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
Adaptive radiation involves the rapid divergence of a single ancestral species into a group of species each occupying a different ecological niche. Differences between species are the result of... Show moreAdaptive radiation involves the rapid divergence of a single ancestral species into a group of species each occupying a different ecological niche. Differences between species are the result of trade-offs in the ability to exploit different environments to avoid competitive interactions. The many species of wildfowl and their diversity in feeding methods are believed to represent an example of adaptive radiation. Frequently used feeding methods are filtering small food items from water and terrestrial grazing. However, few if any anseriform species apply both feeding methods. I have shown that duck-species have a higher performance for filter-feeding than for grazing. Geese and swans, on the other hand, have a higher performance for grazing than for filter-feeding. Structures of the tongue and roof of the upper bill are clearly related with feeding method and the corresponding mechanisms to transport food items through the bill to the oesophagus. These adaptations for either grazing or filter-feeding are incompatible and prevent maximal performance of both feeding methods at the same time. A study on the shape of the skull revealed several adaptations to feeding method. The size of the jaw muscles is exclusively related to size of the bill and only indirectly to feeding method. Show less
