Parallel cases of ring speciation within a rapid adaptive radiation of cichlid fishes

Abstract

Ring species provide a valuable framework for studying intermediate steps of speciation resulting in two new species. Classic examples of ring species have involved large spatial and temporal scales - and this is partly why few idealised cases exist. In contrast, we focus on two species complexes of cichlid fishes from Southern Lake Victoria that evolved rapidly within the last 15,000 years across relatively small geographic space. We reconstruct the path to speciation between two forms of Pundamilia and Neochromis species that exist in a ring-like distribution around deep water, which acts as a dispersal barrier for these shallow water adapted fishes. We test key predictions of ring-like biogeography, population connectivity, terminal overlap and differentiation in mating and ecological traits. Our results indicate that both species complexes show a gradual change in craniofacial morphology along a ring-like distribution, corresponding to diet shifts. This phenotypic change is correlated with geographical distance in both rings, although they appear to have spatially different evolutionary trajectories. Sympatric species at the ends of both rings exhibit signs of convergent and divergent ecological character displacement in several craniofacial traits, potentially explaining their coexistence. In contrast to craniofacial morphology, male nuptial colouration changes abruptly in both rings, with the change occurring at the end of the ring in Pundamilia and early in Neochromis. We discuss the influence of environmental factors on the observed phenotypic variation and how resistance to connectivity might alter the isolation-by-distance pattern. Our study provides compelling evidence of how concerted changes in mating and ecological traits during isolation by distance can lead to new species and highlights the importance of parapatric speciation in adaptive radiation.