Reproductive complexity in plants did not occur gradually, but in two distinct pulses

Plant reproductive complexity increased in two, widely separated pulses likely driven by important evolutionary innovations, including the origin of seeds and flowers, according to a new study. These findings show that, in contrast to many aspects of animal morphological diversity, which often occurred early in evolutionary history, most complexity in plants occurred much later. The evolution of land plants has been characterized by an increase in the complexity of reproductive structures. The earliest groups produced simple reproductive structures, whereas some later taxa evolved to produce complex and intricate flower structures with many specialized parts and arrangements. However, when and how reproductive complexity arose in plants is poorly understood. Andrew Leslie and colleagues devised a novel approach for analyzing the morphological complexity of reproductive structures in vascular land plants across their evolutionary history. The method divides plant reproductive structures into their basic morphological components – parts like pollen sacs or flower petals, for example – and scores the complexity of a given taxon based on the number of part types and their organization. To analyze changes in plant complexity over time, Leslie et al. applied this method to a dataset of 1,338 living and fossil plants spanning the paleontological record. According to the findings, reproductive complexity in plants did not increase gradually but rather in two distinct pulses, corresponding to key evolutionary innovations in plant reproductive biology and separated by roughly 250 million years of relative stasis. The first pulse occurred in the late Devonian (~375 million years ago), during the initial radiation of vascular plants; it culminated with the appearance of non-flowering seed plants. The second pulse occurred during the mid-Cretaceous (~125 million years), corresponding with the origin and rapid diversification of flowering plants.