If forces are similar, there is slow divergence. If populations are reproductively isolated, there may be subtle selective forces that cause one population to diverge more strongly from another, despite geographic proximity. Geographical, ecological, temporal, behavioral, and mechanical isolations exist to the formation of hybrids. Species hybrids are at a fitness disadvantage when the hybrids are sterile, are nearly sterile, or are less well-adapted to survive in the environment. The disadvantage to the hybrid is that they often do not survive, and if they do, they do not pass their genes on to following generations.
Adaptive radiation refers to groups of closely related species which evolved from a common ancestor.
An example would be Darwin's finches or Hawaiian Drosophila. Behavioral isolation maintains the integrity of the species clusters of Drosophila in the Hawaiian Islands. Sexual selection is differential reproduction resulting from variable success in obtaining mates due to combat or courtship. Varied habitats in close proximity to one another and rapid changes in climate foster rapid evolution.