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Some Astyanax mexicanus, a river-dwelling fish found in northeast Mexico, have colonized caves in the region. Although there is little genetic difference between river and cave A. mexicanus and all members of the species can emit the same sounds, biologist Carole Hyacinthe and colleagues found that the context and significance of those sounds vary by location—e.g., the click that river-dwelling A. mexicanus use to signal aggression is used by cave dwellers when foraging—and the acoustic properties of cave fish sounds show some cave-specific variations as well. Hyacinthe and colleagues note that differences in sonic communication could accumulate to the point of inhibiting interbreeding among fish from different locations, suggesting that ______ 
Which choice most logically completes the text? 
Difficulty: Hard

although A. mexicanus living in rivers are genetically similar to those living in caves, river fish rely on sonic communication less than cave fish do. 


although A. mexicanus is a single species at present, it could be in the process of splitting into distinct populations with different characteristics. 


although all A. mexicanus emit sounds, the fish living in rivers produce some sounds that the fish living in caves do not, and vice versa. 


although A. mexicanus from different locations can interbreed currently, river fish and cave fish are sufficiently genetically distinct that they can be considered separate species.

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