Evolutionary genetics of endemic cichlid fishes (Pisces: Cichlidae) in Lake Malawi, Africa
The evolutionary genetics of nine species of cichlid fishes endemic to Lake Malawi, Africa was investigated by starch-gel electrophoresis. Genomic variation was homogeneous among species and similar in magnitude to previous observations for other bony fishes. Genic similarities among genera were unusually high and estimated divergence times between species were very low. These observations suggest that tachytelic speciation in these African cichlids does not involve abnormally high mutation rates nor major genomic modification.
From field observations and basic biological considerations, population size was estimated for several species of endemic fishes in the Mbuna species complex. These species are trophically and behaviorally restricted to algae covered rocks around the lake. Low effective population sizes and absence of gene flow between populations of these fishes isolated by sand barriers suggested that stochastic processes (such as genetic drift) may dominate genetic differentiation in these populations. It was predicted that allozyme frequencies among isolated but contiguous geographical populations would be significantly and nonsystematically heterogeneous. Statistical examination of allele frequencies at two polymorphic marker loci and frequencies of common color morphs revealed patterns of geographic variation which differed among species. Pseudotropheus tropheops and P. zebra exhibited geographic differentiation consistent with stochastic interpretation, while Labeotropheus fulleborni [sic] showed evidence of strong selective influence. Historical variation in lake level cannot sufficiently explain the present differentiation of conspecific populations. It was concluded that the relative contributions of stochastic and selective processes to geographic differentiation may differ among species of these fishes.
Although differentiation consistent with allopatric speciation has been convincingly
established in many cases for African lake cichlids, sympatric speciation in this fauna
is a recurrent theme in the evolutionary literature. The observed geographic differentiation
in allozyme and color morph frequencies in Lake Malawi supports the contention that
differentiation in the African cichlid fauna, though perhaps intralacustrine, is more
consistent with allopatric expectations. In accord with parsimonious hypothesis formation,
sympatric speciation need not be invoked to explain the origin and diversity of these fishes.