In the Oceans on Either Side of the Isthmus of Panama

question 18

Multiple Choice

In the oceans on either side of the Isthmus of Panama are 30 species of snapping shrimp, 15 species on the Pacific side and 15 different species on the Atlantic side. Species live at different water depths. Morphological and genetic data show that Atlantic and Pacific species that live at similar depths are sister species. Geological evidence indicates that the Isthmus of Panama started rising about 10 million years ago and the oceans were completely separated by the isthmus about 3 million years ago. If the isthmus indeed formed gradually, which of the following patterns of genetic differences is most likely in these species pairs?

• A) similar percentages of difference in DNA sequences between all pairs of sister species
• B) greater percentage of difference in DNA sequences between sister species that inhabit deep water than between sister species that inhabit shallow water
• C) greater percentage of difference in DNA sequences between sister species that inhabit shallow water than between sister species that inhabit deep water
• D) greater percentage of difference in DNA sequences among Atlantic species than among Pacific species

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