 The steepness, or gradient, of a cline reflects the extent of the differentiation in the character across a geographic range.For example, a steep cline could indicate large variation in the colour of plumage between adjacent bird populations.It has been previously outlined that such steep clines may be the result of two previously allopatric populations with a large degree of difference in the trait having only recently established gene flow, or where there is strong selection against hybrids.