<p>a) Empirical analyses reveal that selection can act differently on genes in parents and offspring in the longer term to favour a different optimal balance between offspring size and brood size. Optimal outcomes for each party are labelled: offspring ‘win’ and parents ‘win’. b) Fluctuating ecological conditions can temporarily favour one party by changing the positioning of the size-number trade-off. In some scenarios, it may be temporarily aligned closer to the offspring’s optimum (as illustrated here), in others it remains closer to the parent’s optimum. c) In some situations, ecological conditions might even impose an outcome that is closer to the offspring’s optimum and against the parent’s evolutionary interest. d) Alternatively, fluctuating ecological conditions might change the gradient of the trade-off. At one extreme (shown with the green line), caused by very high food abundance for example, it may remove any conflict over offspring size completely because the optima for parents and offspring are temporarily closely aligned. At the other extreme (shown with the red line), caused by sudden limited food availability for example, it may temporarily prevent offspring from ever attaining investment close to their optimum.</p
