Convergence and plasticity in the adaptive radiation of cichlid fishes

Ever since Charles Darwin and Alfred Wallace (1858) propelled our understanding about the importance of natural selection in the transformation of species, researchers endeavoured to use this intellectual foundation to explain larger patterns of biodiversity. One pattern emerging from the observation of phylogenetic relationships and ecological adaptations of species is the abundance of lineages, which are apparently rapidly diversifying, resulting in ecologically diverse clades of species. Most of the biodiversity we know is made up by such clades, being the result of so-called adaptive radiations. Phenotypic diversification and lineage accumulation in adaptive radiations have received considerable attention and great progress has been made in understanding these aspects. Adaptive radiations can be triggered by an ecological opportunity, i.e. a newly formed or colonized habitat lacking competing species or the formation of a key-innovation, a novel trait that allows for the invasion of a completely novel set of niches. The radiation of East African cichlid fishes, and other groups of fishes, are hypothesized to have been triggered by a key-innovation, namely a reorganisation of the pharyngeal jaw apparatus (Liem 1973). Liem’s hypothesis attributes the evolutionary success of groups with certain pharyngeal jaw modifications to an increased versatility in exploiting resources. Although morphological descriptions of the pharyngeal jaw apparatus for many taxa of fishes abound in the literature, and studies with functional, biomechanical or ecological perspectives are numerous as well, as of yet no concise treatise about the evolutionary implications of the different aspects and characteristics of the pharyngeal jaw has been published. This gap I thrive to close with the first chapter of this thesis.
The course of adaptive radiations might be influenced by a phenomenon only little studied in this context so far. Phenotypic plasticity, the ability of a genotype to produce different phenotypes depending on environmental cues, might increase a founding populations chance of persistence. Novel niches might also be invaded more quickly, since the phenotypic shift due to plasticity might place a population in the ‘realm of attraction’ of a peak on the adaptive landscape. If plasticity is only exhibited in some directions in morphospace, but not in others it has the potential of biasing evolutionary trajectories in adaptive radiations. To better understand if phenotypic plasticity in the pharyngeal jaw might have influenced the adaptive radiations of cichlids, I studied the Nicaraguan Midas cichlid in a common garden experiment. My demonstration of plasticity in the cichlids’ pharyngeal jaw, reported in chapter 2, suggests it as a factor to be considered in answering the question of why there are so many cichlid species.
The concept of adaptive radiation is intimately related to ecological adaptation by means of natural selection. Thus, one would not be surprised if phenomena indicative of natural selection would be common in adaptive radiations. One of the strongest cases for the action of natural selection, since the birth of the idea, has been made with the argument of convergent evolution. Separation in time or by geography was, however, assumed to be necessary due to competitive exclusion (Osborn 1902). This principle, later formulated by Gause (1934), was questioned to be applicable to some communities of organisms, one of them being the cichlid species flocks of East Africa. Ernst Mayr (1984) asked:
“The coexistence of hundreds of closely related species in the same lake poses some fundamental questions concerning competition and resource utilization. To what extent, if any, is the existence of fish flocks in freshwater lakes in conflict with the concept of competitive exclusion?”
This question is investigated in chapter 3, which is concerned with convergence within the cichlid radiation in Lake Tanganyika. The revealed abundance of ecomorphological convergence without geographical or chronological separation indeed seems to defy Gause’s principle. Furthermore, it suggests the facility of coexistence of convergent species to be another key factor for the cichlids’ species richness that has been previously overlooked.
The large overlap in morpho- and ecospace between subclades of Tanganyikan cichlids is not unique, but emerges as a common feature of adaptive radiations. This is exemplified by the adaptive radiation of Antarctic notothenioid fishes, the topic of chapter 4, comprising several families, which diversified in parallel along the benthic-pelagic axis. Thus, an adaptive radiation of fishes, taking place in a most different setting than the tropical, confined, freshwater environment in which cichlids diversified, nevertheless exhibits intriguing parallels in subclade overlap. Convergence might hence be a feature of radiations in general.