Our results uncovered different patterns of genetic structure and geographic distribution of genetic diversity in the two study species. Using a combination of different methods, we found support for our hypotheses that the two species interact differently with the same landscape factors. Jointly, our results demonstrated the individual influence of geographic distance and past and current environmental variation on the genetic structure of the study species, suggesting that genetic differentiation in D. oliveirai has been driven mainly by IBE, whereas IBD has driven the genetic differentiation in P. cuvieri. Such findings are another demonstration that selective and neutral evolutionary mechanisms act simultaneously in the same region, but with different strengths for each species. It also supports the idea that Northeastern Brazil is a mosaic of organisms with adaptations to contrasting environments such as mesic forest and xeric habitats (Gehara et al. 2017). Considering that we collected data from the same localities at the same time for both species, our investigation disentangles any effects of different current and historical landscape features which could confound our analyses and interpretations of different results between species.
The relationships between genetic variation and geographic distance, past climatic variation, and current environment in D. oliveirai demonstrate that IBD and IBE explain together the current distribution of genetic variation in this species, probably with IBE being weightier. Current and historical barriers to gene flow, as well as environmental heterogeneity and climatic changes, can cause genetic drift, increasing genetic distances among previously connected populations, while local adaptation occurs when local environmental features favor different alleles in each locality through selection pressures (Nosil et al. 2005; Balkenhol et al. 2019). Given that neutral and selective mechanisms can act together resulting in IBE (Wang and Bradburd 2014), we believe that both genetic drift and local adaptation conduce the genetic differentiation in D. oliveirai. Although our study is based on presumed neutral genetic markers, an adaptive process can still be indirectly inferred from the data (Andrew et al. 2012; Manel and Holderegger 2013). Additionally, the effect of the past climatic variation only on the genetic variability of D. oliveirai confirmed that past climatic instability had an important role in the past gene flow and genetic diversity of species adapted to mesic conditions. Such influence of historical climatic variation on genetic differentiation of tropical amphibian species is already known (e.g. Carnaval et al. 2009; Gehara et al. 2017; Brusquetti et al. 2019), especially for those adapted to mesic climates and/or living in highland forests (e.g. Carnaval and Bates 2007; Amaro et al. 2012; Guarnizo and Cannatella 2013; Oliveira et al. 2021). Our findings indicate that D. oliveirai is better adapted to mesic habitats and that climatic fluctuations, probably aridification, during the Pleistocene caused the isolation of populations in separated forest microrefugia. Today, the connectivity between these populations seems to be mainly influenced by the east-west climatic/environmental gradient, which results in genetic differentiation longitudinally.
Genetic variability in P. cuvieri was mostly explained by IBD with a very small contribution from the environmental variation, which is also expected for wide-range species (see Jenkins et al. 2010). Being widely distributed from the south to the north of South America (Haddad et al. 2008; Frost 2023), P. cuvieri experiences a variety of habitat conditions, which could shape its genetic variation according to a latitudinal gradient. A possible important factor is the latitudinal asynchrony of the rainy season (see Bates et al. 2008; Silva et al. 2019), which is important to explosive breeding amphibians (Aichinger 1987; Duellman 1995; Prado et al. 2005; Ulloa et al. 2019). Such asynchrony could lead to asynchrony in breeding season among populations (mate mismatch) in different latitudinal localities, which in turn could result in genetic differentiation, even among short geographic distances, especially in regions of high topographic relief (Martin et al. 2009). In other words, it is possible that the strong association between geographic distance and genetic variation in P. cuvieri includes the effect of asynchrony in the rainy season along this latitudinal gradient (see Maes et al. 2006; Thomé et al. 2021). Similar patterns were previously described for a widely distributed anuran endemic to the Caatinga, Rhinella granulosa (Thomé et al. 2021), and for Amietia wittei, a tropical Afromontane frog (Zancolli et al. 2014).
Here, we found that geographic distance is the most important landscape feature influencing genetic differentiation in the habitat generalist (P. cuvieri), whereas the genetic differentiation of the habitat specialist (D. oliveirai) was highly influenced by a combined effect of past climatic variation, current environment, and geographic distance. Barring any particular unknown historical reasons, the contrasting levels of genetic structure that we found between D. oliveirai and P. cuvieri result from their different abilities to cross the dry lowlands between highland forests. Our findings join the literature of landscape genetics in tropical regions suggesting or demonstrating that landscape features act on genetic variation depending on species-specific traits (e.g. Blair et al. 2013; Engler et al. 2014; Zancolli et al. 2014; Nowakowski et al. 2015; Sandberger-Loua et al. 2017; Monteiro et al. 2019; Nali et al. 2020). Such studies usually indicate elevation, forest cover, rivers, and geographic distance as the main factors responsible for resistance or permeability to gene flow, but always at different degrees of importance for different species. In our previous work investigating acoustic variation and its drivers in the same populations we demonstrated that geographic and environmental distance have, respectively, moderate and higher association with acoustic distance among P. cuvieri populations, while environmental distance is moderately related to acoustic distance among D. oliveirai populations, with body size and genetic differentiation as covariates (Andrade Lima et al. 2024). Since IBD and IBE imply, respectively, genetic drift and divergent natural selection (Dobzhansky 1937; Crispo et al. 2006; Lee e Mitchell-Olds 2011; Wang 2013, Sexton et al. 2014), combining our current and past findings, we suggest that selective and neutral evolutionary mechanisms conduct intraspecific genetic variation in this Neotropical region, with varied relative importance depending on the ecology of the species. Future investigations involving spatiotemporal demographic analyses using genomic data from these populations will make it possible to confirm local selective processes on the habitat specialist species.
Genetic structure and diversity among localities suggest different levels of landscape permeability for each species. Body size and reproductive mode are life-history traits that can determine habitat preferences, dispersal potential and, consequently, genetic structure (Wollenberg et al. 2011; Paz et al. 2015; Lourenço et al. 2019). Considering that D. oliveirai and P. cuvieri have distinct reproduction habits and ecological niches, perhaps these traits are contributing to the differentiation in their population genetic structure. Although D. oliveirai is not a strict forest dependent species, this tiny treefrog has a reproductive mode strongly related to ponds with long hydroperiod and herbaceous vegetation, which are common places in montane forests in the study region but usually rare in the surrounding lowlands. This potentially led to geographic isolation or lower gene flow between populations. Patterns of limited dispersal and high genetic structure resulting from the absence of suitable environments for reproduction have been seen in other montane amphibians, such as Scinax treefrogs in the central Atlantic Forest (Santana et al. 2024), Melanophryniscus and Brachycephalus toads in the southern Atlantic Forest (Pie et al. 2018), and Ensatina salamanders in the western United States (Devitt et al. 2013). On the other hand, we found less phylogeographic structure in P. cuvieri, as expected for habitat generalist species (Miller et al. 2015; Kort et al. 2021), especially those adapted to breeding in low hydroperiod water bodies in dry areas (Ortiz et al. 2018). Besides its association with dry habitats, this species uses temporary ponds to reproduce, with large clutch sizes and fast larval development (Barreto and Andrade 1995; Haddad and Prado 2005; Aguiar et al. 2014). Such life-history traits may increase dispersal ability in amphibians (Van Bocxlaer et al. 2010; Streicher et al. 2012). As such, the genetic structure found in P. cuvieri by our study is probably a result of its dispersal capability and not of environmental barriers.
Despite differences in the patterns of genetic structure between D. oliveirai and P. cuvieri, we found a putative congruent phylogeographic break close to 8° latitude, between northeastern and central localities, where the Capibaribe and Ipojuca rivers are located. Although very subtle, similar phylogeographic patterns were previously documented in other anurans, Proceratophrys renalis and Pristimantis ramagii, in the same region (Carnaval 2002; Carnaval and Bates, 2007). Since rivers could represent complete or partial barriers depending on the dispersal ability of each species (Coelho et al. 2022), such congruences could suggest a vicariant role (more or less pervasive) of the Capibaribe and Ipojuca rivers for different species. These rivers are the only physical landscape elements near that latitude, and have never been suggested to be or investigated as a barrier before. Thus, we highlight that even those smaller rivers could also account for the current genetic structure of different species in northeastern Brazil. Another comparative study involving more animal species from different groups, or even a most comprehensive genetic dataset of the present study species, could investigate this question more confidently. Regardless of the rivers possibly affecting the genetic differentiation of the present study species, the influence of geographic distance was clearly demonstrated to both species in our analyses. Therefore, the phylogeographic breaks resulting from any local barrier between the northeastern and central localities do not account for the full relationship between geographic and genetic distances.