Abstract. Regional coexistence of ecologically similar species is facilitated when fluctuations in environmental conditions favor different species at different times or places. However, why species with similar ecology should vary in their response to environmental change is unclear. In this study, we explore the role of a life-history divergence in causing changes in relative fitness across environmental conditions experienced by populations of two closely related Ficedula flycatchers on the Baltic island of Ö land, Sweden. We compared patterns of nestling survival between Pied (Ficedula hypoleuca) and Collared (F. albicollis) Flycatchers in relation to two factors known to influence the environment experienced by nestlings: natural variation in their parents' onset of breeding and artificial manipulation of the brood size. Possible differences in the location of the nests (i.e., microhabitat differences) or in habitat use (i.e., feeding patterns) by the adult birds were controlled for by partial crossfostering of young between the two species. We found that nestling mortality was relatively higher among Collared Flycatchers and that this difference increased with later breeding. Mass gain, which predicted survival probability, of nestling Collared Flycatchers did not respond to the seasonal decline in environmental conditions when they were raised in nests with reduced brood size (i.e., where sibling competition was experimentally reduced). This latter result suggests that the smaller clutch size of Collared Flycatchers reflects an adaptive adjustment to their offspring's higher sensitivity to environmental change. We discuss the possibility that the divergence in life-history traits between the two species represents adaptation to different environments experienced during their recent evolutionary history. We conclude that the survival of nestling Collared Flycatchers is more sensitive to harsh environment and that this is likely to limit where and when the more aggressive Collared Flycatchers are able to displace Pied Flycatchers. Our results provide support for models of species coexistence that emphasize the importance of spatial or temporal heterogeneity in relative fitness or life-history divergence. More precisely, our results demonstrate that variation in life-history adaptations may result in changes in relative fitness of species across environments despite their use of similar resources.

Research on the role of sexual selection in the speciation process largely focuses on the diversifying role of mate choice. In particular, much attention has been drawn to the fact that population divergence in mate choice and in the male traits subject to choice directly can lead to assortative mating. However, male contest competition over mates also constitutes an important mechanism of sexual selection. We review recent empirical studies and argue that sexual selection through male contest competition can affect speciation in ways other than mate choice. For example, biases in aggression towards similar competitors can lead to disruptive and negative frequency-dependent selection on the traits used in contest competition in a similar way as competition for other types of limited resources. Moreover, male contest abilities often trade-off against other abilities such as parasite resistance, protection against predators and general stress tolerance. Populations experiencing different ecological conditions should therefore quickly diverge non-randomly in a number of traits including male contest abilities. In resource based breeding systems, a feedback loop between competitive ability and habitat use may lead to further population divergence. We discuss how population divergence in traits used in male contest competition can lead to the build up of reproductive isolation through a number of different pathways. Our main conclusion is that the role of male contest competition in speciation remains largely scientifically unexplored.

Character displacement can reduce costly interspecific interactions between young species. We investigated the mechanisms behind divergence in three key traits-breeding habitat choice, timing of breeding, and plumage coloration-in Ficedula flycatchers. We found that male pied flycatchers became expelled from the preferred deciduous habitat into mixed forest as the superior competitor, collared flycatchers, increased in numbers. The peak in food abundance differs between habitats, and the spatial segregation was paralleled by an increased divergence in timing of breeding between the two species. Male pied flycatchers vary from brown to black with brown coloration being more frequent in sympatry with collared flycatchers, a pattern often proposed to result from selection against hybridization, that is, reinforcement. In contrast to this view, we show that brown male pied flycatchers more often hybridize than black males. Male pied flycatcher plumage coloration influenced the territory obtained in areas of co-occurrence with collared flycatchers, and brown male pied flycatchers experienced higher relative fitness than black males when faced with heterospecific competition. We suggest that allopatric divergence in resource defense ability causes a feedback loop at secondary contact where male pied flycatchers with the most divergent strategy compared to collared flycatchers are favored by selection.

No abstract

Competition-driven evolution of habitat isolation is an important mechanism of ecological speciation but empirical support for this process is often indirect. We examined how an on-going displacement of pied flycatchers from their preferred breeding habitat by collared flycatchers in a young secondary contact zone is associated with (a) access to an important food resource (caterpillar larvae), (b) immigration of pied flycatchers in relation to habitat quality, and (c) the risk of hybridization in relation to habitat quality. Over the past 12 years, the estimated access to caterpillar larvae biomass in the habitat surrounding the nests of pied flycatchers has decreased by a fifth due to shifted establishment possibilities, especially for immigrants. However, breeding in the high quality habitat has become associated with such a high risk of hybridization for pied flycatchers that overall selection currently favors pied flycatchers that were forced to immigrate into the poorer habitats (despite lower access to preferred food items). Our results show that competition-driven habitat segregation can lead to fast habitat isolation, which per se caused an opportunity for selection to act in favor of future "voluntarily" altered immigration patterns and possibly strengthened habitat isolation through reinforcement.