Climatic changes result in a marked decrease in rainfall over an area that is covered by continuous forest. Over time, the forest gradually disappears and is replaced by grassland, except in the valleys created by rivers that flow across the area. Populations of a species of forest-living animals that were once continuously distributed over the area are now cut off from one another and isolated in the river valleys. According to modern evolutionary theory, which of the following changes in these isolated populations will most likely occur over time?
a. Members of all populations will adapt in a similar way to environmental change so that all animals in the area will continue to form a single species.
b. The populations will genetically diverge as each population adapts to its own unique environment, and new species may emerge.
c. Members of isolated populations will become better adapted to grassland habitats in order to maintain contact with other populations and ensure the survival of their species.
d. The species will soon become extinct due to inbreeding within isolated populations that can no longer exchange genes over a wide area.