No abstract

Most models of optimal progeny size assume that there is a trade-off between progeny size and number, and that progeny fitness increases with increasing investment per young. We find that both assumptions are supported by empirical studies but that the trade-off is less apparent when organisms are iteroparous, use adult-acquired resources for reproduction, or provide parental care. We then review patterns of variation in progeny size among species, among populations within species, among individuals within populations, and among progeny produced by a single female. We argue that much of the variation in progeny size among species, and among populations within species, is likely due to variation in natural selection. However, few studies have manipulated progeny environments and demonstrated that the relationship between progeny size and fitness actually differs among environments, and fewer still have demonstrated why selection favors different sized progeny in different environments. We argue that much of the variation in progeny size among females within populations, and among progeny produced by a single female, is probably nonadaptive. However, some species of arthropods exhibit plasticity in progeny size in response to several environmental factors, and much of this plasticity is likely adaptive. We conclude that advances in theory have substantially outpaced empirical data. We hope that this review will stimulate researchers to examine the specific factors that result in variation in selection on progeny size within and among populations, and how this variation in selection influences the evolution of the patterns we observe.

Summary1. Ecologically significant evolutionary change, occurring over tens of generations or fewer, is now widely documented in nature. These findings counter the long-standing assumption that ecological and evolutionary processes occur on different time-scales, and thus that the study of ecological processes can safely assume evolutionary stasis. Recognition that substantial evolution occurs on ecological time-scales dissolves this dichotomy and provides new opportunities for integrative approaches to pressing questions in many fields of biology. 2. The goals of this special feature are twofold: to consider the factors that influence evolution on ecological time-scales -phenotypic plasticity, maternal effects, sexual selection, and gene flow -and to assess the consequences of such evolution -for population persistence, speciation, community dynamics, and ecosystem function. 3. The role of evolution in ecological processes is expected to be largest for traits that change most quickly and for traits that most strongly influence ecological interactions. Understanding this fine-scale interplay of ecological and evolutionary factors will require a new class of eco-evolutionary dynamic modelling. 4. Contemporary evolution occurs in a wide diversity of ecological contexts, but appears to be especially common in response to anthropogenic changes in selection and population structure. Evolutionary biology may thus offer substantial insight to many conservation issues arising from global change. 5. Recent studies suggest that fluctuating selection and associated periods of contemporary evolution are the norm rather than exception throughout the history of life on earth. The consequences of contemporary evolution for population dynamics and ecological interactions are likely ubiquitous in time and space.

Inbreeding-environment interactions occur when inbreeding leads to differential fitness loss in different environments

Males and females of nearly all animals differ in their body size, a phenomenon called sexual size dimorphism (SSD). The degree and direction of SSD vary considerably among taxa, including among populations within species. A considerable amount of this variation is due to sex differences in body size plasticity. We examine how variation in these sex differences is generated by exploring sex differences in plasticity in growth rate and development time and the physiological regulation of these differences (e.g., sex differences in regulation by the endocrine system). We explore adaptive hypotheses proposed to explain sex differences in plasticity, including those that predict that plasticity will be lowest for traits under strong selection (adaptive canalization) or greatest for traits under strong directional selection (condition dependence), but few studies have tested these hypotheses. Studies that combine proximate and ultimate mechanisms offer great promise for understanding variation in SSD and sex differences in body size plasticity in insects.