Present addresses: K. Rassmann, SCIL Diagnostics, Fraunhoferstr. 15, 82152 Martinsried, Germany. U. Schliewen, Zoologische Staatssammlung München (ZSM), Münchhausenstr. D-81247 Munich, Germany.
Genetic and ecological divergence of a monophyletic cichlid species pair under fully sympatric conditions in Lake Ejagham, Cameroon
Article first published online: 21 DEC 2001
Volume 10, Issue 6, pages 1471–1488, June 2001
How to Cite
Schliewen, U., Rassmann, K., Markmann, M., Markert, J., Kocher, T. and Tautz, D. (2001), Genetic and ecological divergence of a monophyletic cichlid species pair under fully sympatric conditions in Lake Ejagham, Cameroon. Molecular Ecology, 10: 1471–1488. doi: 10.1046/j.1365-294X.2001.01276.x
- Issue published online: 21 DEC 2001
- Article first published online: 21 DEC 2001
- Received 2 October 2000; revision received 11 December 2000; accepted 11 December 2000
- adaptive radiation;
- assortative mating;
Although there is mounting evidence that speciation can occur under sympatric conditions, unambiguous examples from nature are rare and it is almost always possible to propose alternative allopatric or parapatric scenarios. To identify an unequivocal case of sympatric speciation it is, therefore, necessary to analyse natural settings where recent monophyletic species flocks have evolved within a small and confined spatial range. We have studied such a case with a cichlid species flock that comprises five Tilapia forms endemic to a tiny lake (Lake Ejagham with a surface area of approximately 0.49 km2) in Western Cameroon. Analysis of mitochondrial D-Loop sequences shows that the flock is very young (approximately 104 years) and has originated from an adjacent riverine founder population. We have focused our study on a particular pair of forms within the lake that currently appears to be in the process of speciation. This pair is characterized by an unique breeding colouration and specific morphological aspects, which can serve as synapomorphic characters to prove monophyly. It has differentiated into a large inshore and a small pelagic form, apparently as a response to differential utilization of food resources. Still, breeding and brood care occurs in overlapping areas, both in time and space. Analysis of nuclear gene flow on the basis of microsatellite polymorphisms shows a highly restricted gene flow between the forms, suggesting reproductive isolation between them. This reproductive isolation is apparently achieved by size assortative mating, although occasional mixed pairs can be observed. Our findings are congruent with recent theoretical models for sympatric speciation, which show that differential ecological adaptations in combination with assortative mating could easily lead to speciation in sympatry.