Morphological evolution in Australian frogs

Abstract

The study of morphological diversity and the processes that have driven it is a topic of uppermost importance in evolutionary biology. Frogs and toads offer a study group that is remarkably diverse in body shape and ecology, particularly in Australia, where biome diversity and isolation have provided excellent opportunities for comparative analyses of broad-scale macroevolutionary patterns. My PhD thesis focuses on morphological evolution in Australian frogs. I gathered an extensive dataset based on external morphology and climatic data, for almost all frog species in Australia (97%), including the two extinct gastric-brooding frog species. I collected x-ray micro-CT scans for representative species of each genera of myobatrachid frogs, which provided detailed 3D morphological information of skeletal structures. I also obtained locomotion and jumping performance data for 61 species of Australian frogs, which represents 67% of the clade diversity and includes all four native families, as well as the invasive cane toad Rhinella marina. I employed phylogenetic comparative methods throughout all of my analyses in order to add evolutionary context. Finally, I describe a simple geometric rigid rotation method, which removes the effect of random translation and rotations, enabling consistent morphological analysis of articulated structures in 3D. The main aim of my thesis was to understand body size and shape evolution in Australian frogs, by asking a series of questions concerning morphological adaptation to environmental pressures, convergence, niche opportunities, integrative and modular evolution of body structures, and the association between form and function. I provide evidence for a close association between morphology and environment through and behaviour and performance, which can lead to convergence patterns between distantly-related lineages. I found that frog species from the same ecotype displayed morphological and physiological similarities, which were even more conspicuous in fossorial species, due to adaptations to arid environments. Despite phylogenetic conservatism, adaptation to similar environments can result in morphological diversification and convergence both between and within clades, erasing the signature of ancestral morphotypes. Specifically, I found that most of the overall body shape differences were concentrated on the limbs: morphology of fore- and hind-limbs was highly integrated and shaped by ecology, locomotion, and burrowing behaviour, whereas skull shape was highly phylogenetically constrained and conserved within subfamilies. Locomotor modes and jumping performance were extremely diverse across Australian frog species. Jumping performance had no phylogenetic signal, and was not directly associated with body size or shape. However, there was a strong pattern of convergence in locomotor modes among species from different families in the same ecotypes. Disparity of locomotor type differed among ecotypes, and fossorial species were confined to the smallest region of the morphospace, probably due to their specialised fossorial morphology. I also demonstrated that the invasive cane toad Rhinella marina occupies an empty morphological niche in Australia, despite overlapping in environmental niche with most native species. This undoubtedly contributed to its extreme invasion success, and shows the importance of morphological niches. To conclude, I discuss morphological evolution of Australian frogs in the context of their ecology, locomotion, burrowing behaviour, and phylogenetic relationships.