How to become larger: ontogenetic basis of among‐population size differences in a moth

Abstract

AbstractEvolutionary studies on animal body size have primarily focussed on selective pressures operating during the adult life. In contrast, ontogenetic pathways leading to differently sized adults have received less attention. In the present study, based on a common garden experiment, we report considerable genetic differences in body size among European populations of Ematurga atomaria (L.) (Lepidoptera: Geometridae). In terms of body mass, the moths from a southern (Georgian) population are twice as large as their northern (Estonian) conspecifics. Detailed monitoring of larval growth schedules revealed that the size difference arises through a longer development period of the Georgian larvae, with no difference in the number of instars. Differential (instantaneous) growth rates of the larvae do not differ between the populations. Eggs and newly hatched larvae are larger in the Georgian population but the difference vanishes in the second instar. The larger size of the Georgian moths is regained through higher relative mass increments during each of the three final instars. Such gradual ‘accumulation of the difference’ confirms the idea about constraints on substantial evolutionary changes in growth patterns within a single instar. The larger Georgian moths were found to be considerably more fecund which implies a strong selection for large female size. It remains unclear which counteracting selective pressures have favored the smaller size of Estonian conspecifics. As the associated difference in egg size appears not to be carried over to larger larval size, the adaptive value of larger eggs is not likely in contributing to the prospective large adult size. The larger eggs of the Georgian population should have an adaptive value per se, or represent a mechanistic consequence of larger maternal body size.