Adaptive Radiation
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Biol. 23, 150–155. http://dx.doi.org/10.1016/j.cub.2012.11.056 Adaptive Radiation: Convergence Insights into these long-standing questions, as often in evolutionary and Non-equilibrium biology, have come from insular systems, habitats isolated by surroundings that are inhospitable The spectacular adaptive radiation of cichlid fish in Lake Tanganyika to the respective organisms, encompasses extensive morphological convergence and co-occurrence as these provide discrete settings, of ecologically similar species, forcing a reevaluation of non-equilibrium often replicated over space and dynamics in community assembly. time. Moreover, with the advent of sophisticated genomic, isotopic, Rosemary G. Gillespie constrained such that the appearance and visualization tools, it has of certain forms at a given time and become possible to understand the The diversity of life is bewildering, but place becomes ‘‘very probable, if not detailed history of lineages, and the two age-old questions remain: first, is inevitable’’ [2]. With regards to extent to which patterns of species origination dictated by chance patterns of species composition, the differentiation are linked to shifts in evolutionary events or constrained to debate concerns the ‘balance of ecology and associated morphology. follow fairly predictable trajectories? nature’ — the idea that the overall In a recent issue of Current Biology, Second, does the observed diversity diversity at a site tends toward Muschick et al. [3] summarize and composition of species reflect an a relatively steady state. This the results of an extraordinarily equilibrium, and if so, over what time supposition was questioned with the comprehensive study of the period? In terms of species origin, recognition of the importance of adaptive radiation of cichlid fish in one argument is that evolutionary the dynamic nature of biodiversity, the African Great Lakes, in which outcomes are shaped by the whims and that patterns may be governed the ecological identity of species, of immediate events: ‘‘The divine tape more commonly by non-equilibrium and their occurrence within a given player holds a million scenarios . [and] processes in which species diversity community, is frequently predictable. the end results are so different’’ [1]. is inherently unstable and changing The study represents the most The alternative is that outcomes are over time. extensive quantitative analysis to date Current Biology Vol 23 No 2 R72 change in response to different diets, Algae scraper Shrimp eater Picker (invertebrates) placing the animal under a different selective regime; this plastic response may then serve as the basis for subsequent adaptive radiation [14]. Other fish can show similar flexibility. In particular, individuals of anadromous sticklebacks are limnetic EEretmodusretmodus ccyanostictusyanostictus NeolamproloNeolamprologusgus prochiluprochiluss GGnathochromisnathochromis ppfefferifefferi when young, becoming more benthic with age: it appears that selection on the timing of this developmentally plastic response has served as the basis for repeated evolution of pairs of benthic and limnetic species as the fish have moved into freshwater lakes [13]. Telmatochromis temporalis Ctenochromis benthicola Tylochromis polylepis The intriguing point is that actual changes in gene frequency can be Current Biology achieved through ecological or developmental plasticity, Figure 1. Tanganyika cichlid ecomorphs. a phenomenon known as ‘genetic Set of three of about a dozen primary ecomorphological types of cichlid fish; each pair accommodation’, and that co-occurs within Lake Tanganyika [3]. Photo credits: Moritz Muschick, Adrian Indermaur, this flexibility may facilitate rapid Walter Salzburger. adaptation while constraining it to specific trajectories. of the ecological underpinnings of of similar genetic changes to allow The second major point made by adaptive radiation in cichlids. rapid and repeated adaptive change, Muschick et al. [3] is that similar The authors [3] make two major including the colonization of freshwater ecomorphs (though not closely points: the first relates to how diversity glacial lakes by marine sticklebacks related species) often co-exist in originated, the second to the dynamics with the repeated evolution of similar Lake Tanganyika, which might be of the communities thus generated. benthic and limnetic forms [9], and the surprising given that theory predicts In the context of the ecological establishment of lizards and mice in competitive exclusion should prevent predictability of species origination, desert environments of North America co-occurrence of taxa that might the authors show that morphological with repeated evolution of traits occupy the same niche. The authors characters, including body and jaw associated with adaptation to the light suggest that the relatively advanced shape, are correlated with the trophic and dry habitats [10]. Likewise, small age of the adaptive radiation in Lake niche (as indicated by stable isotopes changes in a single gene appear to be Tanganyika allows similar ecomorphs and gut content). The important point responsible for the repeated evolution from distant lineages to coexist simply here is that convergence to similar form of similar coloration in the mimicry because they have accumulated is strongly associated with the trophic complexes of Heliconius butterflies differences in other attributes, and niche of the organisms (Figure 1). This [11]. This kind of recurrent evolution of so may occupy slightly different has long been suspected, and has similar forms controlled by the same niches. This pattern is reminiscent served as the basis for the idea of genetic pathways can span