About 3 million years ago, the Isthmus of Panama (a narrow strip of land connecting North and South America) formed, dividing marine organisms into Pacific and Caribbean populations. Researchers have examined species of snapping shrimp on both sides of the isthmus. Based on the morphological species concept, there appeared to be seven pairs of species, with one species of each pair in the Pacific and the other in the Caribbean. The different species pairs live at somewhat different depths in the ocean. Using mitochondrial DNA sequences, the researchers estimated phylogenies and found that each of these species pairs, separated by the isthmus were indeed each other's closest relatives. The researchers investigated mating in the lab and found that many species pairs were not very interested in courting with each other, and any that did mate almost never produced fertile offspring. Which process led to the formation of the species pairs of Pacific and Caribbean snapping shrimp

Allopatric speciation

Explanation:

Allopatric speciation refers to the geographic separation of a continuous genetic background, originating two or more new geographically isolated groups. These divergences of the original population might be related to migration in different directions, extinction of geographically intermediate populations, or geological events. During these events of speciation, emerging new barriers impede genetic flow between groups, as the two new populations that are separated can not get together and mate anymore. These barriers might be geographical or ecological.  

Different stages are involved in the allopatric speciation process:

•The emergence of the barrier.  

•Interruption in the genetic flow

•Slow and gradual differentiation due to the occurrence of mutations in each population and their accumulation in time.  

•Genetic divergence by natural selection and reproductive isolation,  impeding the two groups to mate even if the barrier disappears.  

•Prezigotic isolation mechanisms favored by selection a secondary contact between the new species occurs.

The Panama isthmus is an example of allopatric speciation caused by an extrinsic geological barrier that interrupted the genetic flow between species driving to genetic changes and reproductive isolation.  

Two different groups that evolved in allopatry might meet again in secondary contact if the barrier that separated them, disappears. In the Panama isthmus example, the groups that did mate, could not produce any fertile progeny because of postzygotic reproductive isolation. And the groups that did not mate, were not attracted to each other because of prezygotic reproductive isolation.  

b is the correct answer