A frustrating aspect of dialectology is that variation is often gradual, rather than clearly demarcated, and so speakers considered to be speaking different dialects may actually have numerous language characteristics in common. The linguist must evaluate all the different variables and then try to firmly determine whether the duo speak different dialects. There often is no clear answer. One factor that makes determining dialect less difficult is the existence of significant geographical phenomena, such as mountain ranges or islands. Like Darwin's finches and their physiological variation, speakers isolated by geography will tend to have clearly demarcated speech differences, allowing the linguist to draw the isogloss firmly, proudly, and with a clear conscience.
Dialectology is important for a variety of reasons.
Most importantly from a linguist's perspective, dialects have the humbling but salutary effect of exploding generalization. Linguistic theory can often become bogged down in abstraction and generalizations that in practice do not apply as universally as their proponents might wish. Since the linguist cannot simply ignore data, she must adjust or further hone the theories until they accomodate all the variation in the language. In addition, anthropological information is often revealed in language. Since speech and speech choices vary according to social class and social situation, as well as other factors such as race and gender, the speech of a person reveals a great deal about their position in a society.