Genetic diversity of endemic fish in Lake Malawi and Lake Tanganyika
This thesis studies the evolutionary processes of endemic fish in the East African Great Lakes Tanganyika and Malawi. The pelagic clupeids and nile perches of Lake Tanganyika were studied with the DNA method RAPD, to reveal the possible genetic substructuring of the species. The smaller nile perch species Lates stappersii possessed a differentiated, local strain in waters surrounding the delta of the large river Malagarazi. The larger nile perch Lates mariae strains sampled during the dry and rainy seasons were differentiated, and locally separated. The northern strain occurred sympatrically with other strains during the dry season. The small clupeid Stolothrissa tanganicae possessed five strains, two sampled during the dry season and three during the rainy season. The dry season samples of the larger clupeid Limnothrissa miodon were differentiated from the rainy season samples, which showed separation of fish from southern and northern waters.
The pattern in speciation and stability of niches of endemic cichlid species in Lake
Malawi was determined from lipids, scales and enzymes. The gas-liquid chromatography
of fatty acid composition confirms that the niches occupied by the species studied are
established and stable. The scale micromorphology obtained by scanning electron
microscopy grouped species from different types of niches. Variation in isoenzymes
allowed grouping of the species based on genetic identities. Species with similar
niches are not the closest relatives. The results suggest that, although niche is a
conservative character, fundamentally new niches have been invaded several times during
the evolution of the species flock. A single basin structure offers no strict
geographical barriers for allopatric speciation, but lake sediments reveal that the
seemingly homogeneous environment is, in fact, historically heterogenous. Niche switches
may be products of sympatric speciation resulting in the release of competition after
local fish kills, which are recorded in the sediment data.