Extremely fine-scale genetic partitioning has recently been detected among populations of
Lake Malawi's rock-dwelling cichlids through the study of microsatellite loci. Understanding
the mechanisms of genetic differentiation that operate in this rapidly speciating group
requires further investigation of the geographic patterns of gene flow and the congruence
between morphological and genetic divergence. In pursuit of this goal, genetic variation
at four microsatellite loci and variation in male breeding coloration were examined in
several populations of Labeotropheus fuelleborni from southern Lake Malawi.
Significant genetic differentiation exists among populations (overall F-ST=0.063; p=0.0002).
While migration appears unrestricted within continuous rocky patches, deep waters and sandy
bays more than 2 km wide act as strong barriers to gene flow. Dispersal of L. fuelleborni
appears to follow a stepping-stone model in which the distribution of habitats often constrains
migration to one dimension. It is hypothesized that clinal colour variation in the study area
has resulted from the secondary contact of divergent lineages, although reproductive isolation
between colour variants is not apparent. Relative to shoreline populations, reduced levels of
gene flow among populations inhabiting isolated, deep-water islands provides greater
opportunities for drift, adaptation to local conditions, or sexual selection to effect
genetic differentiation in this species.