Season's fleetings

proposed, that of isolation of populations through temporal separation alone, not requiring geographical, host or habitat separation: so-called 'allochronic' speciation. This mechanism was proposed following observations of fi eld crickets, but subsequent work failed to support the idea. But new work by Helena Santos at the Technical University in Lisbon and colleagues both in Lisbon and France, reported in the Proceedings of the Royal Society B (published online), describes genetic evidence of separation of populations of one European moth linked to different timings in their development. The pine processionary moth (Thaumetopoea pityocampa) belongs to a genus of moths that show a diverse range of developmental timings, with larval development occurring either in the summer or in the winter, depending on species. The pine processionary moth is native to the Mediterranean basin and has a typical winter larval development. The moth is responsible for damaging attacks on pine and cedar forests in the region and is the most signifi cant leaf-eater of both native and introduced pines throughout Italy, especially amongst young and ornamental trees. It gets its name, like other processionary moths, from the observation that the caterpillars can move through trees in long procession, one tagging on behind another. The adults lay eggs on pine leaves in summer and caterpillars feed on needles during autumn and winter. They pupate in the soil in late winter or early spring, and newly emerged adults disperse to reproduce during summer. Ten years ago, an unusual population with summer larval development was discovered in the National Pinewood at Leiria, Portugal. This population existed amongst moths showing the usual winter larval development, so Santos and her colleagues investigated whether the two populations showed A continuing challenge for evolutionary biologists is to explain how species might diverge and develop into new species. The dominant view over recent decades has been that geographical separation of populations has been a major cause of speciation events. On the contrary, sympatric speciation — which occurs without geographical separation through ecological isolation of breeding populations — has been more controversial. Yet many studies as well as mathematical models have provided evidence that sympatric speciation can occur. The best documented case is that of the cichlid fi shes, which speciated by way of ecological specialisation and sexual selection. And several plant-eating insect populations appear to have diverged through host plant or habitat specialisation. But as early as 1960, another mechanism was …