Trophic Novelty Is Linked to Exceptional Rates of Morphological Diversification in Two Adaptive Radiations of Cyprinodon Pupfish

Abstract

Adaptive radiations are known for rapid morphological and species diversification in response to ecological opportunity, but it remains unclear if distinct mechanisms drive this pattern. Here, we show that rapid rates of morphological diversification are linked to the evolution of novel ecological niches in two independent Cyprinodon radiations nested within a wide-ranging group repeatedly isolated in extreme environments. We constructed a molecular phylogeny for the Cyprinodontidae, measured 16 functional traits across this group, and compared the likelihoods of single or multiple rates of morphological diversification. We found that rates of morphological diversification within two sympatric Cyprinodon clades containing unique trophic specialists are not part of an adaptive continuum with other clades, but are instead extreme outliers with rates up to 131 times faster than other Cyprinodontidae. High rates were not explained by clade age, but were instead linked to unique trophic niches within Cyprinodon, including scale-eating, zooplanktivory, and piscivory. Furthermore, although both radiations occur in similar environments and have similar sister species, they each evolved unique trophic specialists and high rates of morphological diversification in different sets of traits. We propose that the invasion of novel ecological niches may be a key mechanism driving many classic examples of adaptive radiation.

Rates of speciation and morphological evolution are correlated across the largest vertebrate radiation

“…Unlike punctuated equilibrium, a coupling between evolvability and speciation provides an immediate explanation for the observation that speciation rates themselves vary widely among clades (Figs 3-5), as well as the phylogenetic predictability of adaptive radiation 45,46 . We are only beginning to understand the developmental genetic and ecological mechanisms that might underlie clade-specific differences in evolvability 40 , but some evidence suggests that clades vary substantially in their capacity to evolve novel phenotypes 43,47,48 . At present, we are unable to distinguish between punctuated equilibrium and evolvabilitydiversification correlations as mechanisms underlying the observed positive relationship between rates of speciation and of phenotypic evolutionary rates 49 .…”

Biomechanical trade-offs bias rates of evolution in the feeding apparatus of fishes

“…It is well documented that traits do not respond equally to selection, and understanding the mechanisms that underlie biases in rates and directions of phenotypic evolution is a major goal in evolutionary biology. Rate biases between body parts are often attributed to variation in selection pressure, when selection gradients affect specific functional systems [20,39]. Even in the presence of selection, genetic variance is necessary for traits to evolve, and so serves as a proxy for the immediate evolutionary potential of a trait.…”

The role of ecological opportunity in shaping disparate diversification trajectories in a bicontinental primate radiation

“…Although generalist herbivores have high rates of net diversification and transition to other trophic niches [82], specialist species are expected to have elevated rates of speciation and extinction owing to increased susceptibility to environmental change [83]. Unique trophic specialists can also have elevated rates of morphological evolution [84]. In this case, folivory may be an evolutionary 'dead-end' for colobines owing to foliage being a much less nutritious food than other vegetative items [85].…”