Evolutionary ecology of progeny size in arthropods

Most models of optimal progeny size assume chat there is a trade-off betwee n progeny size and number, and that progeny fitness increases with increasi ng investment per young. We find that both assumptions are supported by emp irical studies but that the trade-off is less apparent when organisms are i teroparous, use adult-acquired resources for reproduction, or provide paren tal care. We then review patterns of variation in progeny size among specie s, among populations within species, among individuals within populations, and among progeny produced by a single female. We argue that much of the va riation in progeny size among species, and among populations within species , is likely due to variation in natural selection. However, few studies hav e manipulated progeny environments and demonstrated that the relationship b etween progeny size and fitness actually differs among environments, and fe wer still have demonstrated why selection favors different sized progeny in different environments. We argue that much of the variation in progeny siz e among females within populations, and among progeny produced by a single female, is probably nonadaptive. However, some species of arthropods exhibi t plasticity in progeny size in response to several environmental factors, and much of this plasticity is likely adaptive. We conclude that advances i n theory have substantially outpaced empirical data. We hope that this revi ew will stimulate researchers to examine the specific factors that result i n variation in selection on progeny size within and among populations, and how this variation in selection influences the evolution of the patterns we observe.