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Modularity among traits is thought to drive morphological evolution and diversification, with more modular species often showing greater morphological disparity and faster evolutionary rates. However, recent studies suggest this pattern is not universal, as higher integration can sometimes be linked to faster rates of evolution. In adaptive radiation, modularity likely facilitates morphological divergence, but its specific role in trait diversification within these events remains uncertain. Antarctic icefishes (Perciformes: Notothenioidei) have undergone adaptive radiation in the frigid Southern Ocean, yet the role of modularity in their craniofacial evolution remains poorly understood. Emerging from a common ancestor 22 Mya, these fishes developed unique morpho-physiological adaptations, such as antifreeze glycoproteins, that contributed to their evolutionary success, but the contribution of cranial modularity to their diversification is still unexplored. Here, we analyze skull shape across 172 perciform species using micro-CT scanning and geometric morphometrics to investigate the tempo and mode of skull evolution in 80 notothenioids versus 92 perciform relatives. Notothenioids exhibit considerable cranial shape diversity, with skull shapes ranging from short to long faces. Fast rates of skull shape evolution occurred in smaller subclades following the emergence of cranial elongation, a derived trait within notothenioids. They also exhibit elevated evolutionary modularity relative to their perciform relatives, with reduced covariation among skeletal elements over time, likely corresponding with Miocene cooling events and the formation of the Antarctic Circumpolar Current. We propose that greater phenotypic modularity in notothenioid skulls represents a pivotal innovation, facilitating their evolutionary response to new ecological opportunities in the Antarctic.
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Large morphological transitions underlie exceptional shape diversification in an adaptive radiation
Adaptive radiations are characterized by an increase in species and/or phenotypic diversity as organisms fill open ecological niches. Often, the putative adaptive radiation has been studied without explicit comparison to the patterns and rates of evolution of closely related clades, leaving open the question whether notable changes in evolutionary process indeed occurred at the origin of the group. Anolis lizards are an oft-used model for investigating the tempo and mode of adaptive radiations. Most of the prior research on the diversification of Anolis morphology has focused on the post-cranium because of its significance towards subdivision of the arboreal habitat. But the remarkable diversity in head shape in anoles has not been as thoroughly investigated. It remains unknown whether the tempo or mode of head shape diversification changed as anoles diversified. We performed geometric morphometric analysis of skull shape across a sample of 12 Iguanian families (110 species), including anoles. Anolis lizards occupy a unique area and a wider region of morphological space compared to the 11 other families examined. We did not find a difference in the evolutionary rate of head shape diversification between anoles and their relatives. Rather, the extraordinary amount of skull diversity arose through a distinct mode of evolution; anoles moved into novel regions by relatively large morphological transitions across morphological space compared to their relatives. Our results demonstrate that traits not directly tied to the adaptive shift of a lineage into unique ecological spaces may undergo exceptional patterns of change as the clade diversifies.
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- Award ID(s):
- 1942250
- PAR ID:
- 10586350
- Publisher / Repository:
- Nature
- Date Published:
- Journal Name:
- Scientific reports
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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