Introduction
Generation time is a key parameter describing the pace of key biological
processes. In age-structured populations, generation time is determined
by age-specific survival and reproduction (Cochran & Ellner 1992), so
it has been suggested as a good descriptor of the pace of life-history
strategies (Gaillard et al. 2005). Generation time is also in
allometric relation with key characteristics of organisms such as its
body size and its metabolic rate (Brown et al. 2004). It is
related to the rate of mutation (Lehtonen & Lanfear 2014) and the time
a population needs to replace itself (Bienvenu & Legendre 2015).
Evolutionary responses to selection of a trait per unit time depend upon
a population’s generation time
(Lande 1982), and as
such it is a measure that connects the demographic and phenotypic
characteristics of a population with the potential rate of evolutionary
change. Furthermore, generation time is related to the susceptibility of
organisms to stochastic fluctuations in the environment (Sæther et
al. 2005) and it is a key component of evolutionary rescue models
(Chevin et al. 2010). Understanding the ecological processes
affecting the generation time is therefore essential for predicting the
rate at which organisms can adapt to environmental change.
Generation time is generally defined as a population-level attribute.
There are several definitions of generation time for age-structured
populations, all of which can be calculated from survival and
reproduction projection matrices (Cochran & Ellner 1992; Bienvenu &
Legendre 2015). For instance, generation time has been defined as the
time it takes a population to grow by a factor of its net reproductive
rate (Coale 1972), and also as the average time between birth events in
an ancestral lineage going from daughter to mother (Bienvenu & Legendre
2015). Other definitions relate to the average age of mothers of
newborns in the population
(Charlesworth 1994),
such as “mean age of the parents of offspring produced in a particular
time period” (Cochran & Ellner 1992), “the mean age at birth events
for all individuals in a cohort” (Caswell 2001) or a generation
(Steiner et al. 2014). Importantly, all formulations provide
similar relative metrics when applied to a range of scenarios (Ellner
2018). In this study, we focus on understanding the evolutionary
potential and ecological factors shaping generation time of individuals,
measured as the weighted mean age of successful contribution to the
breeding population (i.e. the mean age of recruit-producing parents
weighted by the number of recruits they produce at each age; Cochran &
Ellner 1992; McGraw & Caswell 1996).
Among-species comparisons have shown that generation time predicts the
position of species on the fast-slow continuum of life-history
covariation
(Gaillard et
al. 2005, 2016). At the fast end are organisms prioritizing current
reproduction; that mature early, have high reproductive rates, short
lifespans and short generation times. At the slow end of the continuum
are organisms prioritizing survival over reproduction; characterized by
high survival rates, low reproduction rates and long generation times.
Generation time thus reflects how organisms resolve life-history
trade-offs between current versus future reproduction, such as the
trade-off between reproduction and survival (Gaillard et al.2005; Wright et al. 2019). Despite its evolutionary importance,
surprisingly little research has focused on understanding the sources of
within-species variation in generation time. This is probably because
generation time has generally been considered as a static attribute at
the population or species level. However, generation time as an
individual-level life-history trait can be estimated from an
individual’s age dependent reproduction (McGraw & Caswell 1996), and
has been suggested as a potential measure of an individual’s position on
the fast-slow continuum of life-history covariation
(Araya-Ajoyet al. 2018). Studies quantifying the sources of variation in
generation time at the among-individual and among-population levels
should provide important insights into its evolutionary potential and
how it is shaped by ecological conditions (McGraw & Caswell 1996;
Araya-Ajoy et al. 2018; Wright et al. 2019).
Ecological conditions are expected to shape the generation time of
organisms. The role of density dependence in determining the tempo of
life-history strategies has been a long-standing research topic in
evolutionary ecology (Pianka 1970; Stearns 1976; Boyce 1984; Wrightet al. 2019). Density dependence was introduced as a driver of
life-history strategies in the context of r - versusK -selection after island colonization
(MacArthur &
Wilson 1967). The general idea was that when populations are growing,
density-independent selection will favor fast life-history strategies,
but as populations approached their carrying capacity, density-dependent
selection will favor slower life-history strategies. More recently,
age-structured models of density-dependent evolution have been shown to
provide general predictions concerning the factors that may affect the
optimal age of reproduction
(e.g. Engen &
Sæther 2016). Therefore, while generation time may be constrained by
reproductive and developmental trade-offs, it can also be shaped by
selection in response to ecological pressures.
Our aim in this study is to determine the evolutionary potential of
generation time as a measure summarizing patterns of life-history
covariation, and to assess the role of population dynamics in
determining generation time. We studied variation in generation time in
a metapopulation of house sparrows (Passer domesticus ) in
northern Norway. We first estimated individual generation times based on
individual projection matrices and decomposed its variation into genetic
and environmental sources. Next, we studied the relationships between
individual generation time and first age of successful reproduction,
lifespan, reproduction rate, lifetime reproductive success and
individual expected growth rates (McGraw & Caswell 1996). We then
studied how age- and density-dependent survival and reproduction in turn
affected generation time. Finally, we studied how fluctuations in
population growth affected the mean age at reproduction to test the
prediction that when populations are expanding, generation times should
be shorter, whereas when populations are decreasing or near their
carrying capacity generation times should be longer.