Lineages that exhibit little morphological change over time provide a unique opportunity to explore whether nonadaptive or adaptive processes explain the conservation of morphology over evolutionary time scales. We provide the most comprehensive evaluation to date of the evolutionary processes leading to morphological similarity among species in a cryptic species complex, incorporating two agamid lizard species (Diporiphora magna and D. bilineata). Phylogenetic analysis of mitochondrial (ND2) and nuclear (RAG-1) gene regions revealed the existence of eight deeply divergent clades. Analysis of morphological data confirmed the presence of cryptic species among these clades. Alternative evolutionary hypotheses for the morphological similarity of species were tested using a combination of phylogenetic, morphological, and ecological data. Likelihood model testing of morphological data suggested a history of constrained phenotypic evolution where lineages have a tendency to return to their medial state, whereas ecological data showed support for both Brownian motion and constrained evolution. Thus, there was an overriding signature of constrained evolution influencing morphological divergence between clades. Our study illustrates the utility of using a combination of phylogenetic, morphological, and ecological data to investigate evolutionary mechanisms maintaining cryptic species.K E Y W O R D S : Agamidae, morphological evolution, morphological stasis, nonadaptive, phylogenetics.

Aim Lineages in many parts of the world have distributions that have been shaped by range contraction in the face of deteriorating climatic conditions through the Neogene. When such lineages now consist of widely spaced vicariant populations (vicars) they provide opportunities (1) to compare and contrast the timing and pattern of diversification within and between major geographical regions, and (2) to examine how differential interactions with geography, climate and localized adaptation may have shaped idiosyncratic patterns of genetic diversification. Location Australian Arid Zone (AAZ) and Australian Monsoonal Tropics (AMT). Methods We compiled and analysed mitochondrial DNA, nuclear DNA and allozyme datasets to determine the evolutionary history and genetic diversity of isolated populations of a gecko lineage (the Oedura marmorata complex and related taxa) distributed across the Australian monsoonal and arid biomes. Results Three widely allopatric lineages in the AAZ diverged during the late Miocene and Plio‐Pleistocene. Populations in the AMT display much greater genetic turnover than those in the AAZ over equivalent geographical distances, and also include several lineages that are of equal or greater age than all diversity in the AAZ. Saxicoline lineages tend to show higher genetic diversity and/or have more restricted ranges than arboreal relatives in both biomes. Main conclusions Lineages of Oedura show contrasting patterns of diversification in the two major biomes they inhabit. Diversity in the arid biome is younger and restricted to widely separated refugia, reflecting the younger age and pervasive harshness of this environment. Higher diversity in the AMT is also indicative of persistence, but at much finer geographical scales and broadly over a longer period of time, perhaps reflecting the more continuous spatial and temporal distribution of equitable habitats within this region. Finally, the wider distribution of arboreal lineages in both biomes suggests that ecological flexibility has also shaped the contrasting patterns of distribution in related lineages, and supports the contention that an association with rocks has shaped high lineage diversity in many taxa from the Australian Monsoonal Tropics.

Hybrid zones provide a rare opportunity to explore the processes involved in reproductive isolation and speciation. The southern hybrid zone between the southeastern Australian tree frogs Litoria ewingii and L. paraewingi has been comprehensively studied over the last 40 years, primarily using reproductive compatibility experiments and male advertisement calls. We used mitochondrial DNA (mtDNA) and eight nuclear microsatellite markers to characterize this hybrid zone along a historically studied transect and to test various dispersal-dependent and dispersal-independent hybrid zone models. The species are genetically distinct and the level of hybridization within the contact zone is low, with the majority of admixed individuals representing later-generation hybrids. Based on previous experimental genetic compatibility studies, we predicted that hybrids with L. paraewingi mtDNA would be more frequent than hybrids with L. ewingii mtDNA. Surprisingly, a greater proportion of the identified hybrids had L. ewingii mtDNA. Geographical cline analyses showed a sharp transition in allele frequencies across the transect, and both the mtDNA and microsatellite data showed concordant cline centres, but were best supported by a model that allowed width to vary. Overall, the L. ewingii-L. paraewingi hybrid zone is best characterized as a tension zone, due to the narrow cline width, concordant genetic clines and low levels of hybridization.

Spatio-temporal studies of hybrid zones provide an opportunity to test evolutionary hypotheses of hybrid zone maintenance and movement. We conducted a landscape genetics study on a classic hybrid zone of the south-eastern Australian frogs, Litoria ewingii and Litoria paraewingi. This hybrid zone has been comprehensively studied since the 1960s, providing the unique opportunity to directly assess changes in hybrid zone structure across time. We compared both mtDNA and male advertisement call data from two time periods (present and 1960s). Clinal analysis of the coincidence (same center) and concordance (same width) of these traits indicated that the center of the hybrid zone has shifted 1 km south over the last 40 years, although the width of the zone and the rate of introgression remained unchanged. The low frequency of hybrids, the strong concordance of clines within a time period, and the small but significant movement across the study period despite significant anthropogenic changes through the region, suggest the hybrid zone is a tension zone located within a low-density trough. Hybrid zone movement has not been considered common in the past but our findings highlight that it should be considered a crucial component to our understanding of evolution. K E Y W O R D S :Advertisement call, ancient DNA, cline, historical, hybridization, speciation.