Historically, before people started studying evolution, when species were thought to be unchangeable, people defined species simply as all individuals of a certain type, and would describe a type specimen of each species and categorize everything that looked like that type as belonging to the same species and everything different as belonging to other species. We can't define species based on one description of what they should look like because individuals within species vary. To understand the problems we may have in defining species, we need to consider some of the different ways in which species can vary. There may not be any obvious pattern to such variation but sometimes there is. One common pattern of geographic variation is clinal variation.
Clinal variation refers to a gradual change in some feature across geography.
For example, if you go from south to north in the northern hemisphere you frequently find that populations of mammals are smaller in the south and gradually as you go north you find that within a species the individuals are larger and larger. Such a pattern of gradual change is called a cline so this form of variation is called clinal variation. Hybrid zones are geographic areas where two distinctly different forms of organism contact each other and interbreed. Generally, outside this zone, the two forms retain distinct differences from one another but within the zone intermediate forms between the two occur as a result of interbreeding. For example, a kind of woodpecker, flickers, show a hybrid zone in the United States.