The Galapagos Islands (see image above) are home to giant tortoises that feed on green vegetation. Originally tortoises lived only on Isabela Island and, although there was some variation in shell shape, most of them had dome-shaped shells or carapaces (see figure 1 below). This is still true today. Isabela has a relatively wet climate and varied plant life including many low-lying grasses and plants.
Tortoises are buoyant and can float for long periods of time. They eventually populated other islands after being carried there by ocean currents. However, today typical tortoises on some of the islands look distinctly different from those on Isabela. Initially all tortoises on on small islands such as Pinta, Espanola and Hood all tortoises had dome-shaped shells (carapace) as they arrived from Isabela. Eventually a mutation arose giving some tortoises a 'saddleback' carapace (see figure 2). Today on islands such as Pinta, Expanola and Hood, 100% of tortoises have saddle-back shells.
The 'saddleback' carapace is elevated above the neck and flared above the hind feet. On these islands the climate is dry and there is almost no ground vegetation. Here the predominant plant species is the prickly pear cactus, a plant that is tree-like in form with a woody trunk that holds its fleshy green parts high off the ground.
Use this information to answer the questions to follow.
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Question 1
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If you were to graph this information, the independent varialble goes on the x- axis and dependent variable on the y- axis. Which is the independent variable in this scenario?
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Question 2
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Which is the dependent variable in this scenario?
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Question 3
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Make a claim to answer this question:
Scientific question: Why do tortoises on Pinta island have the saddleback shell?
****Reminder as per box one Claim = for the "because" portion of you claim, you need an environmenntal factor because we are trying to prove, through these case studies that, natural selection (selective environmental factors) cause adaptations.
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Question 4
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Provide evidence to support your claim above (for evidence please use the data provided in the background information.
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Question 5
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Provide a reasoning to connect scientific theory and evidence to your claim. In 6 numbered steps write your reasoning. (Use the boxes below as a template for what to write on each step to ensure you touch on aspects of natural selection and adaptataions and provide a thorough reaasoning- remember you are not copying what is in the boxes, you are using it as a framework to apply to this scenario).
A key is posted In: Objective 18 Materials--> Case Study Keys