Over the course of an individual’s lifetime, luck often explains a
large fraction of the between-individual variation in lifespan or
lifetime reproductive output (LRO) within a population, often
contributing much more to LRO variation than variation in individual
traits or “quality.” While good traits may be necessary for
exceptional success, they have not been sufficient in the cases we
have considered: you also need to be lucky. But how does this balance
between luck and traits play out over the course of an individual’s
life? Do the contributions of luck and trait variation to LRO
variation peak at the same ages/sizes/states, or does the balance
shift over the course of an individual’s life? And is it always more
important to be lucky than to be good, or are there ages or stages
when good traits are the key to success? We develop mathematical
tools to partition the variance of LRO into contributions from luck
and traits at each age and to further partition luck into
contributions from state trajectory variation, fecundity variation,
pre-natal luck, and whether or not an individual bred vs how many
times they bred.
Results/Conclusions:
Our case studies confirm previous findings that luck early in life is
most important, and throughout life, luck is more important than trait
variation. In contrast, the importance of trait variation
relative to luck can peak early or late in life. At all ages, most luck comes from variation in state
trajectories: if life is a series of forking paths, individuals follow
different routes. For species with
long reproductive spans, early luck often consists of two distinct
peaks: very early events affect whether or not an individual ever
breeds, and less early events drive variation in lifetime reproduction
among those who breed at least once. As individuals age, more
survivors have bred, so state trajectory luck comes increasingly from
variation among breeders.
The crucial ages/stages when Luck has the most impact are generally
not the same as those identified as most important by eigenvalue
elasticity analysis. Elasticity analysis measures the impact of
changing the expected outcome over many similar individuals, analogous
to slightly improving the average test performance of every student in
a classroom. Luck is about variation among individuals, and may
identify opportunities for targeted interventions that shift
particular individuals towards or away from particular forks in the
road.