Evolution of pace-of-life
Generation time can be used to predict a species’ position along the
slow–fast continuum of life-history variation (Gaillard et al.2005; Araya-Ajoy et al. 2018). Classic life‐history theory
(Charlesworth 1994) suggests that variation in the mean age at
reproduction evolves as consequence of compromises between age‐specific
survival and reproduction, resulting in some combination of life-history
traits that optimizes mean fitness under particular ecological
conditions. The contemporary evolution of the generation time, and thus
the pace-of-life of an organism, hinges upon there being genetic
variation underpinning the traits that determine generation time. When
we estimated the additive genetic variation in individual measures of
generation time, we found that evolutionary potential or evolvability
was within the range of values estimated for other life-history traits
(Hansen et al. 2011). Individuals with longer generation times
had greater lifetime reproductive success, but not necessarily a greater
individual growth rate, and thus it is not clear that selection should
favor the evolution of longer or shorter generation times.
The evolution of generation time is predicted to be constrained by
trade-offs between reproduction and survival (Stearns 1992; Roff 1993).
In this study, we found the expected negative covariance between
reproductive rate and generation time, and between reproductive rate and
lifespan (Figures 1B and 1F). This is partly unexpected, because
variation in acquisition of resources is expected to mask life-history
trade-offs in observational studies (van Noordwijk & de Jong 1986). To
further explore resource acquisition versus allocation in our
metapopulation, we analyzed the patterns of covariation between
individual life-history traits using principal component analyses (Table
S3). We found that the most important driver of the first axis of
variation was lifetime reproductive success, with positive loadings for
reproductive rate and lifespan, reflecting among-individual variation in
resource acquisition. Whereas the strongest driver of the second
principal components axis was generation time, where the opposing signs
of reproductive rate and life span revealed the classic survival versus
reproduction trade-off.
To further explore evidence of allocation trade-offs, we partitioned the
covariance between annual reproduction and survival at the among- and
within-individual levels (Table 3). If there was a trade-off between
survival and reproduction, then in years where individuals invested more
in survival, they cannot have invested a lot in reproduction. This
should generate a negative within-individual covariance due to resource
allocation trade-offs. Variation among individuals in resource
acquisition should, in turn, be manifested as a positive
among-individual covariance, where individuals with higher than average
recruit production will also have a greater chance of surviving, because
they are able to acquire more resources. We found a positive covariance
between survival and reproduction at both levels, and therefore we find
no evidence of an allocation trade-off at the within-individual level.
The positive covariance between survival and annual reproduction partly
appears to contradict the results above concerning the trade-off between
lifespan and recruitment rate. We addressed this contradiction using
statistical simulations based upon the estimates of annual survival and
reproduction (see Appendix S3). We explored how the demographic
characteristics of this metapopulation could have resulted in a negative
covariance between lifespan and reproductive rate, in the presence of a
positive covariance between survival and annual reproduction (see Figure
S3). We found that even when there was variation among individuals in
acquisition of resources, causing a positive covariance between survival
and annual reproduction at the within- and among-individual levels, the
resulting distribution of individual life-histories in the population
tended to generate a negative covariance between lifespan and
reproductive rate. Given the observed patterns of age- and
density-dependent survival and reproduction, even when individuals that
had a greater survival probability also had a higher reproduction rate,
a negative correlation between lifespan and reproduction rate was
possible, simply as a result of the distribution of individual
life-histories in the population.