Abstract of Doctoral Dissertation

Lake Malawi harbours the largest and most diverse known assemblage of fish endemic to any comparable size area. Observations on the rocky shore ‘mbuna’ species suggest that speciation has taken place allopatrically on isolated patches of rocky habitat within the lake. However, many other haplochromine species endemic to Lake Malawi are not mbuna, and are not restricted to rocky shores. Therefore, a different model of speciation may be needed for these species. This thesis focuses on the genetic structure of three cichlid species of the genus Protomelas: the rocky shore P. taeniolatus, the sandy shore species P. similis and P. fenestratus, which inhabits the interface between rocks and sand. All three species are confined to shallow areas and may be among the best candidates to test whether the allopatric model is equally applicable to rocky and sandy shore non-mbuna species. The results suggest that closely related Malawian haplochromine species can show very different patterns of population structure that can be related to their habitat preferences. Protomelas taeniolatus showed high levels of population structure, while P. similis showed no significant structure over distances up to 21 km. Protomelas fenestratus, showed intermediate levels of population structure. These results suggest that the model of allopatric speciation based on habitat fragmentation may be applicable to rocky shore non-mbuna, but that an alternative model is required to explain speciation among sandy shore species. Examining sex-specific dispersal patterns has also revealed the utility of analysing males and females separately when estimating the population structure of organisms. Male-biased dispersal between populations is demonstrated in three rock-dwelling cichlid species and these results are also discussed with respect to the role of sexual selection in speciation. Finally, based on mitochondrial DNA sequences, evidence that a sequential invasion of niches has occurred within Protomelas is provided. The implications on the origin and patterns of diversification of cichlids from Lake Malawi are discussed.