The cichlids of Lake Malawi present one of the most spectacular examples of adaptive radiation in the vertebrate world. Over 1000 species have evolved, entirely within the lake, in the last 2 million years. The group that Irv Kornfield and I are most interested in, the rockfishes or "mbuna", consists of at least 200 species, and may be less than 100,000 years old.
They're also really pretty fishes, so here are some photos:
P. zebra


This is Pseudotropheus zebra, a common and widely distributed member of the mbuna species flock. In addition to the common blue-barred (BB) morph shown here, there exist literally a couple of dozen variant races, each differentiated by some subtle aspect of male reproductive coloration (for instance the dorsal fin might be yellow). As far as can be determined via field observation, all of these variants constitute good biological species. Photo: Ad Konings