Short Answer 
Allopatric speciation occurs when a population is divided by geographical barriers, leading to reproductive isolation as groups adapt to different environments. Behavioral isolation further solidifies their separation due to distinct mating behaviors, as seen in the example of California salamanders that evolved unique traits and reproductive habits.
Step 1: Understanding Allopatric Speciation
Allopatric speciation occurs when a population is divided by geographical barriers, leading to reproductive isolation. This process begins when a species is separated into two or more groups that are geographically isolated from one another. As these groups adapt to their unique environments, they evolve independently, resulting in significant genetic and phenotypic differences.
Step 2: Role of Behavioral Isolation
With the evolution of distinct features, the groups may also develop unique reproductive behaviors. Behavioral isolation happens when two species, despite being capable of mating, do not do so because they have different mating rituals or preferences. Over time, these differences can become pronounced enough that members of one group will no longer recognize members of the other group as potential mates, further solidifying their separation.
Step 3: Example of Salamanders in California
An example of allopatric speciation can be seen in certain salamander species in California. These salamanders were separated by geographical changes, which forced them to adapt to different environmental conditions. As a result, they developed unique reproductive behaviors and became behaviorally isolated, leading to their inability to mate with one another. This illustrates the process of speciation caused by geographical and behavioral changes.