Question

3. Which of the following situations is least likely to lead to geographic isolation and allopatric speciation?

Answer 1

Answer:

b. A deep, narrow river divides a population of hummingbirds.

Explanation:

Hummingbirds can fly across the river. The deep and narrow river can not separate a population of hummingbirds into two patches since hummingbirds from one side of the river can fly and reach to the other side of the river.  

Without any geographical isolation, hummingbirds can continue to interbreed among themselves. The continued interbreeding would not allow their reproductive isolation and thereby would prevent the allopatric speciation.