Selection towards different adaptive optima drove the early diversification of locomotor phenotypes in the radiation of Neotropical geophagine cichlids

Background/Question/Methods

Adaptive radiation, the diversification of an ancestor into several lineages that exhibit diverse ecological adaptations, is believed to be a major source of biodiversity. Simpson proposed a conceptual model of adaptive radiation in which lineages diversify into “adaptive zones” within a macroevolutionary adaptive landscape. Despite the prominence of Simpson’s model only a handful of studies have empirically investigated the adaptive landscape and its consequences for our interpretation of the underlying mechanisms of phenotypic evolution.

The evolution of locomotor phenotypes may be an important dimension of adaptive ecomorphological evolution in fish radiations given the implications of locomotion for feeding and habitat use. Neotropical geophagine cichlids represent a newly identified adaptive radiation and provide a useful system for studying patterns of locomotor diversification and the implications of selective constraints on phenotypic divergence in general. We collected functional morphological data from museum specimens belonging to 32 species representing each of the major geophagine lineages. Multivariate ordination was used to investigate the distribution of geophagine species in locomotor morphospace. Models of phenotypic evolution and posterior predictive approaches were then used to investigate the macroevolutionary adaptive landscape and patterns of locomotor phenotype divergence over time.

Results/Conclusions

The evolution of locomotor phenotypes was characterized by selection towards at least two distinct adaptive peaks and the early divergence of modern morphological disparity. One adaptive peak included most of the geophagine lineages, which consist of benthic and epibenthic invertivores with deep, laterally compressed bodies that optimize precise, slow-swimming manoeuvres. The second adaptive peak resulted from a single shift to a new optimum in the species-rich ram-feeding/rheophilic Crenicichla-Teleocichla clade that is characterized by species with streamlined bodies that optimize fast starts and rapid manoeuvres. Evolutionary models and posterior predictive approaches favoured an early shift to a new adaptive peak over decreasing rates of evolution as the underlying process driving the early divergence of locomotor phenotypes.

Our findings confirm that locomotion was an important dimension of phenotypic divergence during the geophagine adaptive radiation. They further suggests that the commonly observed early burst of phenotypic evolution during adaptive radiations may be better explained by the concentration of shifts to new adaptive peaks deep in the phylogeny rather than overall decreasing rates of evolution. Accordingly, models that incorporate the macroevolutionary adaptive landscape provide a better-fitting and perhaps biologically more informative account of the historical underlying mechanisms of phenotypic evolution in adaptive radiations.