Divergent natural selection: driving force of 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 four previous grants, from which more than 30 publications in peer-reviewed international journals emerged. Our focus turns towards the understanding of the pathways of adaptive evolution, by specifically tackling the processes driving rapid speciation. We will address three major topics: (1) What is the relative importance of divergent natural selection due to competition as driving force of explosive speciation? We have chosen sets of model species in Lake Tanganyika and Malawi to provide empirical evidence for the reality and importance of natural selection-driven adaptation as major driving force of speciation, aside of sexual selection on male mating color. We intend to study quantitative trait evolution in the wild, through joint analysis of relatedness and quantitative trait variation at the population level. By assessing patterns of neutral genetic variation in relation to morphological variation of selected quantitative traits we intend to to test for character displacement due to the action of (directional) selection on quantitative traits. We will compare the variation of selected quantitative traits of the oral jaw apparatus (QST) to the null-hypothesis of evolution by random genetic drift (FST). Our approach is new to cichlid evolutionary research and is intended to complement ongoing studies focusing on quantitative trait loci, identified by laboratory crossing experiments. (2) We plan to extend our comparative demographic studies of Lake Tanganyika cichlids on the basis of chemically tagged otoliths by surveying the age structure of four sympatric rock-dwelling cichlid species in the South of Lake Tanganyika: Also, we propose to address the stability of the age structure of cichlid populations over time. (3) We address the importance of reticulation in phylogeny due to hybridization. From our previous studies, evidence is accumulating that introgression and hybridization might be a third pathway of speciation in cichlids under particular circumstances. We thus plan to continue our studies of the role and importance of introgression and hybridization for speciation in Lake Tanganyika cichlids. We will take a two-fold approach: first, we will study two populations of Tropheus in which secondary admixis of two ancient mtDNA lineages occurred. Second, we plan to continue our work on lamprologine cichlids which seem to be especially prone to hybridization.

The research project had three sections, the first focusing on the demonstration of natural selection acting on wild populations, the second on demographic aspects of cichlid populations and the third on phylogenetic patterns and the role of hybridization during the evolution of species flocks. The first section was particularly challenging, as we shifted from phylogenetic and population genetic research to tackle the genetic basis of the divergence of natural populations in morphology. We asked the key question if and to which extent natural selection is responsible for the evolution of these differences. Therefore, we combined geometric morphometric, population genetic and quantitative genetic approaches to analyze model populations of our best-studied model for allopatric speciation, the Tanganyikan genus Tropheus. To complement findings on wild populations we carried out breeding experiments and generated F1- and F2 offspring in a standardized pond environment to demonstrate a genetic basis of the morphological differences observed in natural populations, in addition to heritability-calculations of related versus unrelated population members in the wild. Aside of producing pond-offspring of 2 populations, we also hybridized 2 population-pairs. The project P 17680 has produced exciting insights in all three project sections. 37 grant-relevant peer-reviewed publications appeared in renowned international journals. We completed and published the mtDNA phylogenies for all Tanganyikan cichlid tribes, as well as for the small species flock of squeaker catfish of the genus Synodontis. We co-authored one article in Nature and one in the Proceedings of the National Academy of Sciences of the USA, and published a comprehensive evolutionary study on the origin and spread of the haplochromine cichlids, who originated in Lake Tanganyika to spread all over Central- South- and East-Africa, as well as our study on the role of hybridization during the evolution of gastropod shell breeding cichlids. These publications are also highlights of cooperative work. We established innovative comparative morphological techniques by using computer scans of anestethized fish for geometric morphometrics in conjunction to the study of disarticulated bones of the viscerocranium and acquired the expertise to utilize the animal model for the study of natural fish populations. Six master theses were completed during the grant period, one more is in progress. The research of two African Ph. D. students with an OAD-fellowship was funded by this grant, as well as the salary and research of two Austrian Ph. D. students who graduated during the grant period. Also, one post- doctoral scientist (Christian Bauer) worked for one year for this project, but then found a permanent position in a Fisheries Institute of the Austrian Ministry of Agriculture, and was replaced by two new Ph-D. students.