Lake Victoria cichlids show the fastest vertebrate adaptive radiation known which is why they function as a model organism to study evolution. In the past 40 years, Lake Victoria experienced severe... Show moreLake Victoria cichlids show the fastest vertebrate adaptive radiation known which is why they function as a model organism to study evolution. In the past 40 years, Lake Victoria experienced severe environmental changes including the boom of the introduced, predatory Nile perch and eutrophication. Both environmental changes resulted in a decline of haplochromine cichlid species and numbers during the 1980s. However, during the 1990s and 2000s, some haplochromine species recovered. With the use of the unique Lake Victoria cichlid of collection of the Naturalis Biodiversity Center, this thesis showed how the haplochromines have adapted their ecomorphology to the changed environment. In response to a decrease in water clarity, increased predation, larger prey and low dissolved oxygen levels, these fish adapted their eyes, body shape, upper jaw and gills in a way that would be beneficial to the survival of the fish. These adaptive responses could be the result of phenotypic plasticity or genetically based changes such as natural selection or hybridization or a combination of these three mechanisms. This research showed that the Lake Victoria cichlids are able to adapt extremely fast which is likely to have contributed to their extreme fast adaptive radiation. Show less
Mate choice by female cichlid fish from Lake Victoria plays an important role in speciation and the maintenance of species. Females are expected to select against males that are intermediate in... Show moreMate choice by female cichlid fish from Lake Victoria plays an important role in speciation and the maintenance of species. Females are expected to select against males that are intermediate in their phenotype during the process of speciation driven by sexual selection. To test this, we hybridized two species of cichlids that differ in male nuptial coloration. Male hybrids were intermediate in nuptial coloration compared to the parental species. Hybrids had a comparable fitness to the parental species and did not suffer from inviability, sterility, skewed sex ratios, reduced survival or reduced growth rates. Females preferred males of their own species over males of the other species. This implies that female mate preference can drive speciation. Females of one species were able to distinguish between males of their own species and hybrid males; females of the other species however, were not able to see this difference. When there is hybridization in nature, for instance when coloration becomes invisible by an increase in turbidity of the water, there would be asymmetrical selection against hybrids. An understanding of these processes is crucial in speciation research and for the maintenance of biodiversity. Show less
Nile perch was introduced in Lake Victoria in the 1950s and exploded in number during the 1980s. The process of colonization of the lake by this predatory fish is described and explained. The... Show moreNile perch was introduced in Lake Victoria in the 1950s and exploded in number during the 1980s. The process of colonization of the lake by this predatory fish is described and explained. The changes in a number of other fauna elements of Lake Victoria are described and explained. Not only Nile perch is to blame for these changes. Human influence by introducing other fish species, fisheries and agricultural practices have their impact Show less
The several hundred species of cichlid fish endemic to Lake Victoria (East-Africa) are textbook examples of explosive speciation. This thesis evaluates the hypothesis that sexual selection by... Show moreThe several hundred species of cichlid fish endemic to Lake Victoria (East-Africa) are textbook examples of explosive speciation. This thesis evaluates the hypothesis that sexual selection by female mate choice has contributed to this process of divergence, by investigating the mechanisms that drive the evolution and divergence of mating preferences for colour patterns. I studied two representative model systems, the highly polymorphic species Neochromis omnicaeruleus and the species pair Pundamilia nyererei and P. pundamilia. I used observational and experimental approaches in both laboratory and field. My work indicates that the interaction between sexual selection and habitat heterogeneity, in terms of photic environment and parasite exposure, can promote population divergence in male nuptial coloration and female preferences. In contrast, predation pressure and water turbidity may constrain the evolution and persistence of conspicuously coloured morphs and species. Show less
