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 A105 Human Origins and Prehistory
Assignments

Sample Answer is now appended to the questions; there are general comments on the assignment at the bottom of the page.

Assignment 1: Natural Selection in Modern Humans

 We often think of natural selection as a process that produced the species we see today over many millions of years (this is called macroevolution).  However, natural selection can also work quickly on single traits (called microevolution).  For this assignment you’ll be examining one example of microevolution in modern humans.  This example also illustrates the way in which humans’ culture can influence our biology.

 Attached to this page is an excerpt from an introductory physical anthropology text; it describes the emergence of sickle cell anemia, a very harmful genetic disorder.  The genetic trait that leads to this disease is quite common in tropical areas of the world where the introduction of agriculture has led to the destruction of forest trees.  Removal of these trees created conditions conducive to standing water, which enabled mosquito populations to explode, thereby allowing malaria to spread much more easily.  As you’ll see, malaria is important in the emergence of sickle cell anemia.

 The excerpt describes the evolutionary process that led to sickle cell anemia appearing in human populations.  Using the excerpt (and your textbook if needed), answer the following four questions.  Your answers should be typed and double-spaced, using a 12-point font and 1-inch margins.  You may answer each question separately or you may write a short essay that gives a COMPLETE answer to all four questions.  It should take no more than about 2 pages (600 words) to complete this assignment.
 

                                                    Questions

  1. Which condition confers the greatest advantage on a person living in
       tropical Africa or Asia, the homozygous normal (AA); homozygous sickle
       (SS), or heterozygous (AS)?  Explain why. 

      The condition that confers the greatest advantage on a person living in the tropic of Africa or Asia is the heterozygous condition (AS).  This is because the sickle allele, S, causes the red blood cells to take on a sickled shape, which protects against the malaria parasite.  Having two S alleles (SS) results in sickle cell anemia, which usually results in death during childhood, but having only one makes the blood inhospitable to malaria without causing anemia.  A genotype of AA, on the other hand, risks death from malaria.

 2. Why doesn’t the sickle allele (S) disappear? 

     The S allele does not disappear due to its advantage in malarial areas.  People with a single copy of the S allele are less likely to be affected by malaria, as explained above and are thus likely to have more children than those without the S allele (genotype AA).  Thus, S becomes more frequent in the population.  The frequency never passes a certain point (about 10-20%) because those with the SS genotype are selected against.  This phenomenon, in which the two homozygotes are selected against, is called balancing selection.  In this case, because sickle cell anemia is so much worse than malaria, populations tend to stabilize at about 15% frequency of the S allele.

 3. What do you think would happen to the sickle allele if malaria were
     completely wiped out tomorrow?  Explain your answer using evidence from
      the reading.

     If malaria were to disappear, so would the selection pressure that maintains the S allele.  In the absence of malaria, AA is the most fit genotype, because without malaria's presence, the S allele confers no advantage but imposes a high cost (the potential for having SS offspring, which often die in childhood).  Therefore, the S allele would probably decline in frequency; however, it would be unlikely to disappear completely -- mutation would reintroduce the allele, and selection would continue to remove it.

 4. In your own words, describe how sickle cell anemia came to exist.  That
     is, provide a possible scenario for the emergence of the disease, from the
     appearance of the very first gene for sickling to now.  Be sure to structure
     your answer so that it includes Darwin’s three observations and the first
     three natural selection principles that we discussed in class (i.e., variation,
     inheritance, etc.). 
     
     The S allele was probably introduced into various populations many times by mutation, but it was not advantageous, so it rarely attained high frequencies.  However, when this mutation occurred in the African and Asian tropics, where farming practices had encouraged mosquito proliferation and the spread of malaria, it WAS favored.  In malarial areas, the S allele confers an advantage by changing some of the red blood cells in a way that makes them difficult for the parasite to infect.  This mutation introduced variation into the population, as some individuals had the S allele (AS genotype) and others did not (AA).  Those with the S allele had greater reproductive success, meaning that more of their children (who inherited the favorable allele) survived into adulthood due to their helpful variation than the children of those with AA genotypes.  The frequency of S in the population began to rise, so that eventually there were enough heterozygous (AS) individuals that they began to have children together.  Statistically, approximately one of every four children born to two AS individuals will have a genotype of SS, which results in sickle cell anemia.

You might also have focused on Darwin's principles:
     This story is an example of natural selection as described by Darwin.  First, we know there is a struggle for survival; in this case the struggle involves trying to survive in a malarial area.  Second, in this struggle, certain individuals will possess traits -- here, the S allele -- that make them better suited to succeed in their environment. Finally, these successful individuals will have more offspring who will have inherited their adaptive traits, as in this case, where the success of AS individuals led to high frequencies of the S allele, and eventually to the emergence of sickle cell anemia.  The deadliness of this disease ensures that the S allele will never exist in more than about 15% of a given population; SS individuals are selected against. 

In general, either approach would earn you full credit; the best were answers that incorporated both approaches.

General Comments About the Assignment

Naturally, your answer didn't have to look exactly like the one above for you to get full credit; all that was necessary was that I be able to tell that you knew what was in the answer.  On your papers I marked more errors than I deducted points for.  For instance, THIS TIME I didn't deduct points for incorrect statements that had no effect on whether you completely answered the question.  Next time, factual errors will cost you.  And please please please proofread your work -- typos don't lose you points in my class, but it's good practice to eliminate as many as you can!

Overall, as a class you did quite well with this assignment; especially for those with less of a biology background, there were lots of concepts and facts to assimilate, so good job!  You averaged about 16.5 out of 20 possible points.  Of course, there's always room for improvement.  To that end, some comments:
 

  • It's in your best interest to explain things as thoroughly as possible, so that I can tell what you know.  It may help to write as if you were telling someone who was not familiar with the topic. 
  • A few of you paraphrased bits of the text and placed them in spots where they really didn't belong. So, you would have parts of an answer that were correct, but parts that were irrelevant to the question at hand. This is likely to cost a little bit on future assignments that involve referring to a text.   
  • Quite a few of you phrased your answers in such a way that it sounded as if tropical populations had intentionally evolved the sickle trait to prevent malaria -- a few came right out and said it was intentional.  It's an easy error to make, because of course it seems like individuals should do what they can to evolve better adaptations. Remember, though, organisms can't "try" to evolve like this; individuals can only live and reproduce or die. And no, mosquitoes can't infect you with the S allele!
  • Many of you omitted the last step in answering question 4, neglecting to describe how sickle cell anemia appeared after explaining how the allele became common.  I'm going to nag you a little bit here -- read the question carefully!  This kind of thing will be more costly in the future. 
  • If any of this assignment (or anything else we've covered) does not make sense to you, please come to my office hours or e-mail me so we can set up an appointment to talk about it! I want you to have every opportunity possible to get a handle on the material and do well in this class, so don't be afraid to talk to me.  
CLICK HERE for the text excerpt.