Final answer:
The study likely tested the hypothesis that reproductive isolation increases with geographic distance, leading to speciation in dusky salamander populations. This concept aligns with allopatric speciation, where environmental differences and lack of gene flow cause species divergence.
Step-by-step explanation:
The hypothesis tested in the study suggests that reproductive isolation increases with the geographic distance between dusky salamander populations. The concept is grounded in the understanding of allopatric speciation, where physical separation between populations leads to species divergence due to different environmental pressures and lack of gene flow.
Speciation can also occur without geographic separation, known as sympatric speciation, through mechanisms such as polyploidy or other reproductive isolations that arise within the same habitat. Reproductive barriers both prezygotic and postzygotic work as mechanisms enforcing the reproductive isolation necessary for the development of new species.
Examples that illustrate speciation include the variation of species like Darwin's finches and the adaptive radiation seen in the rise of mammals following the extinction of dinosaurs. In this context, since the question specifically references dusky salamanders, the study likely addresses patterns similar to those observed in Ensatina salamanders and their allopatric speciation through ring speciation evident in California.