This "Science Student" entry in the seemingly never ending "How 'blank' sees it" series is taken from the Facebook page of a Malaysian high school student I met at a camp run by the US Embassy while I was living over there last year. It is interesting how similar things are around the world!! Actually, this is an interesting example of a rapidly spreading, yet probably short-lived, meme. How much longer do you think that people will be posting these?
Brief Report on Essay Grading
The TAs, Dr. Dini, and I spent most of Friday afternoon grading essays from the first midterm.
I am sorry to say that I was surprised and disappointed by the quality of the answers on this portion of the exam. The scores were much lower than I had expected (because the test is graded in pieces, I can't give you an exact estimate of the means on the two essay questions). I will post the answer key and then discuss what I think we need to do to try to improve the scores on future exams.
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a) Draw the graph that shows how the population growth rate varies over time in logistic growth when the initial population size is much larger than the carrying capacity.
worth 5 points
b) describe, in words, all of the information held in the graph you drew in part a
worth 5 points
Initially the population growth rate is very negative. Over time the population growth rate becomes less negative until the population growth rate equals zero where it remains.
c) Why is exponential growth an unrealistic model of population growth for most species?
worth 10 points
If populations are growing exponentially then their population sizes will increase over time at ever faster rates. Because we are not knee-deep in “name your favorite species” it appears that the exponential growth is not a realistic model of population growth for most species Exponential growth can be described by the following equation, dN/dt = rN when r is a constant (where dN/dt is the population growth rate, N is the population size, and r is the per capita growth rate). Thus, the assumption of a constant r must be too simple to explain what is happening in most populations.
The per capita growth rate can be calculated by subtracting the per capita death rate (d) from the per capita birth rate (b). Because in reality, both b and d should depend on population size, then it is not surprising that r would also depend on population size.
The per capita birth rate should be negatively correlated with population size. Larger populations should have more competition for resources resulting in each female receiving fewer resources. Because female reproductive success is resource limited, females with fewer resources are expected to produce fewer offspring resulting in an overall lower per capita birth rate.
Alternatively, the per capita death rate should be positively correlated with population size for several reasons. First, as population sizes increase more competition for resources should reduce in each individual receiving less food which might increase their probability of starvation. Second, when population sizes are large individuals live in closer proximity to each other than in smaller populations. Thus, fatal diseases might be more easily spread throughout the population when population sizes are larger. Finally, predators might be attracted to areas high prey density so the probability of predation might increase in larger populations.
Thus, because b decreases and d increases as population size increases, r decreases as population size increases. Eventually, the growth rate of the population becomes zero when the population reaches their carrying capcity at the population size at which b = d.
Comments- I thought that this question was extremely straightforward. All of the questions came directly from the Expected Learning Outcomes. I discussed all of these questions both in class an in Powerpoint on the blog. There was a 21 slide Powerpoint presentation entitled "Why exponential growth is unrealistic?" I would have expected that the average score on this page should have been over 15/20. Sadly, I think we fell well short of that mark.
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The Resplendent Quetzal is a topical bird found in rainforests in Central America. In general, Quetzals are weak fliers so they are at risk for predation from predators such as eagles, hawks, and owls. Male Quetzals have tails that are almost twice as long as their body lengths!
Assume that the long tails of the male Quetzals can be explained by the handicap principle. Describe, in detail, how the process of natural selection produced long tails in male Quetzals under this scenario.
20 points
The unusually long tails of the male Respledent Quetzal have been produced by natural selection. Unlike many tails produced by natural selection, the long tails of male quetzals do not increase the survival of the birds. Instead, the tails have been selected because they provide a mating advantage to the males. If, as the question states, the handicap principle helps to explain why females prefer to mate with males, then females will benefit from choosing to mate with males with extreme tails because their sons and daughters will have higher survival ability.
Female fitness would be enhanced if they were able to choose to mate with the male with the “best genes” because when these “good genes” were passed on to her offspring they would have higher survival or reproductive success. The handicap principle is a hypothesis developed to explain why females choose to mate with males with extreme traits. According to this idea, males carrying extreme traits are at a distinct survival disadvantage because the extreme trait increases their chance of mortality. Thus, if males with extreme traits have survived in spite of carrying a severed “handicap” then their underlying genetic quality must be quite good. Thus, if these good genes are transmitted to the next generation then both sons and daughters should exhibit increased survival. Thus, given the choice, females should prefer to mate with males with the most extreme traits in a population.
If female quetzals prefer to mate with the males with the longest tails then there is a positive correlation between tail length and mating success. Because there is a fitness differential between males with different tail lengths, then natural selection will occur any time there is heritable variation in males.
If natural selection is occurring then the trait frequency of the offspring generation will differ from that of the parental generation. Because males with longer tails have greater mating success, we would expect the mean tail length to increase from one generation to the next. If this process continues over time, then we would expect direction selection to cause tail length in males to increase over time. Once heritable genetic variation was exhausted, selection could only continue if a mutuation allowed the production of an even longer tail.
Thus, over time directional selection and regular mutation that increased maximum tail length in males would lead to an increase in tail length in male quetzals. Today the males tails are twice as long as their body length. It is possible that directional selection is still occurring such that tail length will continue to increase in the future. Alternatively , it is possible that the tails have gotten so long that the mating benefits of increased tail length are offset by a survival disadvantage associated with longer tails so that stabilizing selection is now occurring in the population.
Comment- I thought that this was also a relatively straightforward question. The question arises directly from an Expected Learning Outcome. We discussed, in detail, how natural selection produced the long neck in giraffes during the first and second class periods. I was surprised how few people recognized that they needed to talk about that information to answer the question.
I think that the low scores on this question arose from a combination of incomplete understanding of the material and a failure to answer the question. This question was "Describe, in detail, how the process of natural selection produced long tails in male Quetzals under this scenario." Thus, the key element of this question was to describe the process of natural selection. Unfortunately, too many students assumed that the focus of the question was "the handicap principle". Many students spent virtually all of their answers writing about the handicap principle without addressing the required answer to the question.
Far too many answers contains only a single paragraph (this question was worth 1/5th of the total points on the test!!). Remember, the questions asked you to "describe, in detail, how the process of natural selection produced long tails in male Quetzals under this scenario." They key term here is "in detail". I hope that you can see from looking at the answer key that a one paragraph answer does not provide the level of detail or the appropriate organization of material required to show a deep level of mastery of the material.
Final Thoughts- Hopefully, the scores on the first midterm will not be as low as I fear. You should be able to calculate your score on the MC test from comparing your answers with the posted answer keys and we will return your essays to you on Monday. However, I think that there is a lot of room for improvement in the future. Luckily, we have just had the first of four midterms, and there is the opportunity for students to take a "retest" of the first exam on Tuesday. Thus, it is not time to panic, but it is time to start seriously thinking about how to modify your studying and test-taking techniques to try to improve on your performance on future midterms.
Retest- The retest on Tuesday is intended to be equivalent to the first exam in scope and difficulty. The retests offers you an opportunity to improve your score, but that improvement will only occur if you plan to do something significantly different while preparing for or taking the exam. For example, if you have never take a BIOL 1403/1404 exam before you might now have a better idea about what is required to earn a good score (for example think more deeply when answering the multiple choice questions and include many more details in your essay answers). Maybe you had an Organic Chemistry midterm and a 15 page paper due on the same day and you were not able to study enough. Finally, you might be ready to try a new way of studying the material to see if you can improve your mastery of the material. If you have a plan about how you think you can significantly improve your score (by that I mean 10 - 15 points) then I think that taking advantage of the opportunity for a retest is a good strategy. However, if you are unhappy with the score on your first midterm, but don't have a plan for how you are going to score higher on the retest (other than hoping that the test will easier, will ask questions about the topics you understand, or that you will not make careless mistakes) then I don't recommend taking the midterm. Instead, you should focus your efforts on learning the material for the next test. We will start talking about Photosynthesis so you should start getting a handle on that material so you will perform better on the next exam.
What Can I Do To Help?
Clearly, I did not help to prepare you to succeed as well on this exam as I could have either because I did not make it clear the level of understanding and performance needed to succeed on the test or I did not present the material clearly enough and help you to think about the material in a way that helps you to master the material.
Only a handful of people have asked me questions either in my office or via email. Also, only a relatively small number of people have attended my Group Office Hours (I hope that Group OH regulars found that attending was helpful). Please come and see me if you have any questions. I encourage all of you to take advantage of SI. I know that Jeffrey and Suzanne are working hard to help you learn the material.
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