In recent decades, ecological factors have received increasing attention as a driver of lineage diversification and speciation (e.g., Wiens et al., 2010; Peterson, 2011; Broennimann et al., 2012; Zink, 2014; Hu et al., 2016; Fei et al., 2017; Wang et al., 2017; Gutiérrez-Ortega et al., 2020; Wei et al., 2020; Benício et al., 2021; Engler et al., 2021; Yin et al., 2021). In this debate, one of the main questions is whether newly developed species or clades exhibit signs of niche conservatism or divergence (Kozak and Wiens, 2006; Peterson, 2011; Wang et al., 2017; Zhu et al., 2020; Grossman, 2021; Meirmans, 2021; Van Dijk et al., 2021). Niche conservatism refers to a species’ or clades’ tendency to keep ancestral ecological niche characteristics over time (Wiens, 2004; Wiens and Graham, 2005; Warren et al., 2008; Wiens et al., 2010). Conversely, the process of niche divergence occurs when a species occupies a niche that is distinct from its original one (Cuervo et al., 2021). Therefore, related species or clades are more divergent within their niche space than by chance (Hua and Wiens, 2010). Nevertheless, despite the fact that recent studies have presented ecological niche metrics clearly and statistically (e.g., Broennimann et al., 2012; Brown and Carnaval, 2019; Warren et al., 2021), the relative significance of niche conservatism vs. niche divergence in driving lineage evolution in many species that pass through diverse environments and putative geographical boundaries remains poorly understood (Wiens and Graham, 2005; Alexander Pyron and Burbrink, 2009; Peterson, 2011; Hu et al., 2015).

The evidence supporting niche divergence or conservatism appears to be conflicting at first glance (Hu et al., 2015). However, there is a considerable structure when the patterns are time-structured. The temporal frames of the study systems and niche tests (i.e. niche similarity vs niche equivalency tests) influence results of niche conservatism or divergence (Peterson, 2011; Broennimann et al., 2012). The equivalency tests are more important to determine whether niche models are transferable across time and space, whereas niche similarity tests are more appropriate to test evolutionary and biogeographic assumptions (Peterson, 2011). Niches generally tend to conserve over time in terms of distribution patterns and speciation, whereas niche divergence considers being the driving force behind species diversity along ecological gradients (Graham et al., 2004; Raxworthy et al., 2007). Short-term and recent events, such as distributional shifts over short periods or species invasions, demonstrate significant tendencies toward conservatism. Longer-term events, such as phylogenetic differentiation, however, demonstrate a tendency for conservatism to break down (Wiens and Graham, 2005; Peterson, 2011). According to evidence for niche divergence, divergent natural selection promotes diversity by allowing individuals to adapt to new environments (Schluter, 2009).

The Hyrcanian wood (or brown) frogs, Rana pseudodalmatina (Anura, Ranidae), have been recorded from southeastern Azerbaijan to the Golestan National Park in northeastern Iran (Najibzadeh et al., 2018; Kidov and Litvinchuk, 2021). This study focuses on Iranian frog populations in the Hyrcanian (or Caspian, temperate) forests at altitudes ranging from −22 to 2850 meters above sea level along the northern slopes of the Alborz Mountains and the southern edge of the Caspian Sea. Based on two partial mitochondrial markers (cytochrome b and 16S rRNA), Najibzadeh et al. (2018) clearly showed the existence of two clades in the west and east of the Hyrcanian forest for this species (Najibzadeh et al., 2018). In another study using similar markers, Amiri et al. (2021) demonstrated that there existed two regional patterns within the species distribution range in the west and east, which diverged in the Pleistocene (1.6 Mya) (Amiri et al., 2021). Using species distribution modeling (SDMs hereafter), this study proposed the hypothesis of “refugia-within-refugia” in this area (Amiri et al., 2021). Based on biogeographic analyses, they suggested that dispersal and vicariance (rise of Caspian Sea water levels) processes may have influenced this species’ genetic structure (Amiri et al., 2021).

The recent study employed environmental niche models (ENMs hereafter), which combine GIS layers with species occurrence data to answer basic questions about niche evolution, speciation, and ecological diversification within clades (e.g., Kozak and Wiens, 2006; McCormack et al., 2010; Peterson, 2011; Broennimann et al., 2012; Zink, 2014; Gutiérrez-Ortega et al., 2020; Rodríguez-Rodríguez et al., 2020; Wei et al., 2020; Ahmadi et al., 2021; Cordobés et al., 2021; Purcell and Stigall, 2021). ENMs based on niche theories allow for the quantification of niches at unprecedented temporal and spatial scales (Pearman et al., 2008). ENMs may also be used to solve a wide range of evolutionary and ecological issues, including identifying niche similarities across species or clades and projecting taxonomic niches in response to changing environmental conditions (Soberón, 2007; Kozak et al., 2008; Guisan et al., 2014).

For this purpose, the goal of this study was to investigate whether the R. pseudodalmatina clades occupied the equivalent or similar niche, and if not, what role niche divergence or conservatism had in their evolution. Specifically, we used a maximum entropy approach (MaxEnt) to create ENMs for each clade based on large-scale environmental factors and occurrence data. Following that, using environmental and occurrence data, we investigated whether these two clades occupy more similar environments than estimated based on background (Hyrcanian regions) environmental differentiation using niche overlap, niche equivalency, and niche similarity tests based on principal component analysis (PCA_env). The results of these studies, when combined with the phylogeographic relationships (Najibzadeh et al., 2018, 2021; Amiri et al., 2021), give a comprehensive and multidimensional perspective of niche variation and differentiation in the Hyrcanian wood frog.

Several studies have focused on the genetic structure, phylogeny, and habitat suitability of the Hyrcanian wood frog, R. pseudodalmatina (Najibzadeh et al., 2017, 2018, 2021; Amiri et al., 2021; Kidov and Litvinchuk, 2021). However, the present study, using ENMs and multivariate niche analyses, for the first time, investigated environmental differentiation between the WRC and ERC of the Hyrcanian wood frog in the Hyrcanian forests. The hypothesis focused on whether R. pseudodalmatina lineages preserved similar niches after divergent, which might reveal new details about Hyrcanian wood frog diversification. For this purpose, niche divergence or conservatism and its relationship to lineage diversification were explored based on recent phylogeographic understanding (Najibzadeh et al., 2018, 2021; Amiri et al., 2021). The results of this study found evidence of niche divergences between WRC and ERC that may have accelerated the evolution of clades of R. pseudodalmatina. On the other hand, this research by ecological niche comparisons may provide new insights into the exploration of species diversification processes in the Hyrcanian forests. Furthermore, understanding the major ecological constraints to species distribution is critical for current conservation efforts as well as future research into the effects of climate change on biodiversity (Aguirre-Gutiérrez et al., 2015).

The factors that impact species distributions must be addressed and recognized when evaluating differences in species’ environmental niches (Guisan and Thuiller, 2005; Colwell and Rangel, 2009). Therefore, this study focused on the gradients of abiotic factors such as climate, soil, hydrology, land cover, topography, sunlight, and human impact (Supplementary Table 2) that can constrain species ranges (Gaston, 2003). In the Hyrcanian regions, the annual precipitation decreases from west to east as well as from lowlands to highlands (Tohidifar et al., 2016). The mean annual temperature also decreases from the lowlands to the mountain ranges. According to the CHELSA database,⁸ annual precipitation in the Hyrcanian area is around 1430 mm in the west (Gilan province) and around 280 mm in the east (north of Khorasan province) (Karger et al., 2017a,b). The mean annual temperature in the lowland belt is around 19.2°C (at an altitude of about −20 m a.s.l. in Gilan and Mazandaran provinces), whereas it is about 5.2°C in the upper-montane belt (at an altitude of around 2,500 m a.s.l. in Mazandaran province). According to climate station databases in lowland cities, an increase in the mean annual temperature and a decrease in annual precipitation have been recorded from west to east (Gholizadeh et al., 2020). Furthermore, according to a new bioclimatic classification, the climate in the eastern part of these forests differs from that in the western part and is classified as Mediterranean pluviseasonal oceanic and temperate oceanic, respectively (Djamali et al., 2011). The vegetation in these forests is diverse, as are the geological substrates, which include acidic rock (granite) in the west, dolomite in the center, and sandstone in the east (Siadati et al., 2010; Gholizadeh et al., 2020). Brown soil covers almost 90% of these forests, which can be followed by alluvial, colluvial, rendzina, and ranker soils (Sagheb-Talebi et al., 2014).

According to the findings, climatic and macro-environmental factors have a substantial impact on the distribution of the Hyrcanian wood frog. In more detail, based on percent contribution and Jackknife analysis (Table 1 and Supplementary Figures 4–6), climatic variables, especially precipitation, have a greater impact on the distribution range of western and eastern populations than macro-environmental variables when both BCV and MEV are taken into account. As a result, Bio17 and then Bio18 and Bio14 are the most significant variables influencing the distribution of western populations, whereas Bio18 and then Bio12 and Alt are the most important variables affecting the eastern populations’ range (Supplementary Figure 6). Based on contribution percentages, these factors were Bio18 and then LandCov and slope for western populations, and Bio17 and then Bio1 and slope for eastern populations (Table 1, the section of BCV + MEV). However, it should be noted that biotic interactions such as predators, competitors, and parasites all have an important role in restricting species distribution, which needs attention (Gaston, 2003; Gotelli et al., 2010; Sunday et al., 2011).

Differences in distribution and environmental suitability between the two lineages might be explained by niche differences and divergent reactions to the environment (Peñalver-Alcázar et al., 2021). Evaluating whether ecological features of one lineage can effectively forecast the geographic distribution of another lineage (or itself), as well as vice versa, is a reliable way to investigate this prediction (Peterson et al., 1999; Peterson and Holt, 2003). In this study, ENMs were carried out separately for WRC and ERC as well as at the SL. The habitat suitability predictions for each population were significantly different between the WRC and the ERC. The results clearly reveal that the east of the distribution region is unsuitable for the western populations in terms of BCV, MEV, and BCV + MEV. Eastern populations exhibit similar results, with the exception that in the west, restricted distribution areas for these populations have been found, although they are not highly suitable (Figures 2–4). The results of the probability of species presence show that western populations have evolved in wetter and low-temperature environments that have high precipitation, AET, AI, Alt, and slope, as well as low PET, whereas eastern populations have adapted to drier and warmer conditions with less precipitation, AET, AI, Alt, and slope, as well as high PET (Supplementary Figures 7–15). As a result, these data indicate the potential for intraspecific niche differences in R. pseudodalmatina in geographic (G) space. Therefore, these two regional clades may respond differently to future climate change.

The ecological niche metrics analysis, similar to ENMs results, indicates that WRC and ERC have undergone considerable niche divergence in both climatic and macro-environmental niches (Figures 2–4). As a consequence, the niche overlap observed revealed that the WRC and ERC have little overlap (Schoener’s D-metric), and the rejection of the niche equivalence hypothesis confirmed that the two regional clades exist in distinct environmental niche spaces. The results of the similarity test also support these predictions (Figures 2–4). Furthermore, the PCA_env results in E-space appear to be credible for the Hyrcanian wood frog, as the two regional clades occupy regions with substantial climate and environmental changes in the Hyrcanian forests (Figures 2–4). These findings indicate that niche divergence may accelerate the evolution of western and eastern R. pseudodalmatina populations within Hyrcanian forests. These results follow Brown and Carnaval (2019) conclusions that niches diverged considerably when niche overlap was low, the equivalent test was significant, and the similarity test was significant or not. However, it should be highlighted that without comprehensive physiological investigation, it may be impossible to determine whether two species’ or clades’ fundamental niches differ due to divergence, or whether their fundamental habitats are the same but their prospective niches differ due to biotic variables (Brown and Carnaval, 2019). Therefore, further research is needed on these subjects.

The niche overlap results indicated that the WRC and the ERC share a limited hybrid (or sympatric) region. This finding may confirm the results of the previous study (Amiri et al., 2021), which showed that using Circuitscape analysis, the highest connectivity between the two populations is in the central part of the distribution range (i.e., the westernmost margin of the eastern distribution range and the easternmost margin of the western distribution). According to Amiri et al. (2021), this connectivity might be due to a secondary post-glacial connection. However, one possible reason is that the WRC and ERC have similar environmental requirements in areas where admixture between the regional clades is possible (Hu and Jiang, 2012). This approach could be beneficial for identifying and designating sample zones in other species where allopatric lineages meet and exchange genes, especially in those with a deep phylogenic structure (Peñalver-Alcázar et al., 2021). Furthermore, it emphasizes that connecting multiple sources of knowledge (e.g., ecological and historical biogeography, phylogeographic and phylogenetic techniques) aids in improving the species’ evolutionary and ecological understanding (Wiens and Donoghue, 2004).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethical review and approval was not required for the animal study because this study is modeling and does not require sampling.

SV conceived and designed the study, prepared the data, interpreted the data, made the figures, and wrote, finalized, and revised the manuscript. SV and SR analyzed the data. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.