Studies of snapping shrimp on either side of the Isthmus of Panama have shown that each Caribbean shrimp species is more closely related to a Pacific shrimp species than it is to any other Caribbean shrimp species. The most likely explanation for this result is

Question

A.  snapping shrimp in the Caribbean and the Pacific were never part of the same population.
B.  snapping shrimp species in each ocean diverged prior to the formation of the Isthmus of Panama.
C.  snapping shrimp species in each ocean diverged after the formation of the Isthmus of Panama.
D.  creation of the Panama Canal allowed interbreeding between populations of shrimp.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
 
Gather Content
· What do you already know about sympatric and allopatric speciation?
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  snapping shrimp species in each ocean diverged prior to the formation of the Isthmus of Panama.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses sympatric and allopatric speciation.
· What type of thinking is required?
        o This question is asking you to weigh and judge evidence, or evaluate, to choose the best of the possible answers.
 
Gather Content
· What do you already know about sympatric and allopatric speciation?
        o A population can diverge into two species because the population has been separated geographically – this is allopatric speciation.
        o Or, the population can diverge into separate species even when the populations are not separated – sympatric speciation.
        o In the case of the snapping shrimp, there is a clear geographic separation – the Isthmus of Panama, that separates the Caribbean and Pacific Oceans. However, the question stem states that the species are actually most closely related to a sister species across the isthmus. How should we interpret this information?
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o According to the question, each species of snapping shrimp in Panama is most closely related to a sister species on the other side of the isthmus, rather than to the other species in the same ocean.
        o Does it make sense that snapping shrimp in the Caribbean and the Pacific were never part of the same population? No. Their relatedness suggests that they were part of the same populations at one time.
        o Does it make sense that the formation of the Isthmus of Panama had no effect on speciation in snapping shrimp? No. Clearly it would have been a big factor.
        o Does it make sense that snapping shrimp species in each ocean diverged after the formation of the Isthmus of Panama? No. If that was the case, the species in each ocean would be most closely related to each other.
        o Does it make sense that creation of the Panama Canal allowed interbreeding between populations of shrimp? No. The Panama Canal was only opened in 1914 – too recent to have affected the evolutionary relatedness of multiple shrimp species.
        o Does it make sense that snapping shrimp species in each ocean diverged prior to the formation of the Isthmus of Panama? Yes. That would explain why the closest species are across the isthmus. The species in the same ocean had diverged long ago, but the species across the isthmus only diverged 3 million years ago when it formed.

Reflect on Process · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o The question required you to weigh and judge evidence, or evaluate, to choose the best of the possible answers.
        o Did you recognize that the relatedness of different species can allow us to interpret when and how their speciation occurred?