No abstract

The concept of a trade-off has long played a prominent role in understanding the evolution of organismal interactions such as mutualism, parasitism, and competition. Given the complexity inherent to interactions between different evolutionary entities, ecological factors may especially limit the power of trade-off models to predict evolutionary change. Here, we use four case studies to examine the importance of ecological context for the study of trade-offs in organismal interactions: (1) resource-based mutualisms, (2) parasite transmission and virulence, (3) plant biological invasions, and (4) host range evolution in parasites and parasitoids. In the first two case studies, mechanistic trade-off models have long provided a strong theoretical framework but face the challenge of testing assumptions under ecologically realistic conditions. Work under the second two case studies often has a strong ecological grounding, but faces challenges in identifying or quantifying the underlying genetic mechanism of the trade-off. Attention is given to recent studies that have bridged the gap between evolutionary mechanism and ecological realism. Finally, we explore the distinction between ecological factors that mask the underlying evolutionary trade-offs, and factors that actually change the trade-off relationship between fitness-related traits important to organismal interactions. K E Y W O R D S :Antagonistic pleiotropy, evolution of increased competitive ability, host range, mutualism, parasitism, Y-model.

We document a negative and signi…cant relationship between domestic …nancial development and unemployment volatility in developing and emerging economies (DEMEs). However, there is no signi…cant relationship between these variables in advanced economies (AEs). A labor-search model with production heterogeneity, sectoral …nancial frictions, and inter…rm input credit can rationalize these di¤erential crosscountry results. Unemployment volatility is decreasing in …nancial development, but the quantitative relevance of this relationship is increasing in the extent to which input credit is prevalent in an economy. This insight is consistent with the fact that, empirically, input credit is much more prominent in DEMEs compared to AEs.

Skill-mismatch employment occurs when high-skilled individuals accept employment in jobs for which they are over-quali…ed. These employment relationships can be bene…cial because they allow high-skilled individuals to more rapidly transition out of unemployment. They come at the cost, however, in the form of lower wage compensation. Moreover, an externality arises as high-skilled individuals do not take into account the a¤ect that their search activity in the market for low-tech jobs has on low-skilled individuals. This paper presents a tractable general equilibrium model featuring mismatch employment and on-the-job search to articulate these tradeo¤s. We derive a set of e¢ ciency conditions that describe the labor market distortions associated with these two model features and illustrate how they alter the standard notion of the labor wedges inherent in general equilibrium search models. Finally, we calibrate the model to U.S. data and show that the distortions associated with mismatch employment are largely distributional and can be quantitatively large.

In the United States, the aggregate vacancy-unemployment (V/U) ratio is strongly procyclical, and a large fraction of its adjustment associated with changes in productivity is sluggish. The latter is entirely unexplained by the benchmark homogeneous-agent model of equilibrium unemployment theory. I show that endogenous search and workerside horizontal heterogeneity in production capacity can be important in accounting for this propagation puzzle. Driven by di¤erences in unemployed and on-the-job seekers'search incentives, the probability that any given …rm with a job opening matches with a worker endowed with a comparative advantage in that job exhibits a stage of procyclical slow-moving adjustment. Consequently, so do the expected gains from posting vacancies and, hence, the V/U ratio. The model has channels through which the majority of both the V/U ratio's sluggish-adjustment properties and its elasticity with respect to output per worker can be accounted for.