Darwinian Snails
Prelude
Histograms:
[Question 1.1]
Which snail population has the larger average shell thickness, the population from 1871, or the population from the 1980s?
Answer: The population from the 1980s has the large average shell thickness.
[Question 1.2]
Which snail population has the larger range of variation in shell thickness?
Answer: The snail population from 1871 has the larger range of variation in shell thickness.
[Question 1.3]
Was Seeley’s prediction correct? In other words, is the average shell thickness and/or the range of variation in thickness different for the 1980s snail population versus the 1871 population?
Answer: Seeley’s prediction was correct, because the average shell thickness and the range of variation in thickness are different for the 1980s snail population versus the 1871 population.

Exercise 1: A Model of Evolution by Natural Selection
[Question 4.1]
First, make a prediction. What do you think will happen to the shell thickness distribution in this population of snails as you eat some of them?
Answer: We think the average shell thickness will increase.
[Question 8.1]
Do you notice any differences in the histogram of shell thickness now compared to the staring snail population? If so, explain why this change happened.
Answer: The snails with thinnest shells are decreased (almost disappear), and the average shell thickness is higher. Because we have tried to maximize crab happiness score so that the snails with thinnest shells were eaten first, conversely, snails with the thicker shells are harder to eat.
[Question 12.1]
Compare the staring histogram you saved earlier to this fourth-generation histogram. Has the distribution of shell thickness changed as you predicted in Step 4? Explain.
Answer: Yes, the distribution of shell thickness has changed as our predicted in Step 4. The snails with the thinner shells disappeared. Because the thicker shells snails were less to be eaten, they survived and continued to reproduce.

[Histograms 2]
Starting Population

Generation 4

[Question 16.1]
Which snails tended to get eaten? Which snails tended to survive?
Answer: The snails with thinner shells tended to get eaten and the snails with thicker shells tended to survive.
[Histograms 2]
Before Crabs

After Crabs [Question 17.1]
How does the evolution of the snail population when predatory crabs are present compare to the evolution of the snail population when you acted as predator?
Answer: Only last two types of shell thickness were left when I acted as predator, and the number of thickest snails is very large. However, there was more variety and the snails that were left had a shell thickness that ranged from 5-8 when predatory crabs ate them.

Exercise 2: The Requirements for Evolution by Natural Selection
[Question 4.1]
Make a prediction: Do you think this population of snails will evolve as predators start eating them? Explain why or why not.
Answer: No, we think this population of snails will not evolve as predators start eating them.
[Histograms]
No Variation, Starting Population

No Vairation, Generations 2 [Question 6.1]
Was there a change in the distribution of shell thickness among the snails? Explain why this is different dfrom when you or the crabs were eating snails before. Why does the distribution of shell thickness stay the same now, even though predators were eating just as many snails?
Answer: No, there was no change in the distribution of shell thickness among the snails. When the crabs were eating snails before, there was variation so that crabs eat the thinner shelled snails that were easier to eat.

[Histograms]
No Inheritance, Starting Population

No Inheritance, Generation 4 [Question 9.2]
Do you think that the distribution of shell thickness in the population will change over several snail generations as predators starting eating snails? Why or why not?
Answer: No, we don’t think it will change because the trait is not heritable snails with shell thickness of different sizes may appear at any time.
[Quwstion 13.1]
Why are the starting and ending shell thickness distributions similar to each other in shape, even after your crabs had thress meals of the thinnest-shelled snails?
Answer: Because the types of shell thickness are not associated with any inheritance.

[Histograms]
No Selection, Starting Population

No Selection, Generation 4- Trial 1

No Selection, Generation 4- Trial 2

No Selection, Generation 4- Trial 3

[Question 17.1]
Do you think the distribution of shell thickness in the population will change over time as your super-crab starts having meals? Why or why not?
Answer: Yes, I think the distribution of shell thickness in the population will change over time as my super-crab starts having meals. Because there is no selection and all snails have equal probability of being eaten.
[Question 20.1]
Was there a change in the distribution of shell thickness among the snail population? Is this what you expected? Explain.
Answer: Yes, there was a change in the distribution of shell thickness among the snail population. This is same as I expected because snails have same chance get be eaten.
[Question 23.1]
Are there changes in the shell thinckness distribution from the initial pattern? Are these changes consistent among all three of your trials?
Answer: There are changes in the shell thickness distribution from the initial pattern but the changes are not consistent among all three of my trials.
[Question 23.2]
Try to explain any changes you observed in the populations. Consider whether the changes happen for the same reason as the changes in distribution in Exercise 1 of this lab. Could random chance result in any of the types of changes you’ve seen?
Answer: There are some random changes, I think it is because the genetic drift through the inheritance. The reasons of this and the lab in Exerscise 1 are different, because the distribution were cause by natrual selection, which has certain change trend.

Exercise 3: Darwin’s Theory of Evolution by Natural Selection
[Question 1.1]
Using the data from your experiments, describe the conditions under which the snail population will evolve toward thicker shells and the conditions under which it won’t. refer back to your notes and the histograms you saved as evidence.
Answer: From my experiments, the variation of population is the condition which the snail population will evolve toward thicker shells. the snail population won’t evolve toward thicker shells if miss any one of them.

Exercise 4: The Source of Variation Among Individuals
[Question 2.1]
Is there a limit to how far predatory crabs can drive shell thickness in the snail population? Why or why not?
Answer: Yes, there is a limit. Because the predatory crabs would like to eat the snails with thinest shells at first, at last the option is only the thickest shell snails. There is no variation to select.
[Question 4.1]
Are the children identical to each other (and to the parent)? Are these cases where one of the children is different form the parent? If so, is the change usually towards a thinner shell, a thicker shell, or is it equally likely to be towards either one?
Answer: Most children are identical to each other or the parents, but some children are not identical to each other or to the parent. The change is equally likely to be towards either thinner or thicker shell.
[Question 5.1]
Can you drive the population further towards thicker shells now (with nutations) than you could before (without mutations)? Explain how this can happen, even though there are just as many mutations towards thinner shells as towards thicker shells.
Answer: Yes, I can drive the population further towards thicker shells now, because the trait is heritable.

Exercise 5: What Makes Populations Evolve?
[Question 1.1]
After they were born, did the individual snails ever change their shell thickness or color? If the individuals didn’t change, how was it possible for the population to change?
Answer: The individual snails didn’t change their shell thickness or color after thet were born. The snails with the unfavourable traits would disappear easily, by contrast, the snails with the favourable traits would survive at last. After reproduce, the frequency of traits, which are unfavourable, would increase. Thus, after several generations, the population will changes.
[Question 1.2]
Did snails grow thicker because the snails needed them in order to survive? If not, where did new thickness come from?
Answer: No, snails didn’t grow thicker shells. The new thicknesses come from the natrual selection. The frenquency of traits increased in order to adapt environment and survive. Therefore the new thickness come from the mutations through generations.
[Question 1.3]
What role did the predators play in causing the population of snails to evolve? Did they create a need for the snails to change – a need to which the snails responded? Or did the predatos simply determine which snails survived to reproduce and which didn’t?
Answer:

Exercise 6: CHALLENGE
Evolution by Natural Selection in Flat Periwinkles
[Question 1.1]
What evidence, if any, does Seeley have that the flat periwinkles of Appledore island vary in the thickness of their shells?
Answer: She can collect snails and measure the thickness of snails and found variation of the thickness shells.
[Question 1.2]
What evidence, if any, does Seeley have that snails with thick shells are more likely to survive than snails with thin shells?
Answer: Seeley performed two experiments. She offered each of 8 crabs a thin-shelled snail, and she found that they ate snails in 42 seconds. And then she offered each of another 8 crabs a thick- shelled snail, only one crab was able to crack the snail shell open and it took 8 minutes, and the other crabs gave up. It shows that snails with thick shells are more likely to survive that snails with shells.
[Question 1.3]
What evidence, if any, does Seeley have that shell thickness is heritable?
Answer: When the green crabs arrived, they started eating the thin-shelled snails. This left only the thick shelled snails to reproduce. And when the thick-shelled survivors reproduce, they had thick-shelled offspring, which is same to their parent. It shows that shell thickness is heritable.
[Question 2.1]
For which of the three conditions is the evidence you have seen the weakest? Explain.
Answer: Variation of the trait is the weakest condition I have seen, because the mutations could increase the variety of the trait even though there is no variation.