Ecological Speciation in Nolina parviflora (Asparagaceae): Lacking Spatial Connectivity along of the Trans-Mexican Volcanic Belt

Abstract

The hypothesis of ecological speciation states that as populations diverge in different niches, reproductive isolation evolves as a by-product of adaptation to these different environments. In this context, we used Nolina parviflora as a model to test if this species evolved via ecological speciation and to explore current and historical gene flow among its populations. Nolina parviflora is a montane species endemic to Mexico with its geographical distribution restricted largely to the Trans-Mexican Volcanic Belt. This mountain range is one of the most complex geological regions in Mexico, having undergone volcanism from the mid-Miocene to the present. Ecologically, the Trans-Mexican Volcanic Belt possesses different types of vegetation, including tropical dry forest; oak, pine, pine-oak, and pine-juniper forests; and xerophytic scrub - all of which maintain populations of N. parviflora. Using species distribution models, climatic analyses, spatial connectivity and morphological comparisons, we found significant differences in climatic and morphological variables between populations of N. parviflora in two distinct Trans-Mexican Volcanic Belt regions (east vs. west). This could mean that the geographically isolated populations diverged from one another via niche divergence, indicating ecological speciation. Spatial connectivity analysis revealed no connectivity between these regions under the present or last glacial maximum climate models, indicating a lack of gene flow between the populations of the two regions. The results imply that these populations may encompass more than a single species.

Landscape genomics provides evidence of climate‐associated genetic variation in Mexican populations ofQuercus rugosa

“…Studies of other temperate and subtropical tree species have also identified a greater proportion of SNPs associated with temperature than with precipitation (Cox, Vanden Broeck, Van Calster, & Mergeay, 2011;De Kort et al, 2014;Gugger et al, 2016;Huang et al, 2015;Jaramillo-Correa et al, 2015). In addition, studies of high-altitude co-occurring species along the TMVB have found a strong and significant historical influence of temperature variables in shaping geographic distribution (Ruiz-Sanchez & Specht, 2014;Velo-Antón et al, 2013). But, in oaks, the number of SNPs associated with temperature and precipitation variables varies among species.…”

Biodiversity in the Mexican highlands and the interaction of geology, geography and climate within the Trans‐Mexican Volcanic Belt

“…This has already been partially achieved: several TMVB taxa show a west–east division of sister lineages, or structuring of genetic variation, that are neither congruent with geographical distances among sampling sites, nor with current topographic connectivity (e.g. Bryson & Riddle, ; Bryson et al ., ,b; Parra‐Olea et al ., ; Ruiz‐Sanchez & Specht, ). However, the geographical sampling among these studies is not comparable.…”

“…For example, landscapes can be represented as conductive surfaces, with low resistances assigned to areas that best promote gene flow (McRae et al, 2008), which in turn can be modelled based on elevation or present and past climate data. Both Ruiz-Sanchez & Specht (2014) and Velo-Ant on et al (2013) have applied these methods to explicitly test for population connectivity across the TMVB, and in Fig. 6 we show how elevation data can be used to estimate effective distances under different gene flow scenarios.…”

Pleistocene glacial cycles drive isolation, gene flow and speciation in the high‐elevation Andes

“…Rapid diversification in high-elevation montane habitats has often been assumed to result from increased ecological opportunities owing to the high physiographical heterogeneity in mountains prompting frequent episodes of geographic isolation and allopatric speciation (Simpson, 1964;Hughes & Eastwood, 2006). However, these highly heterogeneous montane habitats may also promote adaptive divergence via ecological speciation (Chapman et al, 2013;Ruiz-Sanchez & Specht, 2014;Contreras-Ortiz et al, 2018). Furthermore, patterns of geographic isolation in mountain ranges have changed dramatically over time most notably during Pleistocene glacial cycles (Flantua & Hooghiemstra, 2018).…”