Speciation can be viewed as the formation of reproductive barriers between different populations. This thesis investigates patterns of natural and sexual selection shaping reproductive barriers between two hybridizing flycatchers (i.e. collared – and pied flycatchers). Behaviorally driven sexual isolation depends on both the availability of conspecific mates and on discrimination ability of individuals. My results demonstrate that these two factors may also interact. Discrimination abilities may change in response to the relative frequency of two interbreeding species. The underlying reason appears to be that male pied flycatchers have a song that incorporates more elements of the song characteristics of male collared flycatchers into their own song repertoires when occurring in areas inhabited predominantly by collared flycatchers. I investigated selection pressures acting on hybrids. In migratory species, hybrid fitness might be reduced as a consequence of intermediate suboptimal migration routes (extrinsic post zygotic isolation). Comparison of stable isotope signatures of revealed that parental species have separate wintering grounds, but hybrids appear to winter at the same location as pied flycatchers. A possible dominance effect in the inheritance of migration direction may hence reduce this potential cost. This interpretation is supported by the absence of a reduction in juvenile to adult survival of hybrids. By further comparing male hybrid fitness to that of the parental species, using lifehistory data, I demonstrate that hybrid males experience a moderate reduction in fitness (mainly through a sexually selected disadvantage). Sexual selection acting on male hybrids can play a major role in the speciation process because when the same characters affect assortative mating as well as hybrid fitness, reinforcement of reproductive barriers becomes more likely. Even when reproductive isolation is completed- the fate of newly formed species may be uncertain since they may strongly compete for ecological space. Long-term persistence of ecologically similar, species requires that there are spatial or temporal variation in their relative fitness. The growth of nestling pied flycatchers is less affected by harsh environmental conditions. We suggest that a regional co-existence of the two flycatcher species is due to a lifehistory trade-off between interference competitive ability and robustness to a harsh conditions. Overall, the studies in this thesis reveal the complexity of the interactions between mate choice and competition in shaping sexual signals. Furthermore, it suggests that natural selection is moderate on hybrid males and that sexual selection may have strong implications for the maintenance of species integrity.

Hybrids are common, being found in about 25% of all plant species, but the isolating barriers which preserve species integrity are poorly studied. I investigated this question in the large New Zealand genus Celmisia Cass. (Asteraceae), which hybridises readily in cultivation, but wild hybrids are relatively rare. My study quantitatively tests four potential reproductive isolating barriers in 12 sympatric species of Celmisia found in the Craigieburn Range, inland Canterbury, New Zealand. I examined two potential prezygotic reproductive isolating barriers (flowering phenology and pollinator specialisation), and two potential postzygotic barriers (pre-dispersal seed predation and hybrid seed germination). I used null models to test whether Celmisia species had temporally segregated flowering times, and found that some Celmisia are temporally segregated and thus less likely to form hybrids. I used experimental pair-wise flowering arrays to observe insect visitation to six different Celmisia species pairs. While I found no difference in the overall pollinator community, several insect families showed preferences for some Celmisia species. Furthermore, I found that subtle floral character differences were driving these insect preferences. In particular, I found scape height to be positively associated with insect visitation with taller Celmisia being favoured over shorter species. Insect preferences did not translate into strong floral constancy, therefore indicating that Celmisia flower visitors are likely to be a weak barrier to hybridisation. I reared a range of insect seed predators from field-collected capitula of the hybrid C. x pseudolyallii and both parent species (C. lyallii and C. spectabilis). There was no overall difference in the number of seed-predators per capitulum between hybrid and parent Celmisia taxa. I collected and sowed seeds from three Celmisia hybrids and their parent species in order to test whether hybrids were less fertile than their parent species. I found no evidence to suggest that the seeds of hybrids had lower germination success than those of their parents. Overall I found evidence for only weak prezygotic reproductive isolation and no evidence for postzygotic isolation in the four barriers I examined in Celmisia.