An early-life survival and reproductive trade-off shapes selection on body size

Evolutionary trade-offs - opposing trait effects on total fitness via
different fitness components - are likely to be widespread. Some key
trade-offs are expected to be the result of chains of causation acting
across an organism's lifetime. For example, a trait imparting
reproductive benefits early in life may trade off against reduced survival
to attain later-life reproductive opportunities. Tools in evolutionary
quantitative genetics have recently been developed to formally
characterise selection acting through different causal pathways throughout
the life cycle and, therefore, to formally characterise evolutionary
trade-offs. We use these methods to investigate a trade-off between early
life reproduction and survival, and how that trade-off affects selection
on body size in the Soay sheep population inhabiting St Kilda (Outer
Hebrides, Scotland). We decompose and quantify the total effects of
first-year female body mass on lifetime fitness, with particular attention
to the effect of body mass on early-life reproduction, and the potential
survival cost of early-life reproduction. Our results establish that the
total effect of body mass on lifetime fitness is positive, despite the
strong negative contribution acting via early life reproduction. Moreover,
we show that the magnitude of the selection on body mass acting through
different causal paths highly depends on population density. At higher
densities, the cost of early-life reproduction is higher and, therefore,
it contributes a strong negative component to the total selection of body
mass - i.e., at higher population density, selection on body mass is
weaker than it is when the population size is smaller. By decomposing
total selection and quantifying selection acting through different causal
paths, we expose the underlying mechanics shaping body mass in Soay sheep
female lambs, and we provide a meaningful contribution to the
understanding of the evolution of body size in this population.