An organism’s life history strategy is shaped by traits that have a
natural hierarchy. The organism’s phenotype determines how the
individual acquires resources [71] through morphological,
physiological, and behavioural adaptations. Development describes
the allocation of those resources to competing processes: supporting
somatic maintenance vs. supporting the germline vs.development in either [72]. The consequences of these allocations
are life history traits . These traits are characterised by
probability distributions of survival and fertility. As survival and
fertility govern the number of individuals in populations, life history
traits govern not just an organism’s fitness but also itsdemography - population size, structure, and dynamics [1].
Traits at all hierarchical levels covary with, and feed-back on one
another, and are subject to constraints imposed by evolutionary history,
physical laws, bauplan, and the environment [73]. Natural selection
acts to favour life history strategies that maximise the organism’s
fitness in its environment, given those constraints [1].
Trait currencies and their units vary considerably among life history
studies. Moreover, these units do not necessarily correspond with trait
types or hierarchies. Physiological traits often measure energyexpenditure (e.g., metabolic rate). Behavioural traits typically
quantify energetic expenditure (e.g., foraging efficiency) or
space usage (e.g. , home range size). However, space units are
also common among morphological traits (e.g., specific root
length). Rates of energetic allocation are used to describe resources
given to developmental processes (e.g ., change in body size over
time), thus combining energy and time currencies. Time is commonly used
to measure life history traits, whether as durations (e.g ., mean
life expectancy), rates (e.g ., number of offspring per year), or
temporal probabilities linked to survival, development, and reproduction
(e.g. , probability of reaching maturity at a given age). Life
history traits are often converted to expectations of life history for
populations or species, which is especially necessary where events occur
only once. For example, an individual only dies once, but the aggregate
deaths in a population yield the distribution of survival for the
population’s average phenotype, from which we can estimate e.g.mean life expectancy. Selection on phenotypic, developmental, or
life history traits can quantify the trait’s contribution to fitness
(e.g ., selection pressure). Shaded areas on the figure are
indicative, but not exhaustive, of common currencies used at each level
and their overlap.