Fundamental constraints on the evolution of vertebrate life histories

Abstract

ABSTRACTThe evolution of vertebrate life histories has been fuelled by key transitions among water and land. Life history diversity evolves according to the interplay of predation on juveniles and adults, the physical properties of the environment, and the developmental context of each lineage. The selection pressures of each environment dictate which life-history strategies persist over evolutionary time and render certain strategies impossible. Despite the well-recognized shifts in selective pressures accompanying transitions among environments, the conditions driving innovation and its consequences for life-history evolution have never been comprehensively tested. Here, we show that the contrasting fitness landscapes operating in aquatic and terrestrial environments result in predictable shifts in demographic and evolutionary constraints that explain differences in vertebrate life histories. Specifically, our results emphasize the reduced potential for life-history diversification on land, especially that of reproductive strategies. Moreover, our study reveals differences between the evolution of viviparity in the two environments. Transitions from egg laying to live birth represents a major shift across life-history space for aquatic organisms, whereas terrestrial egg-laying organisms evolve live birth without drastic changes in life-history strategy. Whilst trade-offs in the allocation of resources place fundamental constraints on the way life histories can vary, ecological setting defines the macroevolutionary space that species can explore.