The cichlid fish Pseudotropheus zebra is endemic to Lake Malawi and exists as four distinct colour morphs. It is shown from field and laboratory observations that Ps. zebra must be considered as two sympatric species with complete sexual isolation. This isolation is most probably based on an ethological mechanism with males of one population discriminating against females of the other. The two mating groups are comprised of BB/OB animals and B/W animals respectively. [BB = barred blue; OB = orange blotched; B = blue; W = white. The B/W group is the "cobalt zebra" of aquarists; the species was later named Pseudotropheus callainos by Stauffer & Hert (1992) and is now usually called Maylandia callainos. -- note by M.K. Oliver]
Slight but significant ecological differences were found between the two populations. B males occupy much larger territories than do BB males, and B animals feed much more frequently on plankton. Furthermore, B animals exhibit a preference for steep rock surface from which to scrape algae, the primary food source of both populations. BB animals show no preference for steepness of rock slope.
A marked deviation from unity was found in the sex ratio of both populations. Three
possible reasons for this are discussed. 1. There is a greater predator pressure on
the males; 2. there is limited space for establishing territories, with the B/W group
exhibiting larger territories and lower population density; 3. sex determinism is of
the WXY type. It is further hypothesized that the B/W gene pool evolved from the
BB/OB gene pool by several stepwise mutational events with ethological mechanisms
enforcing the separation into two distinct sympatric populations.