1. What hypotheses were the researchers testing in this study? What are the inde
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Question
1. What hypotheses were the researchers testing in this study? What are the independent and dependent variables?
2. Why did the research team set up four different treatments?
3. Why did researchers present unshelled periwinkles to the crabs? What do the results of this part of the experiment indicate?
4. Summarize the results of the experiment in words. Do these results support the hypotesis you identified in question 1? Explain.
5. Suggest how natural selection may have affected populations of flat periwinkles in the southern Gulf of Maine over the last 100 years.
SCIENTIFIC SKILLS EXERCISE In a second part of the experiment, the bodies of and presented to crabs from northern and southern Data from the Experiment Understanding Experimental Design and Interpreting Data rn and southern populations were removed from their Is There Evidence of Selection for Defensive Adaptations in Mollusc Populations Exposed to Predators? The fossil record shows that historically, increased risk to prey species from predators is often accompanied by increased incidence and expression of prey defenses. Researchers tested whether populations of the predatory European green crab (Carcinus maenas) have exerted similar selective pressure Y A periwinkle on its gastropod prey, the flat periwinkl Souther periwinkles Northern periwinkles (Littorina obtusata). Periwinkles from southern sites in the Gulf of Maine have experienced predation by European green crabs for over 100 generations, at about one generation per year. Periwinkles from Northerrn Southern Source population of crab northern sites in the Gulf have been inter- acting with the invasive green crabs for elatively few generations, as the invasive Data from R. Rochette et al., Interaction between an invasive decapod and a native gastropod: Predator foraging tactics and prey architectural defenses, Marine Ecology crabs spread to the northern Gulf com- paratively recently Progress Series 330:179-188 (2007), Previous research shows that (1) flat periwinkle shells recently When the researchers presented the crabs with unshelled periwinkles Il the unshelled periwinkles were consumed in less than an hour collected from the Gulf are thicker than those collected in the late 1800s, and (2) periwinkle populations from southern sites in th Gulf have thicker shells than periwinkle populations from northern sites. In this exercise, you'll interpret the design and results of the researchers' experiment studying the rates of predation by crabs on periwinkles from northern and southern populations. INTERPRET THE DATA 1. What hypotheses were the researchers testing in this study? What are the independent variables? What are the dependent 2. Why did the research team set up four different treatments? How the Experiment Was Done The researchers collected peri- winkles and crabs from sites in the northern and southern Gulf of Maine, separated by 450 km of coastline. A single crab was placed in a cage with eight periwinkles of different sizes. After three days researchers assessed the fate of the eight periwinkles. Four differ ent treatments were set up, with crabs from northern or southern populations offered periwinkles from northern and southern popu lations. All crabs were of similar size and included equal numbers of males and females. Each experimental treatment was tested 12 to 14 times 3. Why did researchers present unshelled periwinkles to the crabs? What do the results of this part of the experiment indicate? 4. Summarize the results of the experiment in words. Do these resu support the hypothesis you identified in question 17 Explain. 5. Suggest how natural selection may have affected populations of fat periwinkles in the southern Gulf of Maine over the last 100 years Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology MOExplanation / Answer
1.
The fossil record shows that historically often an increased incidence and the expression of the prey defenses are followed by an increased risk to the prey species from the predators. The researchers tested, whether the similar selective pressure have exerted on the flat periwinkle by the populations of the European green crab that are the predators.
The independent variable in any study is the variable that is not dependent on any of the variables that are used in the study. Therefore, the independent variables in this study are northern or the southern sites present in the Gulf of Maine, which are the source of periwinkles.
The dependent variable in any study is the variable whose value is dependent on the other variable. Therefore, the dependent variables in this study are the number of the periwinkles that are killed by te European green crabs.
2.
The crabs from the northern or the southern populations were organized into four different treatments, in which the periwinkles were offered from the northern or the southern populations. The size of all the crabs were similar and they had uniform numbers of females and males.
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