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BIOL 103 Practice Questions for Test 1

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  1. Captain Kirk brings one Fribble to the planet Quohog, which has plenty of food and no predators.  If each Fribble produces six offspring every eight hours (then the parent dies), how many Fribbles will there be by the end of the week?  What is their doubling time?

             N0 = 1             Q = 6   (not 7, because the parent dies)

    Nn = N0 x Qn

    They reproduce every 8 hours, this means 3 times per day, at the end of the week they will have produced 3 times 7, so n = 21

                N21 = 1 x 621 = 2.19 x 1016 fribbles present after one week

    Doubling time:

    N = 61/8 hours     = 21/d hours

    (1/8 hours) log 6 = (1/d) log 2

    d = 8 x log 2 / log 6 =     3.09 hours

  2. If a town of 10,000 people doubles every five years, how big will it be in 2028?  Suppose instead toxic waste contamination kills off 20% per year; how many will be left?

            N0 = 10,000 = 1 x 104           population doubles every 5 years   2008 to 2028 = 20 years

            N20 years = N0 x 2t/d      N20 years =  (1 x 104) 220/5          N20 years =  (1 x 104) 24          

            N20 years =  1.6 x 105 = 160000 people in the year 2028

    If instead of reproduction, toxic waste kills off 20% per year, then the remaining population each year will be:
    1-0.2 = 0.8 times N the previous year

    After 20 years, the population N will be:
    N20 =     N0 x (0.8)20 = 10,000 x (0.8)20 = 115 people

  3. Well's Time Traveler saw the sun appear to grow large and red. Why did this happen, in the author's view? How will our sun actually turn red some day? Why was the process of stellar evolution essential for human evolution? (Wells didn't know this, but we do.)

    In the nineteenth century, scientists knew that rotating objects (such as the Earth revolving around the sun) eventually lose speed as a result of frictional forces. As the Earth revolves more slowly, its orbit would decrease, and the Sun would appear larger because it it closer. Furthermore, the sun would appear red because it would "burn down," running out of fuel, and it would emit energy at lower wavelength (red instead of yellow).

    Today we know that the Earth's orbit will last a lot longer, because frictional forces are relatively small in outer space. The sun will not "burn down" like a flame; instead, its nuclear reactions will change from H ->> He to He reacting to form carbon and other elements. The sun will grow hotter and expand, forming a "red giant" (the energy of radiation will be lowered because of the expansion of the reaction volume). The red giant in fact will grow so large that it consumes the Earth.

    The process of stellar evolution in other stars was essential for life to exist because our own sun is still at the hydrogen-reacting stage. Other stars had to age and produce carbon, nitrogen and oxygen before these elements could be available to constitute our planet. In larger stars, later stages of fusion lead to explosion as a supernova. The process of the supernova includes late-stage fusion events that create the higher elements of the periodic table, including metals needed for life.
  4. Why does natural selection favor individuals who overpopulate their habitat and cause precipitous population decline?  What is the real reason the lemmings "jump in the sea"? Do the lemmings have alternatives? Explain.

    The lemmings reproduce to large numbers. Natural selection favors lemmings in which high population density triggers the instinct for dispersal to find new habitats. The lemmings migrate in all directions. Some migrate to cliffs over the fjords, where they may jumpt in. Some that jump in may drown--but     the few that make it across the fjord may find new habitat with plentiful resources, and will produce large numbers of progeny. Most will inherit the tendency to disperse.

    At the same time, there is always genetic diversity. A few lemmings will not have the gene for dispersal, and may stay behind. They will produce some offspring, especially after the others have left. So, the genes for both behaviors remain. Thus you can't say there is only one "winning" strategy; both alternatives have advantages and disadvantages.

  5. James Wait (Mr. Flemming) dies of familial hypercholesteremia (a gene copy that fails to make low-density-lipoprotein receptor protein). The condition is dominant and not X-linked.

    If he had a son, what is the son's chance of getting a heart attack in his forties?

    Mr. Flemming has genotype Aa, he has 50% chance of passing on the A allele to his son, assuming the wife is aa (healthy). So the son has a 50% chance of having a heart attack in his forties.

    Suppose in a given population the allele frequency (p) is one in three hundred.
    What fraction of the population shows the disease?

    If p = 1 in 300             p = 1/300 = 0.00333              p + q = 1         q = 1 – p         q = 0.99667

    p2 =  (0.00333)2 =  1.111 x 10-5 = AA frequency

    2pq = 2(0.00333)(0.99667) = 6.64 x 10-3 = Aa frequency

    q2 = (0.99667)2 = 0.99335 = aa frequency

    Check: p2 + 2pq + q2 = 1
    1.111 x 10-5 + 6.64 x 10-3 + 0.99335 = 1.0000


    Suppose we find that the rare individuals who inherit two copies of the LDL defective allele are extremely sick and die by age 30. How does this modify our definition of "dominant" inheritance for this disease?

    In fact, someone who has the misfortune of inheriting two A genes (genotype AA) gets early heart attacks and often dies by age 30. Thus, like many "dominant" conditions, this turns out to show "incomplete dominance" or "codominance."

  6. What is the role of population size, gene frequencies, and genetic drift in evolution of new species? Explain.

    The smaller the population, the more rapid the change in gene frequencies; thus, the more rapidly the population can change its genetic character. For a new species to evolve, it must go through a period in which a small population is isolated, allowing genetic drift as well as propagation of new genes favored by natural selection. The new genes can then propagate throughout the population. As the population grows, the new genes become "fixed" as drift becomes less likely. When the population becomes so different as to prevent natural interbreeding with other descendents of the original population, there is a new species.
  7. If natural selection means "survival of the fittest," than how can one "fittest" species evolve into more than one? How can more than one be "the fittest"? Explain.

    Natural selection is always relative; a particular group produces more offspring than another group. But natural selection depends on the particular environmental conditions. If two groups split off and experience different environments (such as underground vs. living in a tree), then natural selection will drive the populations in different directions. Also, selection involves random genetic drift. If animals end up on different islands, their small populations will experience drift in different directions. For the test, you should be able to apply this question to Galapagos, for example, the evolution of diverse finches or tortoises.

    Note: "Environment" also acts on INDIVIDUALS without changing their genes. On the "genes vs. environment" question, what I am getting at is how an INDIVIDUAL can experience environment; for example, a person's height can be shorter than one's genetic potential, if one receives poor nutrition.
  8. In the X-Files, the giant fluke evolved to look like a primate. What do we call this kind of evolution? Can you imagine (in a followup episode) a possible explanation (however unlikely) for how this might occur? Would the fluke ever be able to interbreed with real primates? Why or why not?

  9. In Galapagos, explain how Mary ensures the future of the human species, and how her actions differ from "Eve." Why does natural selection sometimes favor individuals who help neighbors raise offspring, instead of raising their own? What conditions favor this kind of selection?

  10. Explain how each of the following traits is determined by genes and/or environment: Huntington’s disease; Diabetes; Cancer; Spoken language.

  11. Explain reductive (or degenerative) evolution.  Why does it work?  Cite examples from Wells’s The Time Machine and from Vonnegut's Galapagos.

  12. Explain the difference between genetic and cultural evolution.  Use an example to show how these may be confused.

  13. Why do individuals move out of successful populations (migrate or disperse)?  Give examples of physical and biological modes of dispersion or migration.  For biological modes, give examples involving parasitism or mutualism.  Explain why each is parasitic or mutualistic.

  14. Vonnegut offers several hypotheses to explain how tortoises traveled to Galapagos.  Explain evidence supporting and evidence refuting each hypothesis.  Which hypotheses can neither be refuted nor proved?  Why not?

  15. Suppose you ingest 500 Salmonella bacteria in contaminated turkey, and in three hours you feel sick; there are now 500,000 bacteria. What is their doubling time in your body, in minutes?

           Nt  = N0 x 2t/d              N0 = 500         Nt  = 500,000             t = 3 hours      d = ??

    500000 = (500)23/d
    500000/500 = 1000 = 23/d
    log 1000 = (3/d)(log 2)
    3 = (3/d)(0.30)
    d = 3(0.30)/3
    d = 0.30 hours to double      how many minutes?  (0.30 hours)(60 min/hour) = 18 minutes to double

  16. A male bird is equally likely to offer food to his own chicks, or to his mother's chicks. Why? Explain by calculating his percent relatedness to his own chicks, and to his mother's chicks. (Assume that both hens are 100% faithful to their mates. In real life, about 90% may be typical.)

    The bird shares 50% of his mother's genes with his siblings (equal chance of getting the mother's copy or the father's copy of each gene.) Similarly, the bird shares 50% of his chick's genes (the other half from the chick's mother.) Thus, from the gene's point of view, there is equal chance of the bird's genes getting passed on through a sibling as through the offspring. Natural selection favors kin selection to precisely the extent of genetic relatedness.
  17. (Corrected) According to a study published in the Weekly World News, 1/5 of Americans have been abducted by aliens. Suppose that the trait desired by aliens is genetic, with recessive inheritance; and that every person homozygous for this trait gets abducted.

    What is the allele frequency (p) of the alien abduction trait?

           1/5 Americans abducted by aliens = f(aa) = 0.2

    aa = desired by aliens
    Aa = carriers of abduction trait
    AA = normal, miss all the fun

    Allele frequency of alien abduction trait: if f(aa) = 0.2 = p2           then p = sqrt(0.2) = 0.447

    What percentage of Americans are carriers of the alien abduction trait; that is, they don't get abducted, but could pass it on to a child?

    Carriers have the genotype Aa       f(Aa) = 2pq     if p = 0.447     then q = 1 - 0.447 = 0.553

                F(Aa) = 2(0.447)(0.553) = 0.494

                Double check: q2 = (0.553)2 = 0.305          p2 + 2pq + q2= 0.2 + 0.494 + 0.305 = 1.000,  value should be 1, checks OK
    If some of those abducted never come back, what will happen to the Hardy-Weinberg equilibrium? What do we call this effect?

     If some of the abducted people never return to produce children, then natural selection acts against the abduction trait. Natural selection (negative, in this case) perturbs the Hardy-Weinburg equilibrium and leads to evolution of the population, so that the remaining individuals tend NOT to get abducted. (Remember, this assumes the abduction tendency is genetically inherited by the humans; it would be different if it depends upon the traits of the alien abductors.)

    In one very small, isolated town, the entire population claims to have experienced abduction. What do we call this effect?

    In a small population, random fluctuations in number of offspring lead to random changes in the allele frequencies. The frequencies of alleles are no longer in equilibrium, and they change by genetic drift.           

  18. Suppose that gray-furred lemmings double in one year, but brown-furred lemmings take 10% longer.

    a. If the proportions of gray and brown lemmings start out equal, and if they breed for four years, what fraction of the total lemmings (brown plus gray) will be brown?

    The increase of gray lemmings over their original population is given by:

    Nt/N0= 2t/d = 24 years /1 per year = 16-fold increase

    The increase of brown lemmings over their original population is given by:

    Nt/N0= 2t/d = 24 years /1.1 per year = 12.44-fold increase

    After 4 years, the fraction of brown lemmings over the total is given by:

    Brown/(Brown + Gray) = 12.44/(12.44 + 16) = 0.4374 = 43.74%

    b. Suppose the lemming population crashes after four years, then builds up to the same total as before. After five crashes, what fraction of the total lemmings will be brown?

    In real life, this would be hard to predict exactly, as it depends on how the populations crash. Assuming the proportion of brown and gray remains the same after the crash, the total number of doublings is 4 x 5 = 20, so after 20 years:

    Brown/total = 220 years /1.1 per year / (220 years /1.1 per year + 220 years /1 per year)
    = 0.22 = 22%

    c. What alternative behaviors may lemmings exhibit, instead of crashing?

    Lemmings may stay behind and find alternative resources, perhaps eating foods that most lemmings don't like. Or they may migrate to new territories. They may jump into the fjord and cross the water to fine an empty territory on the other side.

  19. Many human societies promote “cross cousin marriage.”

    a. What fraction of your genes do you share with your first cousin?

    You share 50% of genes with your parent.
    Your parent shares 50% of genes with their sibling (your aunt or uncle).
    Your aunt or uncle shares 50% of their genes with their son or daughter (your cousin).
    The proportion of genes that you share with your first cousin is:
    0.50 x 0.50 x 0.50 = 0.125 = 12.5%

    b. Under what conditions would marrying your cousin be favored biologically? Under what conditions would it be detrimental?

    Marriage to a cousin is favorable genetically if the frequency of recessive lethals is low, and the frequency of shared complementary traits (traits that fit well together, such as large mouth and large teeth) is high. Marriage to a cousin is detrimental if the family has recessive lethal alleles that could come together in one offspring.

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