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Swamp sparrows live in a variety of wetland habitats. Unlike

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Swamp sparrows live in a variety of wetland habitats. Unlike [#permalink] New post 06 Aug 2013, 23:20
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Swamp sparrows live in a variety of wetland habitats. Unlike most swamp sparrows, which live in freshwater habitats, the coastal-plain subspecies lives in tidal wetlands, where freshwater and seawater mix and the mud is gray rather than brown. Coastal-plain swamp sparrows differ from all other populations of swamp sparrows in having plumage that is gray brown rather than rusty brown. DNA analysis indicates several important genetic differences between swamp sparrows that inhabit tidal marshes and other subspecies of swamp sparrows. Therefore there must have been genetic-selection pressure on swamp sparrows in tidal marshes to become darker and grayer.

Select Strengthen for the statement that would, if true, most strengthen the argument, and select Weaken for the statement that would, if true, most weaken the argument. Make only two seletions, one in each column.

(A) None of the genetic differences that have been identified in the genomes of coastal-plain swamp sparrows and freshwater swamp sparrows affect plumage color.
(B) Mud in tidal marshes tends to be graysih because of the presence of iron sulfide, whereas freshwater mud is browner because of the presence of iron oxide.
(C) Some species of birds that live in tidal marshes do not have gray plumage.
(D) The diets of both coastal-plain and freshwater swamp sparrows can change significantly from season to season.
(E) Baby birds of coastal-plain subspecies and baby birds of a freshwater swamp subspecies, all raised on an identical diet under controlled conditions, grew plumage similar in color to that of their respective parents.

[Reveal] Spoiler:
Strengthen: E, Weaken: A
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Re: Swamp sparrows live in a variety of wetland habitats. Unlike [#permalink] New post 09 Aug 2013, 02:33
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Hi,

Information:
Coastal-plain swamp sparrows are gray brown whereas others are rusty brown.
There are a lot of genetic differences between the coastal-plain swamp sparrows and the others.
Conclusion:
Genetic differences must be the reason of of the difference in color.

The gap is that we do not know whether the difference is caused by internal(i.e. genetics) or external (i.e. climate, food etc.) reasons.

Taking these into consideration
A) weakens the argument, because it clearly states that there is no genetic difference between coastal-plain swamp sparrows and freshwater swamp sparrows that affects plumage color, thus the reason of difference in plumage color should be external (for example food)
E) strengthens the argument, because it shows that under same external conditions(“identical diet and controlled conditions”) the birds grew plumage similar in color to that of their respective parents. This shows that external factors have no affect, thus the reason should be genetic.

Hope this helps
Re: Swamp sparrows live in a variety of wetland habitats. Unlike   [#permalink] 09 Aug 2013, 02:33
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