Final answer:
The ranges of two sister species can reflect either a past allopatric speciation event followed by secondary contact or ongoing sympatric speciation. Examples include the allopatric speciation of spotted owls and sympatric speciation among East African cichlid fish.
Step-by-step explanation:
The observation that two sister species currently have parapatric (or sympatric) ranges does not immediately suggest how they became discrete; this may instead be the result of allopatric speciation like dispersal or vicariance, followed by secondary contact between differentiated populations. For instance, allopatric speciation can occur when a geographical barrier, such as the glaciers of the ice age, separates a population into groups that evolve independently, like the two subspecies of spotted owls along the west coast of the United States. In contrast, sympatric speciation occurs without a physical barrier, allowing for species to diverge within the same area, as seen with the East African cichlid fish where mate selection and ecological factors like light penetration affecting color perception promote reproductive isolation, or in cases of chromosomal errors, such as polyploidy.