewhere showing that consistent use of condoms reduces HIV infection significantly. There is no data to suggest the use of condoms increases the problem of HIV/AIDS transmission. Review the process by which the HIV population inside a human host evolves resistance to the drug AZT. What traits of HIV contribute to its rapid evolution? How might a similar scenario explain the evolution of antibiotic resistance in a population of bacteria? - correct answer HIV has a very high mutation rate, a rapid reproductive rate, and an enormous population size. This means that at any given time, a human infected with HIV is carrying tens of millions of HIV virions with millions of different random mutations. Inevitably, a mutation will occur that confers resistance to AZT. (This will typically be a mutation that causes greater selectivity in the active site of the reverse transcriptase enzyme.) Notice that the HIV population has heritable variation for resistance to AZT before exposure to AZT. However, at this stage the resistance to AZT occurs only in one or a few virions out of the billions. In the other examples of this type of evolution, such as in the evolution of microbial resistance to antibiotics, we expect similar outcomes. It would take longer for the evolution of the resistant strains of bacteria, when compared to evolution of the HIV virions, which might happen within an individual patient. However, bacteria easily share their antibiotic resistance genes with other bacteria, via conjugation and other forms of lateral genetics exchange. The main difference will be in the specific nature of the mutation—it will not occur in the gene for reverse transcriptase, but in some bacterial gene. In the early 1990s, researchers began to find AZT- resistant strains of HIV-1 in recently infected patients who had never recieved AZT. How can this be? - correct answer Since an HIV population already has heritable variation for resistance to AZT before exposure to AZT, some patients would quickly develop resistant HIV strains as soon as they began taking AZT. Although early application of AZT in the 1990's showed some success in preventing or slowing replication of most HIV virions, the resistant virions with the right mutation would survive and replicate. In more general terms, there is differential reproductive success among the virions that is linked with a heritable trait, such as AZT resistance. Given the risk of evolution of resistance, why do you think the two patients shown in figure 1.11 were not given high doses of AZT immediately, rather than starting them with a low dose? - correct answer The patients were probably not given higher doses because of the serious side effects of antiviral drugs. Since viruses use their host cell's molecular machinery, any drug that can stop viral replication usually interferes with normal healthy cells as well. AZT can disrupt cell division because it interferes with DNA transcription in healthy cells, not just in HIV-infected cells The idea behind multidrug therapy for HIV is to increase the number of mutations required for resistance and thus reduce genetic variation in the viral population for survival in the presence of drugs. Could we acieve the same effect by using antiretroviral drugs in sequence instead of simultaneously? Why or why not? - correct answer No, we could not achieve the same result if we administer the drugs in a sequence. The unfortunate result would likely be development of resistance to all the drugs. This is because the HIV population would only have to develop resistance for one drug at a time, which is quite easy for it to do. The key to multiple drug therapy is that the drugs are given simultaneously, so that HIV virions must have four or five simultaneous mutations (one for each drug) to survive. Even with HIV's high mutation rate and large population size, the simultaneous occurrence of multiple resistance mutations in one virion is unlikely. Some physicians have advocated "drug holidays" as a way of helping HIV patients cope with the side effects of multidrug therapy. Under this plan, every so often the patient would stop taking drugs for a while. From an evolutionary perspective, does this seem like a good idea or bad idea? Justify your answer. - correct answer From an evolutionary perspective, this is a risky idea. Recall HIV's high mutation rate and large population size. A break in multiple drug therapy allows the surviving HIV virions—most of which will be those with partial resistance to one or two of the drugs—to multiply and generate billions of offspring with new mutations, some of which will confer mutations to additional drugs. However, a break in the therapy might promote an increase in the frequency of "wild type" or non-resistant virions too, which could lead to a more effective treatment. In a monograph published in 1883, Alexander Graham Bell wrote that "natural selection no longer influences mankind to any great extent." Do you agree? What is your evidence? - correct answer This statement of A. G. Bell seems very inappropriate in the light of many challenges humanity faces today, including the evolution of resistance (from the antimicrobial drug resistance to the resistance to pesticides). Think about other examples where the human future depends on the outcomes of natural selection.