Biology 261 – First Exam KEY Fall 2012

READ THESE INSTRUCTIONS: there are five questions in the test. You will answer FOUR of them. Each question is multi-part and worth a total of 25 points. The answer length limit for each section of each question is given with the question. Do not exceed that limit.

A. This is Figure 2a from the Helbig paper. Use this figure to state the following with regard to that paper:

a. (5 pts) (no more than one sentence) What question was asked?

How do migratory birds “know” which direction to fly?

How do migratory birds navigate from the breeding location to the wintering location? NOT AS GOOD: Is there a genetic component to navigation in migratory birds?

b. (5 pts) (no more than one sentence) What hypothesis was being tested?

Directional navigation has a genetic component.

c. (5 pts) (no more than one sentence) What prediction from the hypothesis was tested?

If directional navigation from the breeding to wintering locations is under genetic control, then a hybrid cross between adults of two different populations that migrate in different directions will produce offspring that show migratory movements in an intermediate direction.

d. (10 pts) (no more than half a page) How would Popper view this prediction? Does it meet his standards? Why or why not?

Karl Popper argues that proof does not exist in science, only the possibility of disproof. A good theory must make predictions that are risky; these predictions must “be capable of conflicting with possible observations.” Thus, an outcome that would falsify the theory must be conceivable, and the theory can be dismissed with as little as one falsification. In this case, the prediction was explicit that the offspring would be intermediate with regard to directional navigation. To refute it, the hybrid offspring would show non-intermediate navigation. Thus, it was a “risky” prediction. However, Helbig got lucky, there could have been a genetic component that was not polygenic and controlled by dominance in such a way that no intermediate direction was possible in the offspring.

B. In the paper by Carazo, there are two experiments. Explain the following:

a. (4 pts) (no more three sentences) What is an ultimate explanation for a behavior?

Ultimate causation deals with how a behavior evolved. Specifically, it explains why a behavior is advantageous to the individual with regard to fitness and natural selection.

b. (8 pts) (no more half a page) Which of the experiments dealt with ultimate causation and why?

The choice experiment in which males were given the choice of females in various reproductive states dealt with ultimate causation. Ultimate hypotheses in this case attempt to explain why males choose mature females over immature ones. One possible explanation is the higher fecundity of mature females. This would put a selective pressure on males to choose mature females, since it would provide them with a higher number of offspring. The ultimate explanation for male preference for virgins may also involve an increased likelihood to reproduce. In beetles, sperm competition occurs, where multiple males inseminate one female. Thus, it is possible that mating with a non-virgin female will put the male’s sperm in competition with others, and therefore lower the likelihood of one of its own offspring being born. Thus, there would be a selective pressure for a male to prefer virgin females.

c. (4 pts) (no more three sentences) What is a proximate explanation for a behavior?

Proximate explanations deal with how a behavior occurs and what biological mechanisms cause/allow it to happen.

d. (9 pts) (no more half a page) Which of the experiments dealt with proximate causation and why?

The second experiment dealt with proximate causation. It tested the mechanism by which males discern the reproductive status of females. Specifically, it tested whether chemical cues alone were sufficient to make the “right” choice (by “right”, I mean the choice that would result in the highest reproductive success). Although, they did not identify the specific chemical involved in the process, they did show that chemical cues alone produced the same result as in the first part of the experiment, i.e., that chemical cues are sufficient in giving a male enough information to choose a female mate.

C. The figure to the right shows the sound intensity (volume/power) at which a certain species of moth can detect various frequencies of sound.

a. (10 pts) (no more half a page) How did the concept of “nerve cell thresholds” (from textbook) help the researchers carry out this study?

Nerve cell thresholds describe the sensitivity of individual nerve cells to an external stimulus. In other words, what degree of input is necessary for the nerve cell to “fire”, or send a signal to the central nervous system. This It is important here because the figure essentially shows that in the moth, nerve cells are not equally sensitive across the entire spectrum of sound, in this species of moth. A range of frequencies was tested from 3 to 100 kHz and neuroscientists were able to quantify their intensity threshold (dB). This threshold essentially measures the sensitivity of the moth to different frequencies; the lower the threshold, the more sensitive the moth to the frequency. The nerve cell threshold is quite low for sounds around 5-6kHz, and relatively low for sounds between 40kHz. For all other frequencies of sound, the nerve cell threshold is quite low.

OR

The nerve cell threshold is the amount of change in the voltage across a neuron’s membrane needed in order for the cell to fire. The firing of a neuron, or an action potential, is the most basic molecular basis for any behavior performed by an animal. If a stimulus does not cause a neuron’s voltage to go above threshold, it will not fire and an action will not be performed. According to the figure, the moth is more sensitive to certain frequency ranges than to others, implying that the threshold of the moth’s “hearing cells” varies with frequency. If that is the case, the figure seems to imply that the threshold is lowest at frequencies between 5 and 6 kHz. It is also possible that some sort of mutation has occurred which allows the “hearing cells” to reach threshold more easily in certain ranges.

It is important to note that an action potential is an “all-or-nothing” event, meaning if the cell does not reach threshold, it does not fire, and if it does reach threshold, it does fire. It cannot fire “more” or “less” – no matter how much the cell voltage passes threshold, it will fire the same amount. Therefore, it would not be possible for a neuron to fire “more strongly” according to frequency, as one may expect to happen (it seems logical to think that as frequency increases, intensity increases). Knowing this, researchers knew to look at the entire range of hearing rather than just a sampling of certain ranges such as “high” “medium” and “low.” If they had done this, it would have been possible that they would miss the range which could be heard at the lowest intensity, 4.5-6 kHz.

b. (5 pts) (no more two sentences) What is umwelt?

Umwelt is an animal’s sensory perception of the world. It can be thought of as the sensory input to which the animal is sensitive, which is only a subset of all the potential sensory input in the world. The umwelt reflects what sensory input is relevant to the animal from the perspective of natural selection.

c. (10 pts) (no more half a page) What does the concept of umwelt have to do with this figure?

The figure represents the relevant sound for a moth from the perspective of natural selection. It therefore illustrates the moth’s umwelt w.r.t. sound. There is a filtering process present in the moth. Moths that are more sensitive to the sounds made by potential mates (5-6kHz) and predators (~40kHz) have higher fitness than moths that do not. There is not selective pressure for sensitivity to relatively benign sounds (8-20 kHz). In fact, sensitivity may be selected against in these cases; if the animal cannot differentiate (filter) between the important and unimportant sensory cues then they are at risk of being preyed upon because they may not exhibit predator avoidance behaviors when needed, and it is a waste of energy to unnecessarily react.

OR

Not all animals have the same umwelt. It is shaped by each animal’s needs, dangers, and other selective pressures. Looking at the figure, one can see that this type of moth can detect noises in the 4.5-6 kHz and 40-80 kHz ranges at a lower intensity than noises at different frequencies. It is apparent from this figure alone that the umwelt, at least in regard to hearing, of the moth is much different than that of a human and is most likely different than all other types of animals. It shows the way in which this type of moth perceives the outside world in terms of sound.

These two frequencies most likely correlate to important species-specific stimuli – such as the mating call of other moths or sounds of common predators – which are important to survival and reproductive success. It is unlikely that another species will have a low threshold at the same frequency (an identical umwelt) because if that frequency was also important for another species, the moths would detect this other species’ important mating or warning sounds as well as their own and vice versa, making the frequency range less useful.

D. A Dachshund and a Saint Bernard are both breeds of dog. There are hundreds of breeds of dogs in the world as a result of artificial selection by humans.

a. (9 pts) (no more four sentences) Explain what conditions are necessary for natural selection to occur.

Variable, heritable, fitness. There must be variation in a trait, that trait must have a heritable component, and there must differential reproductive success based on the variation in the trait.

b. (8 pts) (no more four sentences) Explain how artificial selection by humans is similar to natural selection.

When humans breed dogs, there is variation in traits. There is a heritable component to those traits. There is differential reproductive success base on those traits. These are the same components that define natural selection and, like natural selection, artificial selection leads to changes in gene frequencies in a population over time.

c. (8 pts) (no more four sentences) Explain how artificial selection by humans is different from natural selection.

However, that differential reproductive success is based on pre-determined human preferences for the trait, rather than which trait allowed individual dogs to deal with their environment and find mates better than other traits. Specifically, humans choose which dogs will mate, i.e., which genes will be perpetuated, based on the human desire to have bigger, smaller, longer, etc., dogs.

E. A flying squirrel (Glaucomys sabrinu) was placed in cage with a treadmill for 150 days. When the squirrel ran on the treadmill, its activity was recorded. In this way, the squirrel’s daily activity levels were monitored. Figure 1 (right) shows the squirrel’s daily activity levels under various light: dark conditions. DD signifies days in which the squirrel was held in constant dark conditions. LD represents days in which the squirrel was exposed to “normal” light conditions. Dark lines represent periods of high activity. Explain the following:

a. (2 pts) (no more one word) What kind of biological clock (or rhythm) is studied here?

Circadian

b. (4 pts) (no more three sentences) What is a zeitgeber?

The zeitgeber (German for “time-giver”) is the external cue that “sets the clock”, which may be sunlight, the moon, or the tide, all reliable cues with regard to time. The zeitgeber is received by a sensory receptor, which influences a cyclical clock mechanism that may regulate locomotion, feeding, or hormone levels. When the zeitgeber for a biological clock is absent (for example, if an organism is kept in total darkness), the organism will enter on a free running period.

c. (8 pts) (no more half a page) Why is does the onset of activity remain constant from day ~90 through day ~ 130 and not at the beginning and end of the experiment?

At the beginning and end of the experiment, there was no zeitgeber to which the squirrel could become entrained. It could not set its biological clock.

This experiment tests the biological clock mechanism of the flying squirrel in the absence of the light zeitgeber. The squirrel begins the experiment in complete darkness and therefore enters a free-running activity period. Each day, the activity period starts slightly earlier than the previous day. When the zeitgeber is provided (12:12 light:dark cycle,) it immediately entrains to an exactly 24 hour activity cycle. Every day, treadmill running increases immediately after the dark begins and tapers off after approximately 6-9 hours. Thus, it is a nocturnally active animal following a circadian rhythm of activity, as was predicted based on its free running period and Aschoff’s rule. Interestingly, when the shift from dark:dark to 12:12 is made, the length of activity is only about 3-4 hours, but it gradually increases each day. This observation suggests that initially the cessation of activity was still regulated by the free running period even after placing the squirrel on a 12:12 cycle. After 100 days of 12:12 light:dark, the squirrel is shifted back to complete darkness. Despite its “training” on a 12:12 cycle, its free running period remains the same as previous—slightly less than 24 hours.

d. (6 pts) (no more than a paragraph) What role do you think genetics plays in the behavior illustrated by this figure?

Clocks are all about gene regulation. When to turn on and turn off certain genes. We saw examples in class when genes that regulate activity in the pineal gland in rats were expressed relative to the external cues of the zeitgeber (light). Also saw that the fruit fly PER gene in the wild state allows the fruit fly is able to have a normal circadian rhythm. Mutations in that PER gene can cause the fruit fly to not entrain properly to external cues (longer period, shorter, none). Thus, not only do clocks regulate when genes are expressed, genes also code for the clock mechanism more generally.

e. (5 pts) (no more than a paragraph) What role do you think the endocrine system might play in the behavior illustrated in this figure?

The expression of genes is very much involved in the regulation of hormone production. In this case, the clock may regulate the production of a hormone such as melotonin. We saw that gene expression in the pineal gland of rats was correlated with light levels. Spcefically, genes that regulated melotonin production were expressed as light approached. This production of melotonin is related to activity levels of the animal. A similar link between gene expression and may explain the activity levels of the squirrel.