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Sexual conflict is thought to be an important evolutionary force in driving phenotypic diversification, population divergence, and speciation. However, empirical evidence is inconsistent with the generality that sexual conflict enhances population divergence. Here, we demonstrate an alternative evolutionary outcome in which sexual conflict plays a conservative role in maintaining male and female polymorphisms locally, rather than promoting population divergence. In diving beetles, female polymorphisms have evolved in response to male mating harassment and sexual conflict. We present the first empirical evidence that this female polymorphism is associated with (i) two distinct and sympatric male morphological mating clusters (morphs) and (ii) assortative mating between male and female morphs. Changes in mating traits in one sex led to a predictable change in the other sex which leads to predictable within-population evolutionary dynamics in male and female morph frequencies. Our results reveal that sexual conflict can lead to assortative mating between male offence and female defence traits, if a stable male and female mating polymorphisms are maintained. Stable male and female mating polymorphisms are an alternative outcome to an accelerating coevolutionary arms race driven by sexual conflict. Such stable polymorphisms challenge the common view of sexual conflict as an engine of rapid speciation via exaggerated coevolution between sexes.

The genus Lathrolestes Förster (Hymenoptera: Ichneumonidae) from Northern Europe is reviewed. A new species, L. oelandinus sp. n. from Öland (Southern Sweden) is described. Illustrated diagnoses for 23 Northern European species are provided. Lathrolestes soperi Reshchikov, 2010 is recorded for the first time in the Palaearctic region (from the UK) and L. bipunctatus (Bridgman, 1886), L. citreus (Brischke, 1878), L. clypeatus (Zetterstedt, 1838) are new records for Denmark, and L. moravicus Habermehl, 1923 is a new record for Finland, France and Russia. Additionally L. ensator recorded newly from Belorus, L. erythrocephalus from Azerbaijan, L. macropygus and L. pictilis from Austria. Trophic interactions of the genus are discussed.