(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.