Natural selection: driving force of adaptive radiation?

The Great Lakes of East Africa with their species flocks of cichlid fishes provide excellent model systems for the study of adaptive radiation, an evolutionary phenomenon during which a multitude of species emerges within a short period of time. The present proposal builds on a previous grant addressing evolutionary patterns and mechanisms during speciation and adaptive radiation. Our focus turns towards the understanding a critical issue for this process, the role of natural selection as driving force of diversification. We plan to study the role of adaptive evolution in two selected sets of model species, the first at an early stage of radiation in a satellite lake of Lake Victoria, and the second at a mature stage in Lake Tanganyika. Using a combination of comparative morphological, population genetic and quantitative genetic methods we will address three major topics: (1) Do morphological differences among populations or species have an inherited basis? (2) Do the observed eco- morphological differences bear the signature of natural selection? (3) What is the relative importance of divergent and stabilizing selection, enforced by competitive interactions, in evolutionary young and mature stages of adaptive radiation? We reconstruct quantitative trait evolution in the wild, through joint analysis of relatedness and quantitative trait variation at the population level of traits critical for ecological specialization. In a first step the genetic basis of morphological differences among populations or species must be demonstrated by heritability- calculations of related versus unrelated population members, or by hybrid experiments. Then we assess the presence and direction of selection by comparing neutral genetic variation of microsatellite markers to morphological variation of ecologically relevant quantitative traits. We concentrate on characters that have been shown to be relevant for niche differentiation, such as body proportions and the shape of particular viscerocranial bones, and assess them by a combination of geometric morphometric and traditional comparative methods. To demonstrate selection-driven evolution against the null-hypothesis of evolution by random genetic drift we use a combination of GST/QST comparisons by applying the "animal model approach" to natural populations.

The Project P 2094 is a continuation of my previous grant P17680, in which we focused on the degree to which natural selection shapes the evolution of new morphologies and ultimately the origin of new species. To this end we studied natural populations of the rock- dwelling cichlid fish genus Tropheus, of which about 120 distinct geographical races or sister species exist along the shores of Lake Tanganyika, our model for allopatric speciation. We compared patterns in natural populations to those in pond-bred populations of pure natural morphs and their hybrids. In terms of methods we employed our innovative method of generating standardized images from anesthetized fish for geometric morphological analysis, in combination with the analysis of selectively neutral nuclear genetic markers, to detect the relation of morphological and genetic differentiation. We addressed this in spatially separated natural populations and among sympatric pairs of two Tropheus species in sympatry, which are subjected to differential selective frameworks. In this way we gauged the total scope of morphological and genetic variation in the genus Tropheus, as well as the influence of sympatric interactions on the morphology. It became clear that the overall body shape of allopatric Tropheus varies in a relatively narrow shape space, presumably bordered by interspecific interactions of the complex species community in rock habitats, keeping Tropheus in its ecological niche, consistent with the action of stabilizing selection. Our breeding experiments in a standardized pond environment on 4 geographical morphs not only elucidated the impressive degree of phenotypic plasticity in the body proportions of Tropheus but also highlighted a clear signal of genetically based differences among populations, whereby those resulting from plasticity exceeded those among all natural populations studied so far by a factor of 2,5. We generated and studies F1, F2 and F3 generation pond offspring as well as backcrosses of F2 fish with both parental lineages. Our project part on non-sympatric versus sympatric Tropheus gave insights on competitive interactions resulting in consistent eco-morphological differences, suggestive for ecological character displacement. Our studies resulted in exciting new findings published in so far 19 peer-reviewed papers in ISI-listed scientific journals. Two publications are submitted and several in the pipeline. 4 Diploma Theses and 2 PhD Thesis were completed in the framework of this grant, 3 Diploma Theses and 1 new PhD Thesis are ongoing and now financed by my new grant (P 22737).