Clines may extend across the entire geographic range of a species. A step cline is a short, steep cline between two widespread forms within a species, sometimes between subspecies. The colors and patterns differ greatly among subspecies. Clinal variation in the characteristics of a species may vary in parallel or independently of one another. The causes of clines have been debated for a long time.
Most geographic variation is probably, adaptive, hence, attributable to natural selection.
Interbreeding between formerly isolated populations. Secondary contact is contact between formerly isolated populations which diverged from a common, ancestral stock. The area of interbreeding is called the hybrid zone. There was much debate in 1950's about its importance. It now is generally thought to be important in many populations.