Cline: a character gradient in space

(a) A "smooth" cline. If environmental character changes gradually [for example, a north / south temperature gradient], the fitness of the "A" allele (red line) changes gradually: the frequency of the allele changes gradually. (b) A "stepped" cline. If the environment changes abruptly [for example, in an ecotone transition between forest & grassland], the fitness of the "A" allele also changes abruptly. Depending on the degree of gene flow, the cline will be more or less steep.