In the Oceans on Either Side of the Isthmus of Panama

question 7

Multiple Choice

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.

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Why should deepwater shrimp on different sides of the isthmus have diverged from each other earlier than shallow-water shrimp?
Why should deepwater shrimp on different sides of the isthmus have diverged from each other earlier than shallow-water shrimp?

• A) They have been geographically isolated from each other for a longer time.
• B) Cold temperatures, associated with deep water, have accelerated the mutation rate, resulting in faster divergence in deepwater shrimp.
• C) The rise of the land bridge was accompanied by much volcanic activity. Volcanic ash contains heavy metals, which are known mutagens. Ash fall caused high levels of heavy metals in the ocean sediments underlying the deep water, resulting in accelerated mutation rates and faster divergence in deepwater shrimp.
• D) Fresh water entering the ocean from the canal is both less dense and cloudier than seawater. The cloudy fresh water interferes with the ability of shallow-water shrimp to locate mating partners, which reduces the frequency of mating, thereby slowing the introduction of genetic variation.

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