MAIN TEXT
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.