Wednesday, December 7, 2011

Other questions that the class had trouble with, and their explanations

I wanted to go through a bunch of the final exam questions to broadly address topics I wanted you to understand. Below are questions that less than 50% of the class got correct. If you have any questions, comments or concerns, please post a comment.

7. The remarkably high frequency of sexual reproduction in most taxonomic groups is paradoxical given what facts?
a. In sexual populations, females contribute to population growth
b. In sexual populations, individuals contribute only half of their genetic material to their offspring
c. In asexual populations, recombination is more frequent
d. A and B are correct
e. All of the above

The correct answer is D, this is the two-fold cost of sex. However, I gave full credit for all answers but C and E. Unfortunately, I think it would have been clearer if I had put "In sexual populations, ONLY females contribute to population growth".

8.  You find a species of bird with a ridiculously long tail in males that is clearly maladaptive for the organism's niche. You can conclude conclusively that:
a. The tail is a honest indicator of the males genetic quality
b. The tail catches the attention of the females because of sensory bias
c. Females prefer the costly trait because it means that their sons will be sexy
d. All of the above
e. None of the above

The correct answer is E. The key word here is "conclusively". In fact, A, B and C all could be true, but none of them are conclusively true. All three are mechanisms for the evolution of secondary sexual ornaments that may be maladaptive from a viability perspective.

9. Which of the following can result in a decrease in mean fitness in the population?
a. Stabilizing selection
b. Disruptive selection
c. Frequency-dependent selection
d. Balancing selection
e. Adaptive evolution

The correct answer is C, as in the Hawk-Dove game. I sense that most of you chose B because it "sounds bad". Don't fall into that trap, rely on what you know. Disruptive selection increases mean fitness, it just means that the extremes of the population survive the best.

11. The figure above is taken from Hanifin et al. (2009) with regard to prey toxicity of Rough-skinned newts and garter snakes resistance to those newts. Which of the following can you conclude from this figure?
a. Garter snakes are so resistant to TTX that there are rarely any fitness consequences to consuming them.
b. TTX is so high in most newts that the coevolutionary arms race with garter snakes could not be an explanation.
c. The coevolutionary arms race between newts and garter snakes generally ends with garter snakes winning.
d. Stabilizing selection favors an optimum ratio between TTX toxicity and resistance to be about 50%
e. none of the above

The correct answer is C. Not much to say about this one other than it was covered in the worksheet you did in-class, and reading the paper. Answer D was popular, but is gibberish. Stabilizing selection cannot select for a ratio between two different species' phenotypes. Both would have fluctuating or directional selection being applied on them during coevolutionary arms races, not selection to stay the same.

21. A Darwinian demon cannot exist because of the existence of:
a. Semelparity
b. Trade-offs
c. Senescence
d. Unstable equilibria
e. B and D are correct

B is correct. I suspect many of you chose E because of test-taking beliefs rather than because you thought unstable equilibria limit Darwinian Demons. Fundamentally, answer D doesn't actually mean anything in this context.

25. This modeling approach is especially useful for studying frequency-dependent selection
a. Game theory
b. Optimality theory
c. Quantitative genetic theory
d. Neutral theory
e. None of the above

The correct answer is A. Seemingly, every question that involved frequency-dependent selection was a struggle. Suggestions on how I could have made this concept clearer would be appreciated.

41. Helacyton gartleri, or HeLa cells, is a species under all of the following species concepts EXCEPT:
a. Biological species concept
b. Morphological species concept
c. Evolutionary species concept
d. Phylogenetic species concept
e. None of the above

The correct answer was D. Many chose A. However, unless you believe a human will mate with HeLa cells and produce viable offspring, you are wrong. B is wrong because HeLa cells on a petri dish look morphologically different than Homo sapiens. C is wrong because A is wrong, these species are distinct evolutionary species and will never interbreed again. D is the only one in which HeLa cells fail to be a distinct species. While HeLa cells are monophyletic, naming them their own species would make humans paraphyletic.

42. Which of the following is an ultimate explanation for why organisms senesce and die?
a. Antagonistic pleiotropy
b. Shortening telomeres
c. Oxidative stress
d. The failure of cellular repair mechanisms
e. All of the above

Answers B, C and D are proximate explanations. Only A provides an evolutionary explanation for aging.

44. SAT analogy time! Humans : Sexual reproduction
a. Viruses : coinfection
b. Viruses : infection
c. Viruses : Mutation
d. Viruses : Red Queen
e. Viruses : Virulence

I was going for A, but I realized that B is also potentially a correct answer and have given full credit to that question. I was hoping to draw a parallel between both being processes that allow for genetic recombination, but both also are processes that replicate the species.

51. Life history traits (which are usually complex, polygenic traits with many inputs) are characterized by:
a. Low heritability, but high genetic variance.
b. Low genetic variance due to constant directional selection
c. High heritability and low genetic variance
d. High phenotypic variance and high genetic variance
e. Low phenotypic plasticity and high heritability

It turns out that both A and D are correct, and have been given full credit.  B is wrong because life history traits actually have abundant genetic variance. C is impossible given the equation for heritability. E is false on both counts.

57. In general, if genetic drift were the only force acting on populations (there was no natural selection), we would expect to observe:
a. More morphological diversity than we observe today
b. Less morphological diversity than we observe today
c. About the same amount of morphological diversity that we observe today
d. Genetic drift is not strong enough to result in evolutionary change.

A is the correct answer. Many chose B. One student commented on their exam that we have not covered this in class. However, we did cover the example of human cranial capacity, and showed that even this extreme case of rapid evolution was easily explained by genetic drift. In problem set III we showed that genetic drift would change human body size far more than it has. Stabilizing selection and stasis are the dominant patterns we observe in nature, genetic drift would result in far more variability and variance than we observe today.

62. Consider a population with two alleles at a locus and an effective population size of 100. Assuming neutrality, what is the probability that the population will become fixed for only one allele at that locus?
a. 0
b. 1/100
c. 1/200
d. 1
e. It's stochastic so we can't know

Only 11% of you got the right answer. The correct answer is D. This looks similar to a question on the last exam, but you need to read the questions carefully. I don't mean to be asking trick questions here, just testing your understanding. The question on the last exam was "what is the probability of a new mutation drifting to fixation?", which would be in this case, 1/200. However, this question asks "What is the probability that ANY allele drifts to fixation?", which is 1. It's like asking "What is the probability that you become president?" vs. "What is the probability that SOMEONE becomes president?".

    What went wrong?


    One of the questions on the final exam I was a little dismayed to see that only 36% of the class got correct.

    Here is the question:
    49. Humans probably evolved directly from which of the following:
    a. Chimpanzees
    b. Gorillas
    c. Tiktaalik
    d. An ancient unicellular eukaryote
    e. Darwinius masillae (an early primate fossil)

    Across the board, even among the top students in the class, folks chose A. Why? What was confusing about this question? Humans did not evolve directly from chimpanzees, chimpanzees are an existing species. We share a common ancestor with chimpanzees. We did however, necessarily evolve from an ancient unicellular eukaryote (i.e. an ancient unicellular eukaryote is everybody's great ^ 500,000,000 grandparent, no chimpanzee that has ever existed is any sort of grandparent of any human being). As you leave this class, make sure that you understand the common fallacies that are made about evolution. Make sure you directly confront your misconceptions. We did not evolve from any species that exists today!! A less important concept this question was trying to address was that almost all fossils we find are probably not direct ancestors, but side branches close to direct ancestors.