Current perspectives and future directions in animal life history evolution

Abstract

1. A life history strategy, the collection of actions, timings and characteristics individuals employ to optimize fitness, represents the evolutionary answer to a species' ecological problems. From the fatally reproductive salmon to the seemingly immortal jellyfish Turritopsis dohrnii, different species have found vastly different answers to their ecological problems, generating the vast suite of life histories observed across the animal kingdom. To explain this variation, life history theorists have generated and tested specific hypotheses to describe this variance and define what drives it. 2. Since Stearns (1992) and Roff (2002), animal life history evolution has pushed new frontiers. Specifically, insights from theoretical modelling, experiments, fieldwork and comparative studies have elucidated: how to describe life histories, what drives variance in life histories and what are the mechanisms that underlie life history traits. However, despite this progress, gaps in knowledge still remain.3. In turn, here we review current perspectives, developed over the past 20 years, that support much of life history research today. These perspectives include: (1) the two-axes framework to describe life histories across taxa, (2) three different types of variance that impact life history evolution (i.e., variance within time-steps, across time-steps and variance in life history outcomes) and (3) the utility of integrating ultimate and proximate modes of research to understand life history evolution. Subsequently, we outline future directions that represent new frontiers in animal life history evolution. These future directions are targeted at specific gaps in knowledge that offer timely insights for the broader ecology and evolutionary biology community: (1) where does selection act in a 3 life history, (2) a new representation of life histories in variable environments and (3) dealing with time in life history evolution. 4. In summary, this review provides a holistic perspective (from molecules to selection gradients) on how life histories are studied and why life history research requires interdisciplinarity. The further discussion of current perspectives and future directions provides a cross-section of animal life history research today: where we are, how we got here and where we are likely heading.