It's All Life History

different stages in the life cycle and to evolve specialised forms to go with them. This strategy is widely used — from the adult mayfly whose sole task it is to find a mate, to the ever-hungry caterpillar — and involves often dramatic bodily transformations between stages. Before the term 'life history' was even coined, such metamorphoses — like the tadpole turning into a frog illustrated on the issue's cover — were what first caught the attention of naturalists and artists. The physiological and developmental mechanisms underlying this and other metamorphoses in vertebrates are discussed in the review by Vincent Laudet. In the sea, where the main task of larvae is dispersal, the transformations from larva to adult are especially striking: free swimmers become sessile filter feeders, bilateral symmetric animals adopt a secondary radial symmetry and so forth. Yet, such radically different forms are of course made from a single genome. The review by Dustin Marshall and Steve Morgan focuses on how this genetic and morphological linkage between stages may affect the evolution of life history in marine organisms. If marine organisms are the champions of transformation between stages, parasites are arguably the masters of complexity as far as life-cycles are concerned — a fact paid homage to in the review by Mark Viney and Jo Cable. Aside from the trade-offs that free-living animals face, parasite life histories are also shaped by the need to switch between and adapt to ever-evolving hosts. Parasitism is particularly common in nematodes, where it has evolved several times independently, and one reason why this should be so might lie in their life histories. Many nematodes are able to enter a robust, dormant larval stage, the so-called dauer larva, and this phenotypic plasticity — the hormonal control and evolutionary elaboration of which Ralf Sommer and Akira Ogawa discuss in their review — may have facilitated the evolution of parasitic lifestyles. Phenotypic plasticity is also the topic of Stephen Simpson and colleagues' review, which focuses on the ability of many insect species to adapt alternative guises in response to environmental conditions or signals from conspecifics — a phenomenon known as 'polyphenism'; think: winged and wingless aphids, worker bees and queens, migratory and solitary locusts. While life history has been predominantly studied from an evolutionary point of view, studies on genetic model organisms have also led to a deep mechanistic understanding of how life-history characteristics are controlled, …