Classical work on life history theory traces back to the foundations of
evolutionary theory (Charlesworth, 1994; Hamilton, 1966; Stearns, 1992).
In any given age-structured population, individuals have a survival rate
and a fertility rate that both depend on age. The famous Lotka equation
demonstrates that in this scenario the annual population growth rate,
λ,
is a function of age, survival and fertility. Because these three
parameters make up a life history, the Lotka equation provided means to
quantify associations between life history components and the
corresponding values of
λ
that these associations yield. The realization that the population
growth rate,
λ,
was a quantitative definition of population fitness – it is defined by
fitness components – allowed a direct connection between life history
theory and evolutionary theory (Hamilton 1966). Now, estimating changes
in fitness
λ
produced by changes in the life history was made possible. For example,
if changes are assumed to arise from random mutations, consequent
changes in fitness can be interpreted as selection acting on the
mutations (Charlesworth 1994). Crucially, such fitness changes can also
be computed from
λ
and its partial derivatives (i.e., sensitivity; Caswell 2001). The
definition of these quantitative associations between fitness and the
life history directly linked individual phenotypes, to population and
evolutionary dynamics setting the stage for the field of evolutionary
demography.
Because of its flexible applicability grounded in general theory, many
developments to this approach have been made for both ecological and
evolutionary theory (Charlesworth 1994; Caswell 2001). More recent
studies addressing the evolutionary demography of animal populations
have focused on integral projection models (IPMs) as a flexible method
to quantify population and evolutionary dynamics while accounting for
continuous phenotypes (Levin et al., 2021). Since the development of
IPMs (Easterling et al., 2000), studies addressing size-based
demographics by structuring populations into a body size continuum have
become common mostly in mammals and birds (Coulson, 2012; Levin et al.,
2021; Merow et al., 2014; Rees et al., 2014), however few studies have
attempted to incorporate social structures into the model (Kappeler et
al., 2019; Paniw et al., 2021), while studies explicitly incorporating
health structures have just started to emerge (Vincze et al., 2022).