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Complex life cycles are characterized by niche shifts at the time of metamorphosis. Current models predict optimal sizes for metamorphosis based on maximizing growth, minimizing mortality, or some balance of these goals. These models predict optimal sizes that are independent of the time of metamorphosis. Reproduction and other major events in the life history of organisms are often constrained to seasons, and the state (e.g., mass) of the organism at that time is related to fitness. Therefore, an organism's state as well as the time that that state is achieved are central variables in these time—constrained life histories. We extend earlier theory to include explicit time constraints in three, hypothetical, complex life cycles. Dynamic optimization models are constructed to determine optimal time and mass trajectories for niche shifts. First, we consider the habitat shift at emergence in mayflies, where reproduction terminates a growth period in the first habitat and is constrained to a season. Second, we consider the habitat shift at metamorphosis in amphibians, where reproduction terminates a growth phase in the second habitat and reproduction is constrained to a single point in time. Third, we combine the first two effects to allow an extended period of reproduction in amphibians. Here optimal time and mass trajectories are determined for two niche shifts–the shift from aquatic to terrestrial habitat and the shift from a growth phase to a reproductive phase. We present analytical theory that allows both quantitative and qualitative predictions. Problem constructions and solutions are presented graphically to aid intuition in interpreting our results and extending the framework to other parameter values and other life—history examples. The general conclusion is that time constraints on complex life histories lead to optimal sizes for niche shifts that vary with time. In time—constrained life histories, any variation in the state of individuals at some time prior to reproduction will be preserved to some degree at reproduction. Therefore, in time—constrained life histories, we expect optimal switches in habitat use or life history stage to depend not only on state but also on the time that state is achieved.

In coevolutionary 'arms races' between the sexes, the outcome of antagonistic interactions may remain at an evolutionary standstill. The advantage gained by one sex, with any evolutionary exaggeration of arms, is expected to be matched by analogous counteradaptations in the other sex. This fundamental coevolutionary process may thus be hidden from the evolutionist's eye, and no natural examples are known. We have studied the effects of male and female armament (clasping and anti-clasping morphologies) on the outcome of antagonistic mating interactions in 15 species of water strider, using a combination of experimental and phylogenetic comparative methods. Here we present, by assessing the independent effects of both species-specific level of arms escalation and small imbalances in the amounts of arms between the sexes within species, the consequences of a sexual arms race. Evolutionary change in the balance of armament between males and females, but not in the species-specific level of escalation, has resulted in evolutionary change in the outcome of sexually antagonistic interactions such as mating rate.

Recent evidence suggests that sexually selected traits have unexpectedly high genetic variance. In this paper, we offer a simple and general mechanism to explain this observation. Our explanation offers a resolution to the lek paradox and rests on only two assumptions; condition dependence of sexually selected traits and high genetic variance in condition. The former assumption is well supported by empirical evidence. We discuss the evidence for the latter assumption. These two assumptions lead inevitably to the capture of genetic variance into sexually selected traits concomitantly with the evolution of condition dependence. We present a simple genetic model to illustrate this view. We then explore some implications of genic capture for the coevolution of female preference and male traits. Our exposition of this problem incidentally leads to new insights into the similarities between sexually selected traits and life history traits, and therefore into the maintenance of high genetic variance in the latter. Finally, we discuss some shortcomings of a recently proposed alternative solution to the lek paradox; selection on variance.