In the oceans on either side of the Isthmus of Panama are 30 species of snapping shrimp; some are shallow-water species, others are adapted to deep water. There are 15 species on the Pacific side and 15 different species on the Atlantic side. The Isthmus of Panama started rising about 10 million years ago. The oceans were completely separated by the isthmus about 3 million years ago.
In the figure, the isthmus separates the Pacific Ocean on the left (side A)from the Atlantic Ocean on the right (side B). The seawater on either side of the isthmus is separated into five depth habitats (1-5), with 1 being the shallowest.

Which factor is most important for explaining why there are equal numbers of snapping shrimp species on either side of the isthmus?
A) the relative shortness of time they have been separated
B) the depth of the sea
C) the number of actual depth habitats between the surface and the sea floor
D) the elevation of the isthmus above sea level

Question

In the oceans on either side of the Isthmus of Panama are 30 species of snapping shrimp; some are shallow-water species, others are adapted to deep water. There are 15 species on the Pacific side and 15 different species on the Atlantic side. The Isthmus of Panama started rising about 10 million years ago. The oceans were completely separated by the isthmus about 3 million years ago.
In the figure, the isthmus separates the Pacific Ocean on the left (side A)from the Atlantic Ocean on the right (side B). The seawater on either side of the isthmus is separated into five depth habitats (1-5), with 1 being the shallowest.

Which factor is most important for explaining why there are equal numbers of snapping shrimp species on either side of the isthmus?
A) the relative shortness of time they have been separated 
B) the depth of the sea 
C) the number of actual depth habitats between the surface and the sea floor 
D) the elevation of the isthmus above sea level

Quizplus · Accepted Answer

The fact that there are equal numbers of snapping shrimp species on either side of the isthmus suggests that they evolved before the isthmus completely separated the oceans, and have since had the same amount of time to diverge from each other. Therefore, the most important factor is the relative shortness of time they have been separated, which is choice A. The other factors listed (depth of the sea, number of actual depth habitats, and elevation of the isthmus above sea level) may have played some role in the evolution of these shrimp species, but they do not explain why there are equal numbers on either side of the isthmus.

In the Oceans on Either Side of the Isthmus of Panama