Part III - Understanding Natural Selection
Your answer to Question 3 might have gone something like this: those crickets who were silent would not be attacked by the parasitoid, and consequently would have a survival advantage over stridulating male crickets. Therefore, because parasitism was fatal and the risk of parasitism was high, the silent flatwing crickets were more likely to survive (and reproduce, assuming they could still find and court a female) than the normal, calling crickets.
Note: Do not change your answer to question #3 after reading this.
This explanation invokes natural selection, which is the mechanism originally proposed by Charles Darwin that leads to evolution in populations. One framework for natural selection summarizes it as a process that relies on three prerequisites:
1. A population has variation in a trait.
2. At least some of that variation is heritable.
3. The variation affects the likelihood that individuals will reproduce, leading to differential reproductive success.
Therefore, natural selection will occur whenever the three prerequisites exist, i.e., whenever there is heritable variation in a trait that has reproductive consequences by affecting survival, attractiveness, competitiveness, fertility, etc. The most advantageous variations should become more common in subsequent generations because those who have them will produce more offspring (i.e., higher fitness). Less advantageous variations should decline or disappear from the population because those who possess them will not reproduce as often.
Use these ideas about natural selection to answer the questions below.
Questions:
4. Based on what you have learned so far, identify how the following two prerequisites for natural selection are satisfied in our example of Hawaiian T. oceanicus cricket.
a. A population has variation in a trait (3pts).
b. The variation affects the likelihood that individuals will reproduce (3pts).
5. Based on the framework for natural selection above, what would be expected to happen if there had never been variation in wing shape that influenced stridulation? (4pts)
6. While we have not seen direct evidence of heritability thus far, describe what would happen if there was variation in stridulation, but it was not heritable. (4pts)
7. In T. oceanicus cricket populations elsewhere in the Pacific Ocean, there are no O. ochracea flies. In those populations, given that there are no parasitoids, would the prerequisite that "variation affects the likelihood that individuals will reproduce" apply? Explain your answer. (6pts)