Co-distributed species may exhibit similar phylogeographic patterns due to shared environmental factors or discordant patterns attributed to the influence of species-specific traits. Although either concordant or discordant patterns could occur due to chance, stark differences in key traits (e.g., dispersal ability) may readily explain differences between species. Multiple species’ attributes may affect genetic patterns, and it is difficult to isolate the contribution of each. Here we compare the relative importance of two attributes, range size and niche breadth, in shaping the spatial structure of genetic variation in four sedge species (genus Carex) from the Rocky Mountains. Within two pairs of co-distributed species, one species exhibits narrow niche breadth, while the other species has broad niche breadth. Furthermore, one pair of co-distributed species has a large geographical distribution, while the other has a small distribution. The four species represent a natural experiment to tease apart how these attributes (i.e., range size and niche breadth) affect phylogeographic patterns. Investigations of genetic variation and structure revealed that range size, but not niche breadth, is related to spatial genetic covariation across species of montane sedges. Our study highlights how isolating key attributes across multiple species can inform their impact on processes driving intraspecific differentiation.

Sample collection

Samples were field-collected from 10 sites that represented the majority of the range for each species (see maps in manuscript: Fig. 2 and Fig. 3, and SI Table S1). In our experimental design, the small range species were sampled from 10 identical locations and the large range species were also sampled from 10 identical locations. Across all species, an average of 7.8 individuals were collected per sampling location. Individuals were sampled from the same sites for the two small-range species (C. bella and C. chalciolepis) across Colorado, New Mexico, and Utah (a range of approximately 250,000 km²) and for the two large-range species (C. epapillosa and C. pelocarpa) across Colorado, Idaho, Montana, Nevada, Oregon, Utah, and Wyoming (a range of approximately 1,000,000 km²). Due to the close proximity of different habitat types in montane regions, we were able to sample species with different habitat preferences from the same sampling sites. The average distance among sampled plants at a locality was 300m, and the minimum distance between samples was 35m to minimize the chance of sampling siblings or close relatives. For each sampled individual, leaf tissue was collected and placed directly into silica gel desiccant. Leaf tissue was kept in the dark and stored at room temperature until DNA extraction. DNA was extracted with DNeasy Plant Mini Kits (Qiagen, Hilden, Germany) using the manufacturer’s standard protocol.

RAD-seq library construction and processing

We used a restriction site associated DNA sequencing (RAD-seq) approach to generate thousands of anonymous loci using six single-end Illumina HiSeq 2500 sequencing lanes following the protocol of Peterson et al. 2012. We used ipyrad 0.7.20 (Eaton 2014) to demultiplex and filter reads from each library, allowing one mismatch per barcode and using the most stringent filtering to identify and exclude contamination from adapters and/or primers. As some individuals were represented in more than one library, every library was initially demultiplexed and filtered separately (ipyrad steps one and two). Reads corresponding to the same individual were then combined and all reads were trimmed to a length of 41 bp using the fast-x toolkit (http://hannonlab.cshl.edu/fastx_toolkit/). We used a 90% clustering threshold for de novo assembly; all other parameter settings that affect steps three through seven were set as default (see SI Methods for additional details). Across all species, between two and 10 individuals were sequenced per sampling location, for a total of 89 C. bella individuals, 97 C. chalciolepis individuals, 71 C. epapillosa individuals, and 55 C. pelocarpa individuals (SI Table S1, S2). 

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