Self Test Answers

1. 1)What is the storage mechanism for an organism’s genetic information?
   

1. A)DNA
   

2. B)RNA
   

3. C)Amino acids
   

4. D)Proteins
   

DNA is where genetic information is stored.  RNA is created from DNA and encodes the production of amino acids.  Amino acids are the building blocks of proteins.

2. 2)Which of these is NOT one of the forces of evolution?
   

1. A)Genetic drift
   

2. B)Mutation
   

3. C)Fixation
   

4. D)Selection
   

The forces of evolution are mutation, genetic drift and selection.  Fixation is one of the possible results of the evolution of a population.

3. 3)For a random walker with a probability of p to step forward and probability (1 - p) to step backward, what is the probability that the walker will end up 8 steps behind his starting point after 8 coin throws?
   

1. A)p^8
   

2. B)p^4*(1-p)^4
   

3. C)(1-p)^8
   

4. D)p^2*(1-p)^6
   

Since each step is independent and the probability to step back is 1 - p, we can get the probability of 8 consecutive steps backwards by multiplying the probability by itself 8 times, yielding (1 - p)^8

4. 4)How does the width of a distribution of random walkers scale with time?
   

1. A)Proportional to time
   

2. B)Constant (doesn’t depend on time)
   

3. C)Proportional to time squared
   

4. D)Proportional to the square root of time
   

As discussed in the random walk page, the furthest out a walker can get from his or her original position grows proportional to time, so the width of the distribution grows proportional to time.

5. 5)What can be added to the random walk formalism to deal with the case that when an entire population acquires a trait, all offspring will always have that trait?
   

1. A)Fixation
   

2. B)Loss
   

3. C)Selection
   

4. D)Adsorbing boundaries
   

Adsorbing boundaries, or ice cream shops as we represented them in the random walker scenario, represent a position a walker or population cannot leave once it has arrived there, leading to a buildup at those states/positions.

6. 6)What is the term for an entire population ending up without the trait being studied?
   

1. A)Fixation
   

2. B)Loss
   

3. C)Selection
   

4. D)Adsorbing boundaries
   

Loss describes a state where none of the individuals in a population have the trait being studied.  At this point, since none of the individuals can pass on the trait to offspring there will be no individuals at any point in the future with the studied trait.

7. 7)In the theory of natural selection, why does a given trait become more prominent in a population over time?
   

1. A)Individuals with that trait are more likely to live longer
   

2. B)Individuals with that trait die after they produce offspring
   

3. C)Individuals with that trait are more likely to migrate and merge with another population
   

4. D)Individuals with that trait are more likely to produce a larger number of offspring
   

If a trait causes a particular individual to produce more offspring (by making that individual more fertile, able to live longer in order to produce more offspring or by any other method) then the trait will be passed on to a larger number of individuals than  it would be without selection, leading to the increase in the fraction of the population with that trait.

8. 8)Which of these does NOT change with time the random walk discussed representing evolution in a population with selection for the trait being studied?
   

1. A)Width of the distribution
   

2. B)Average of the distribution
   

3. C)Number of peaks in the distribution
   

4. D)Probability of fixation
   

The width of the distribution will grow with time as discussed in problem 4, the average of the distribution will drift with time (since it will be more likely for the walk to move one way than the other) and the number of peaks in the distribution will go from one in the initial distribution to two describing fixation and loss.