Speciation remains a process that is poorly understood. In particular, little progress has been made in elucidating the mechanisms underlying prezygotic isolation. A full understanding requires a knowledge of underlying genetics. To date the only real progress has been made using Drosophila: more model organisms are needed.\ud \ud It is proposed that the zebrafish, Danio rerio, a species well suited to the laboratory environment and with much information available about its genetics, has potential in this area. However, currently little is know about mate choice in this species and whether its behaviour\ud and morphology exhibit any genetic variation in the wild that may be amenable to further analysis. This thesis addresses these questions by investigating the behaviour and morphology of zebrafish deriving from several wild populations in Nepal and Bangladesh.\ud \ud Mate choice trials in the laboratory revealed several features of the zebrafish that make it less amenable to such studies than several alternative teleost species. These include a lack of strong sexual dimorphism and the absence of behaviours that reliably indicate mate choice.\ud Nevertheless observations in the course of this study suggest that male carotenoid coloration, longitudinal melanophore stripes and symmetry of caudal fin pattern may have behavioural roles in this species.\ud \ud Investigations into the shoaling behaviour of the zebrafish revealed no evidence for association preferences for familiar individuals or kin. The ubiquity of a preference for familiars in shoaling fish may have been over emphasized. This study did, however, find evidence for\ud inter-population variation in grouping behaviour that may provide another avenue for the investigation of the genetics of behavioural adaptations.\ud \ud Documentation of morphological variation revealed heritable inter-population variability, in particular in body stripe pattern, as well as consistent sexual dimorphisms in body\ud shape and anal fin morphology. Further investigation of the quantitative genetic basis of these traits revealed that several features of body stripe pattern were not heritable under laboratory conditions. This may indicate stabilising selection on body stripe pattern. In contrast, elements\ud of anal fin morphology exhibited more underlying genetic variation
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