Secondary contact zones are regions where genetically distinct populations overlap and produce hybrids, and they offer the potential for significant insights into the process of speciation. The central Wet Tropics in north-east Queensland is a region where numerous species have genetic lineages in secondary contact following the expansion of rainforest from historical (Pliocene and Pleistocene) refugia in the northern and southern Wet Tropics. One of these species is the Green-eyed Tree Frog Litoria genimaculata, a medium-sized arboreal frog associated with rainforest streams. The Wet Tropics populations of this species consist of a deeply divergent (13% CO1 mtDNA) northern and southern lineage currently in contact in the central Wet Tropics. This thesis presents a thorough analysis of the outcomes of historical isolation and secondary contact in this species and aims to elucidate the evolutionary processes that have determined these outcomes.
The northern (N) and southern (S) lineages of L. genimaculata overlap at two independent contact zones in close proximity – one between the main ranges of N and S (contact A) and the other, 13 km to the north, where an isolated population of the southern lineage (termed iS) occurs within the northern lineage area (contact B). The iS population is currently geographically isolated from the genetically similar main range of the S lineage. Genetic analyses of the contact zones revealed that hybridization occurs between the two lineages in the field, but it occurs significantly more frequently at contact A (estimate of 3.1-6.8% hybrids) than at contact B (0-1.4%) and also over a broader area (approximately 6.0 km versus 0.6 km). Experimental crosses revealed asymmetric offspring viability between the lineages, with crosses between southern females (S/iS) and N males failing in the early larval stage while the reciprocal crosses succeeded. This is supported by the genetic analyses, which found that none of the potential hybrids carried S mtDNA.
There are no morphological differences between the lineages where the main ranges meet at contact A, but there is a significant difference in male call. However, females from this contact did not show a preference for either lineage when these calls were used in laboratory-based two-choice call trials. In contrast, there is significant divergence in iS male (but not female) body size and call from N at contact B, and mate choice trials revealed highly significant premating isolation between the lineages at this contact. The genetic analyses supported these results in finding the frequency of hybridization to be significantly lower at contact B than at contact A. No differences in ecology or behaviour were detected between the lineages at either contact. The conclusion from these results is that the process of reinforcement (increase in premating isolation due to natural selection against hybridization) has resulted in speciation between the lineages at contact B but not at contact A.
The larva of a Batrachomyia fly parasitise L. genimaculata and the potential impact of parasitism on contact zone dynamics was investigated. Infection prevalence was significantly higher in southern (S/iS) lineage L. genimaculata at the mixed lineage sites in both contact zones, with the magnitude of this effect being similar at both contacts despite the phenotypic divergence and reproductive isolation of iS males. No differential impact of parasitism on host body condition of either lineage, or on host body condition in general, was detected. This suggests that Batrachomyia parasitism has a limited impact on L. genimaculata contact zone dynamics. The rarity of both hybrids and Batrachomyia infection precluded testing for a differential impact of parasitism on hybrid versus pure lineage frogs.
The phenotypic divergence of iS from N due to reinforcement has, as a by-product, resulted in significant divergence in iS male call and body size from the main range of S. Mate choice trials revealed that this call divergence results in highly significant premating isolation between iS and S. This strongly suggests that speciation by reinforcement between iS and N at contact B has also resulted in speciation between iS and the genetically similar main range of S. This provides the first demonstration that reinforcement between lineages can incidentally drive allopatric speciation within one of the lineages. Speciation of iS was deemed to be rapid given the estimated recency of secondary contact between the lineages, and the genetic similarity between iS and S. The iS population is described as a new species, while S and N are retained as a single species, L. genimaculata. The new species is phenotypically distinct from both S and N based on call characteristics and small male body size. This research highlights the limitations of phylogenetics in delimiting species and demonstrates the value of an approach using multiple lines of data. The biology and conservation of the new species are outlined. This research demonstrates the conservation value of contact zones in maintaining the evolutionary potential of regions.