Genetic and ecological divergence of a monophyletic cichlid species pair under fully sympatric conditions in Lake Ejagham, Cameroon

Although there is mounting evidence that speciation can occur under sympatr ic conditions, unambiguous examples from nature are rare and it is almost a lways possible to propose alternative allopatric or parapatric scenarios. T o identify an unequivocal case of sympatric speciation it is, therefore, ne cessary to analyse natural settings where recent monophyletic species necks have evolved within a small and confined spatial range. We have studied su ch a case with a cichlid species flock that comprises five Tilapia forms en demic to a tiny lake (Lake Ejagham with a surface area of approximately 0.4 9 km(2)) in Western Cameroon. Analysis of mitochondrial D-Loop sequences sh ows that the flock is very young (approximately 10(4) years) and has origin ated from an adjacent riverine founder population. We have focused our stud y on a particular pair of forms within the lake that currently appears to b e in the process of speciation. This pair is characterized by an unique bre eding colouration and specific morphological aspects, which can serve as sy napomorphic characters to prove monophyly. It has differentiated into a lar ge inshore and a small pelagic form, apparently as a response to differenti al 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 n ow between the forms, suggesting reproductive isolation between them. This reproductive isolation is apparently achieved by size assortative mating, a lthough occasional mixed pairs can be observed. Our findings are congruent with recent theoretical models for sympatric speciation, which show that di fferential ecological adaptations in combination with assortative mating co uld easily lead to speciation in sympatry.