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Any individual is born, survives, develops, possibly reproduces, and ultimately dies. There are countless pathways through life, and as such there is a tremendous diversity of life history strategies (see Glossary) across species. However, such strategies are not always associated with species’ evolutionary history or environment (Box 1). Central aims of evolutionary ecology include describing and explaining the diversity of life histories across the Tree of Life [1]. In this context, the fast-slow continuum [2] has become the standard framework. Indeed, the fast-slow continuum describes a high amount of life history variation across species in most taxonomic groups studied so far [3–9]. However, beyond this fast-slow continuum, many comparative studies have identified various other important axes of variation, often linked to developmental or reproductive tactics.
Around the inception of the fast-slow continuum, Stearns [2] suggested that there was not yet a “general and reliable theory of life history evolution” and that the field was not“empirically sufficient” to test life history theory. With the emergence of large global databases of demographic and trait data [10–18], we are now much closer in several taxonomic groups to the required sufficiency to test general theories of life history evolution. However, the challenges of identifying, quantifying, explaining, and inferring life history diversity has quickly become lost in a fog of data types, trait currencies, statistical confounds, non-independence, mathematical artefacts, and eco-evolutionary feedbacks.
So although the fast-slow continuum is the main structuring axis of life history variation across species studied to date, it is not by itself an adequate universal theory of life history, and there remains little consensus on how to measure life history [19] and which interspecific patterns deserve explanation. As such, we still lack a“general and reliable” [2] understanding of life history diversity.
Here, we reflect on the state-of-the-art in life history theory and the potential for general theories in the context of further axes of life history variation beyond the fast-slow continuum. We identify several empirical challenges that must be overcome to properly test such theories across the Tree of Life. These challenges include filling taxonomic and biogeographic data gaps, standardising trait units and developing novel analytical techniques to facilitate moving toward hypothesis-based over exploratory analyses. Without universal benchmarks, it will be difficult to determine whether the life history strategies we currently observe in the wild are shaped by allometry, relatedness, physiologies, bauplan , behaviours, lifestyles, trophic levels, or biogeographic realms.